Methodology
All GainTally estimates are built on open, verifiable formulas — no black boxes. This page documents the formula, key assumptions, common edge cases, and primary data sources plus authoritative benchmarks (NREL, EIA, DOE, EPA, ENERGY STAR, IRS, SAE, CFPB) for each of the 20 calculators. Every result can be reproduced with publicly available data. If you find an error or want to discuss our approach, email [email protected].
Formula validation updated: May 2026(may be outdated)
Technical terms used on this page: Payback Period, LCOE, PVWatts, ROI, Peak Sun Hours. See the full Glossary for all definitions.
Solar ROI Calculator
Formula
INPUT:
system_size_kw (number, 3–15 kW)
annual_sun_hours (kWh/kW/yr, from NREL PVWatts API)
electricity_rate ($/kWh, from EIA state average)
system_cost_per_watt ($/W, default $2.75/W mid-tier)
panel_type (monocrystalline | polycrystalline | thin-film)
state_incentive ($, optional flat-dollar rebate — default $0)
CALCULATION:
gross_cost = system_size_kw × 1000 × system_cost_per_watt
net_cost = gross_cost − state_incentive
base_production = system_size_kw × annual_sun_hours × 0.80 × 0.86 (kWh/yr)
lid_factor = 1 − LID (Mono 2%, Poly 3%, Thin-film 5% — year 1 only)
year_n_factor = lid_factor × (1 − annual_degradation_rate)^(n−1)
year_n_savings = base_production × year_n_factor × electricity_rate × (1 + 2.5%)^n
payback_year = first n where Σ savings ≥ net_cost (max 26)
lcoe = net_cost / Σ 25-year production ($/kWh)
total_savings_25yr = Σ gross_savings − net_cost
OUTPUT:
net_cost ($)
payback_year (yr, 1–26)
total_savings_25yr ($)
lcoe ($/kWh)
yearly_projections (array: year, production_kwh, savings, cumulative_savings)
BENCHMARK:
±5–10% of NREL PVWatts v8 annual production estimate
±1.5 yr payback vs EnergySage Solar CalculatorAssumptions
- Performance ratio: 0.80 (NREL modern system median)
- System losses: 14% (PVWatts v8 default)
- Panel degradation — Monocrystalline: 0.5%/yr, Polycrystalline: 0.7%/yr, Thin-film: 0.8%/yr
- LID (Light Induced Degradation) applied in year 1 only: Mono 2%, Poly 3%, Thin-film 5%
- Electricity inflation: 2.5%/yr (EIA historical average)
- 25-year analysis window; payback reported as 26 if not reached
Edge Cases
- Coastal microclimate (e.g., San Francisco fog belt): annual averages may overstate output by 5–10% versus PVWatts hourly data — cross-check with PVWatts hourly export for shaded sites.
- Leased system (TPO/PPA): the homeowner does not own the system, so federal ITC was claimed by the installer (historical only — 25D is $0 after Dec 31, 2025) and electricity-inflation hedging accrues to the lease holder, not the resident.
- Stacked state-plus-federal incentives (NY-Sun + 25D, historical 2025 only): real-world payback was 1–3 years shorter than calculator default — these stacks no longer apply for 2026+ installations.
Data Sources
- NREL PVWatts v8 — annual sun hours by zip code
- EIA — state electricity rates
- EnergySage — payback benchmark validation
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| NREL PVWatts v8 (annual production) | ±5–10% | 2026-05 |
| EnergySage Solar Calculator (payback) | ±1.5 yr payback | 2026-05 |
Solar Panels Needed Calculator
Formula
INPUT:
monthly_kwh (kWh/mo, typical US home ~900 kWh)
annual_sun_hours (kWh/kW/yr, from NREL PVWatts API)
panel_watts (W, 250–440 W per panel)
system_losses (fraction, default 0.14)
CALCULATION:
annual_kwh = monthly_kwh × 12
system_size_kw = annual_kwh / (annual_sun_hours × 0.80 × (1 − system_losses))
panel_count = ⌈system_size_kw × 1000 / panel_watts⌉ (rounded up — no partial panels)
roof_area_sqft = panel_count × 17.5
OUTPUT:
panel_count (integer)
system_size_kw (kW)
roof_area_sqft (ft²)
BENCHMARK:
±10% of NREL PVWatts module sizing methodology
±15% of EnergySage panel count methodologyAssumptions
- Performance ratio: 0.80; system losses: 14%
- Standard panel footprint: 17.5 ft² (typical 60-cell module)
- Panel count rounded up — partial panels are not possible
- Sun hours sourced from NREL PVWatts API by zip code
Edge Cases
- Heavy shading (>30% derate from trees, chimneys, or neighbors): add 30–50% more panels or use micro-inverters / power optimizers — calculator uses unshaded baseline.
- Split east-west roof orientation: panel count rises ~10–15% versus an ideal south-facing array because per-panel production drops with off-axis azimuth.
- High-efficiency 400–440 W panels reduce panel count by ~40–50% compared to legacy 250 W modules but cost 15–25% more per watt at the module level.
Data Sources
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| NREL PVWatts (module sizing) | ±10% | 2026-05 |
| EnergySage panel count methodology | ±15% | 2026-05 |
Solar Installation Cost Calculator
Formula
INPUT:
system_size_kw (kW)
installation_type (roof | ground)
CALCULATION:
system_size_w = system_size_kw × 1000
mount_premium = ground ? ×1.20 : ×1.00
low_estimate = system_size_w × $1.50/W × mount_premium
mid_estimate = system_size_w × $2.75/W × mount_premium
high_estimate = system_size_w × $3.50/W × mount_premium
OUTPUT:
low_estimate ($)
mid_estimate ($)
high_estimate ($)
cost_per_watt ($/W, mid tier)
BENCHMARK:
±10% of SEIA Solar Market Insight national $/W average
±15% of EnergySage state-level marketplace dataAssumptions
- $/W tiers based on SEIA 2024 national installation averages
- Ground-mount adds 20% over roof-mount (additional foundation and racking cost)
- Federal 25D residential solar tax credit: $0 for installations after Dec 31, 2025 (expired per OBBBA, Pub.L. 119-21)
- State incentives and rebates not included — see Solar Tax Credit Calculator
Edge Cases
- Bulk installation discount (>10 kW residential, light-commercial leaning): $/W can drop 10–20% below the Low tier as soft costs amortize over more capacity.
- Premium modules (SunPower Maxeon / LG NeON / REC Alpha, 22%+ efficiency): add 20–40% to $/W versus mainstream 19–20% modules — pays back only if roof area is the binding constraint.
- Post-2025 installations receive $0 federal 25D credit (expired). Out-of-pocket cost equals the gross install price minus any state-only incentives.
Data Sources
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| SEIA Solar Market Insight (national $/W) | ±10% | 2026-05 |
| EnergySage Marketplace state data | ±15% | 2026-05 |
Solar Battery Storage Calculator
Formula
INPUT:
battery_capacity_kwh (kWh, usable capacity)
battery_cost ($)
peak_rate ($/kWh, on-peak electricity rate)
off_peak_rate ($/kWh, off-peak electricity rate)
cycles_per_year (integer, typical 250–365)
battery_life_years (yr, 10–15 per warranty)
CALCULATION:
price_spread = max(0, peak_rate − off_peak_rate)
annual_savings = battery_capacity_kwh × cycles_per_year × price_spread
payback_years = battery_cost / annual_savings
lifetime_savings = annual_savings × battery_life_years − battery_cost
cost_per_cycle = battery_cost / (cycles_per_year × battery_life_years)
OUTPUT:
annual_savings ($)
payback_years (yr)
lifetime_savings ($)
cost_per_cycle ($/cycle)
BENCHMARK:
±5% of Tesla Powerwall warranty spec (manufacturer-rated cycles)
±5% of Enphase IQ Battery spec sheetAssumptions
- Time-of-use (TOU) arbitrage model: charges off-peak, discharges at peak
- Negative spread (off-peak rate > peak rate) results in $0 savings
- Depth of discharge not modeled separately — user enters usable capacity
- No battery degradation curve; conservative for long-term projections
Edge Cases
- Backup-only deployments (rare cycling, primarily power-outage resilience): TOU arbitrage savings are near $0; financial payback is essentially never — value is captured in outage avoidance, not bill savings.
- Low rate-spread utilities (<$0.05/kWh peak vs. off-peak): payback exceeds typical 10–15 year warranty; consider whether storage is justified versus a generator.
- LFP (LiFePO4) vs. NMC chemistry: LFP rated ~6,000 cycles, NMC ~4,000 — multiply Cycles × Battery Life inputs accordingly when comparing real specs.
Data Sources
- Tesla Powerwall — capacity and cycle specifications
- Enphase IQ Battery — spec sheets
- NREL — Residential Battery Storage report
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| Tesla Powerwall datasheet (warranty cycles) | ±5% (manufacturer-rated) | 2026-05 |
| Enphase IQ Battery spec sheet | ±5% (manufacturer-rated) | 2026-05 |
Solar Panel Degradation Calculator
Formula
INPUT:
initial_production_kwh (kWh/yr, year-1 baseline output)
panel_type (monocrystalline | polycrystalline | thin-film)
analysis_years (integer, 1–30)
CALCULATION:
lid_factor = 1 − LID (Mono 2%, Poly 3%, Thin-film 5% — year 1 only)
annual_rate = {Mono: 0.5%/yr, Poly: 0.7%/yr, Thin-film: 0.8%/yr}
year_n_factor = lid_factor × (1 − annual_rate)^(n−1)
year_n_production = initial_production_kwh × year_n_factor
total_loss_kwh = Σ (initial_production − year_n_production) for n=1..years
OUTPUT:
yearly_production (array: year, production_kwh, factor)
total_loss_kwh (kWh over analysis period)
BENCHMARK:
±0.1%/yr of NREL PV Module Reliability Workshop median degradation rate
±1% LID vs IEC 61215 qualification testAssumptions
- LID applied once in year 1: Monocrystalline 2%, Polycrystalline 3%, Thin-film 5%
- Annual degradation rates: Mono 0.5%/yr, Poly 0.7%/yr, Thin-film 0.8%/yr
- NREL median across all panel types: 0.75%/yr (panel-type breakdown is more accurate)
- Linear degradation model — real-world curves are slightly logarithmic but within ±5%
Edge Cases
- Tier-1 versus Tier-2 modules: NREL field data shows ~0.5%/yr for Tier-1 brands and 0.9–1.0%/yr for unknown / utility-overstock modules — adjust the annual rate input.
- Cold-climate thermal cycling (Northern Plains, Mountain West): freeze-thaw stress can accelerate solder-bond and EVA browning failures by 10–20% versus mild climates.
- Year-25 warranty cliff: most manufacturers guarantee 80–85% of nameplate at year 25; modules continuing past this point operate without a warranty backstop.
Data Sources
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| NREL PV Module Reliability Workshop | ±0.1%/yr | 2026-05 |
| IEC 61215 LID qualification test | ±1% LID | 2026-05 |
Net Metering Savings Calculator
Formula
INPUT:
system_size_kw (kW)
annual_sun_hours (kWh/kW/yr, from NREL PVWatts API)
monthly_consumption_kwh (kWh/mo, household electricity use)
buy_rate ($/kWh, retail electricity rate)
sell_rate ($/kWh, net metering export credit rate)
CALCULATION:
monthly_production = system_size_kw × annual_sun_hours × 0.80 × 0.86 / 12
self_consumed = min(monthly_production, monthly_consumption_kwh)
export_kwh = max(0, monthly_production − monthly_consumption_kwh)
annual_net_savings = self_consumed × buy_rate × 12
+ export_kwh × sell_rate × 12
OUTPUT:
monthly_production (kWh/mo)
self_consumed_kwh (kWh/mo)
export_kwh (kWh/mo)
annual_net_savings ($)
BENCHMARK:
policy-level: DSIRE state net-metering policy database
exact tariff: CPUC NEM-3 decision (California export rate)Assumptions
- Performance ratio: 0.80; system losses: 14%
- Monthly averages used — seasonal production variation not modeled
- NEM 3.0 sell rate ≈ 20–30% of retail rate (CPUC NEM-3 decision)
- Traditional net metering: sell rate equals the retail buy rate
Edge Cases
- California NEM 2.0 grandfathered customers (interconnected before April 14, 2023) keep full retail-rate export for 20 years from interconnection — payback typically 2–3 years shorter than NEM 3.0.
- Non-NEM states (Hawaii, Mississippi, Tennessee TVA service): no export credit; payback depends entirely on self-consumption — consider sizing the system to match daytime load only.
- Utility-specific aggregate caps and queue freezes: even within a NEM-friendly state, the local utility may be at its cap and route new customers to a successor tariff with reduced credit.
Data Sources
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| DSIRE state net-metering policy | policy-level (no numeric margin) | 2026-05 |
| CPUC NEM-3 Decision (California export rate) | exact tariff | 2026-05 |
Solar Panel Tilt Angle Calculator
Formula
INPUT:
latitude_deg (°, 0–60 covers continental US and Alaska)
current_tilt_deg (°, 0–90)
system_size_kw (kW)
CALCULATION:
optimal_tilt_deg = latitude_deg (within ±3° of PVWatts output)
reference_production = system_size_kw × 1500 × 0.80 × 0.86 (kWh/yr)
tilt_delta_rad = |current_tilt_deg − optimal_tilt_deg| × π / 180
current_efficiency = cos(tilt_delta_rad)
current_production = reference_production × current_efficiency
optimal_production = reference_production (at optimal tilt)
production_gain_pct = (optimal_production − current_production) / current_production × 100
OUTPUT:
optimal_tilt_deg (°)
current_production (kWh/yr, estimate)
optimal_production (kWh/yr, estimate)
production_gain_pct (%)
BENCHMARK:
±3° of NREL PVWatts azimuth/tilt sensitivity analysis
±2° seasonal vs PVWatts V5 Manual tilt methodologyAssumptions
- Simplified model: optimal tilt ≈ local latitude (within ±3° of PVWatts output)
- Reference sun hours: 1,500/yr (US national average used for fair comparisons)
- Valid latitude range: 0°–60° (covers continental US and Alaska)
- Seasonal rule of thumb: summer tilt = latitude − 15°, winter = latitude + 15°
Edge Cases
- Flat roof (0° pitch): add a 10–15° minimum mechanical tilt for water drainage and self-cleaning rain — the 'latitude' optimum is impractical without ballast or rails.
- Adjustable ground mounts (manual seasonal pivot): summer tilt = latitude − 15°, winter = latitude + 15° captures 5–8% more annual production than fixed tilt.
- High-latitude (>45°N) installations: steep winter tilts (60–65°) are impractical due to snow load and wind uplift — fixed tilt near latitude is the practical compromise.
Data Sources
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| NREL PVWatts azimuth/tilt sensitivity | ±3° | 2026-05 |
| PVWatts V5 Manual (tilt methodology) | ±2° seasonal | 2026-05 |
Solar vs Grid Cost Calculator
Formula
INPUT:
system_size_kw (kW)
annual_sun_hours (kWh/kW/yr)
system_cost ($, total installed after incentives)
electricity_rate ($/kWh)
electricity_inflation (fraction, default 0.03)
years (integer, 1–30)
CALCULATION:
annual_production = system_size_kw × annual_sun_hours × 0.80 × 0.86
annual_grid_cost_n = annual_production × electricity_rate × (1 + inflation)^n
cumulative_grid_cost = Σ annual_grid_cost for n=1..years
solar_total_cost = system_cost + $150/yr × years (flat annual maintenance)
savings = cumulative_grid_cost − solar_total_cost
OUTPUT:
cumulative_grid_cost ($)
solar_total_cost ($)
savings ($)
yearly_comparison (array: year, solar_cumulative, grid_cumulative)
BENCHMARK:
±2% of EIA state retail electricity rates (monthly survey)
±15% of EnergySage 25-year solar vs grid comparisonAssumptions
- Annual system maintenance: $150/yr flat (covers inverter monitoring, cleaning)
- Production held constant across years in this model (no degradation applied)
- Electricity inflation entered by user; default 3%
- System cost entered as total installed cost after any incentives applied
Edge Cases
- Time-of-use (TOU) utility plans: solar savings can be 10–20% higher than flat-rate output suggests, because daytime production aligns with peak-rate hours.
- Heating-dominant homes (cold-climate gas furnace + electric AC): winter heating bill does not shift with solar — sizing should match the summer-electric / annual blended load, not just peak summer.
- Inflation horizon choice matters: EIA 1990–2024 historical average is ~2.5%/yr; the 2008–2024 window is closer to 3.0%/yr — entering 4–5% optimistically overstates 25-year savings.
Data Sources
- EIA — historical electricity rate inflation
- EnergySage — 25-year solar vs. grid comparison methodology
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| EIA state retail electricity rates | ±2% (monthly) | 2026-05 |
| EnergySage 25-year comparison | ±15% | 2026-05 |
Solar Loan Calculator
Formula
INPUT:
loan_amount ($, principal P)
annual_rate_pct (APR %; 0% triggers zero-interest special case)
loan_term_years (yr)
annual_savings ($/yr, estimated solar electricity bill savings)
CALCULATION:
monthly_rate = annual_rate_pct / 12 / 100
n_months = loan_term_years × 12
monthly_payment = P × [r(1+r)^n] / [(1+r)^n − 1] (0% rate: P / n)
total_interest = monthly_payment × n_months − P
break_even_year = first year where Σ (annual_savings − annual_payments) ≥ 0
net_25yr_savings = annual_savings × 25 − monthly_payment × 12 × loan_term_years
OUTPUT:
monthly_payment ($)
total_interest ($)
break_even_year (yr)
net_25yr_savings ($)
BENCHMARK:
±$1 monthly payment vs CFPB standard amortization reference
±$1 monthly payment vs Bankrate Loan CalculatorAssumptions
- Standard amortization formula (verified against CFPB mortgage calculator)
- 25-year analysis window for break-even and net savings calculation
- Annual solar savings assumed constant (no electricity inflation adjustment)
- No prepayment, refinancing, or loan origination fees modeled
Edge Cases
- Secured solar loan via HELOC or cash-out refinance: interest is potentially deductible under IRS Pub 936 'home acquisition or improvement' rules — consult a licensed CPA or tax professional before assuming a deduction.
- Re-amortization after the 2025-only 25D carry-forward: borrowers who claimed the credit in 2025 (last eligible year) and applied it as a principal paydown can request a re-amortized lower payment — model does not auto-adjust.
- Dealer-fee 'no-money-down' solar loans: an 18–22% dealer fee is rolled into principal, raising effective APR by 4–6 percentage points above the advertised rate — request the cash price and finance separately when possible.
Data Sources
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| CFPB amortization reference | ±$1 monthly payment | 2026-05 |
| Bankrate Loan Calculator | ±$1 monthly payment | 2026-05 |
Solar Tax Credit Calculator
Formula
INPUT:
system_cost ($)
include_state_tax_credit (boolean)
state_credit_rate (fraction 0–1; used only if include_state_tax_credit = true)
CALCULATION:
federal_itc_rate = 0.30 if installed ≤ Dec 31 2025, else 0.00
(OBBBA Pub.L. 119-21 — residential 25D credit expired)
federal_credit = system_cost × federal_itc_rate
state_credit = include_state_tax_credit ? system_cost × state_credit_rate : 0
total_credit = federal_credit + state_credit
net_system_cost = max(0, system_cost − total_credit)
OUTPUT:
federal_credit ($)
state_credit ($)
total_credit ($)
net_system_cost ($)
BENCHMARK:
exact statutory: IRS Form 5695 instructions (rate 30% → 0% after 2025)
program-level: DSIRE state incentive databaseAssumptions
- Federal residential solar credit (25D) expired December 31, 2025 per OBBBA (Pub.L. 119-21, July 4, 2025)
- 2025 installations may carry unused credit forward to the 2026 tax year
- State incentive rates sourced from DSIRE database (vary by program year and funding)
- Only direct tax credits modeled; utility rebates and performance payments excluded
Edge Cases
- Installations placed in service after December 31, 2025 receive $0 federal 25D credit. 'Placed in service' generally means the system passes inspection and is operational — confirm with installer documentation and consult a licensed CPA or tax professional.
- 2025 carry-forward of unused 25D credit: any unused portion of a 2025 credit may be carried forward against 2026 tax liability — this is the last year carry-forward is available (no new 25D credit in 2026).
- State and utility incentives are unaffected by the 25D expiration: NY-Sun, MA SMART, CA self-generation rebates, and DSIRE-listed programs continue under their own statutes — federal expiration does not auto-cancel state credit.
Data Sources
- IRS Form 5695 — Residential Energy Credits
- OBBBA Pub.L. 119-21 — credit expiration reference
- DSIRE — state solar incentive database
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| IRS Form 5695 instructions | exact (statutory) | 2026-05 |
| DSIRE state incentive database | program-level (no numeric margin) | 2026-05 |
EV Charging Cost Calculator
Formula
INPUT:
battery_capacity_kwh (kWh, energy added this session = capacity × SoC change)
charging_efficiency (fraction; L1/L2: 0.90, DC Fast: 0.85)
electricity_rate ($/kWh)
miles_per_charge (mi, driving range from this charge)
CALCULATION:
actual_energy_kwh = battery_capacity_kwh / charging_efficiency
cost_per_charge = actual_energy_kwh × electricity_rate
cost_per_mile = cost_per_charge / miles_per_charge
monthly_cost = cost_per_mile × 1,000 (US avg 1,000 mi/mo)
annual_cost = monthly_cost × 12
OUTPUT:
cost_per_charge ($)
cost_per_mile ($/mi)
monthly_cost ($)
annual_cost ($)
BENCHMARK:
±5% of DOE eGallon methodology
±3% kWh vs AFDC Vehicle Cost CalculatorAssumptions
- L1 (120V) charging efficiency: 90%; L2 (240V): 90%; DC Fast: 85%
- DC Fast held conservative vs. EPA range (88–90%) to avoid under-estimating cost
- Average monthly driving: 1,000 miles (US average ~12,000 miles/yr)
- Electricity rate is user-entered or sourced from EIA state averages
Edge Cases
- Time-of-use plans: peak rates ($0.40–$0.50/kWh on PG&E E-TOU-C 4–9 PM) versus super-off-peak ($0.08–$0.12/kWh overnight) can produce a 4–5× cost swing for the same kWh — schedule charging accordingly.
- Apartment / condo drivers without home L2 access: 100% public-charging dependency raises per-mile cost 2–4× over home overnight charging because public DC Fast averages $0.40–$0.60/kWh.
- Highway DC Fast (Electrify America / EVgo road trips): $0.43–$0.56/kWh non-member rates are 3–5× home electricity — the calculator's default home-rate model underestimates road-trip cost.
Data Sources
- DOE eGallon Methodology — charging cost reference
- EPA / FuelEconomy.gov — charging efficiency data
- AFDC Vehicle Cost Calculator — benchmark validation
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| DOE eGallon methodology | ±5% | 2026-05 |
| AFDC Vehicle Cost Calculator | ±3% kWh | 2026-05 |
EV vs Gas Cost Calculator
Formula
INPUT:
ev_purchase_price ($)
gas_purchase_price ($)
ev_efficiency_miles_per_kwh (mi/kWh)
electricity_rate ($/kWh)
gas_price ($/gal)
gas_mpg (mpg)
annual_miles (mi/yr)
years (integer, 1–15)
ev_tax_credit ($, 30D credit; $0 for 2026+ per OBBBA)
CALCULATION:
annual_ev_fuel = annual_miles / ev_efficiency × electricity_rate
annual_gas_fuel = annual_miles / gas_mpg × gas_price
net_ev_purchase = ev_purchase_price − ev_tax_credit
ev_cumulative_n = net_ev_purchase + (annual_ev_fuel + $400 maintenance) × n
gas_cumulative_n = gas_purchase_price + (annual_gas_fuel + $700 maintenance) × n
break_even_year = first n where gas_cumulative ≥ ev_cumulative
OUTPUT:
ev_total_cost ($)
gas_total_cost ($)
ev_savings ($, positive = EV cheaper over analysis period)
break_even_year (yr)
yearly_comparison (array: year, ev_cumulative, gas_cumulative)
BENCHMARK:
±10% of AFDC Vehicle Cost Calculator TCO
±$100/yr maintenance vs AAA Your Driving Costs 2024Assumptions
- EV annual maintenance: $400; ICE: $700 (AAA 'Your Driving Costs' 2024 report)
- Insurance and depreciation assumed equal between EV and gas vehicle
- Federal 30D EV tax credit: $0 for 2026+ (expired per OBBBA, Pub.L. 119-21)
- Fuel prices held constant across years (no inflation applied)
Edge Cases
- Resale-value divergence: EVs held 60–65% at 3 years and gas vehicles held 50–55% (KBB 2024 5-year residuals) — model assumes equal depreciation; reality favors EV for short ownership horizons.
- EV insurance can run 15–25% higher than equivalent ICE premiums due to higher repair cost and limited bodyshop availability — consult a licensed insurance agent for accurate quotes.
- Gasoline volatility: $2.30 (April 2020) to $4.85 (June 2022) US average — modeling at a single constant rate masks the realistic 25–40% variance an ICE owner experiences over a 5-year hold.
Data Sources
- AFDC Vehicle Cost Calculator — TCO methodology
- AAA 'Your Driving Costs' 2024 — maintenance figures
- EIA — electricity and gasoline price data
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| AFDC Vehicle Cost Calculator (TCO) | ±10% | 2026-05 |
| AAA Your Driving Costs 2024 | ±$100/yr maintenance | 2026-05 |
EV Range Calculator
Formula
INPUT:
battery_capacity_kwh (kWh)
efficiency_miles_per_kwh (mi/kWh, EPA-rated)
temperature_f (°F, −20 to 110)
hvac_usage (none | low | high)
speed_mph (mph, typical 25–85)
CALCULATION:
base_range = battery_capacity_kwh × efficiency_miles_per_kwh
temp_factor = linear_interpolate(temperature_f;
−20°F→0.60, 70°F→1.00, 100°F→0.85)
hvac_factor = {none:1.00, low:0.92, high:0.87}[hvac_usage]
speed_loss = max(0, (speed_mph − 65) / 10) × 0.125 (≤ 0.80 cap)
estimated_range = base_range × temp_factor × hvac_factor × (1 − speed_loss)
OUTPUT:
base_range (mi)
temp_factor (fraction)
hvac_factor (fraction)
estimated_range (mi)
BENCHMARK:
±10% of EPA cold-weather EV range study
±10% of AAA EV Range Anxiety Report (2022)Assumptions
- Temperature model: EPA cold weather study validated against AAA 2022 EV range report
- HVAC impact modeled independently from temperature (additive energy draw)
- Application order: base range → temperature → HVAC → speed adjustment
- Speed loss capped at 80% maximum reduction to prevent extreme extrapolation
Edge Cases
- Cold soak below 20°F (overnight outdoor parking, no preconditioning): observed Tesla / Hyundai Ioniq 5 range loss is 25–35% — heavier than the calculator's smooth interpolation suggests until cabin and battery warm.
- Highway A/C use at 75 mph in 95°F+ heat: speed loss and HVAC loss compound — real-world range can drop 35–40% versus EPA label, far more than each factor alone.
- WLTP-rated (European) vs. EPA-rated (US) vehicles: WLTP figures run 15–25% optimistic — adjust battery capacity input downward when modeling a vehicle quoted under WLTP.
Data Sources
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| EPA cold-weather EV range study | ±10% | 2026-05 |
| AAA EV Range Anxiety Report (2022) | ±10% | 2026-05 |
EV Charging Time Calculator
Formula
INPUT:
battery_capacity_kwh (kWh)
current_soc_pct (%, 0–100)
target_soc_pct (%, 0–100)
charger_power_kw (kW; ≥50 kW classified as DC Fast per SAE J1772)
CALCULATION:
energy_to_add = battery_capacity_kwh × (target_soc − current_soc) / 100
if charger_power_kw < 50 (L2):
time_hours = energy_to_add / charger_power_kw
else (DC Fast — taper at 80% SoC):
energy_to_80 = battery_capacity_kwh × max(0, 80 − current_soc) / 100
time_to_80 = energy_to_80 / charger_power_kw
energy_above_80 = max(0, energy_to_add − energy_to_80)
time_taper = energy_above_80 / charger_power_kw × 2 (2× taper factor)
time_hours = time_to_80 + time_taper
OUTPUT:
time_hours (hr)
time_minutes (min)
energy_added_kwh (kWh)
BENCHMARK:
±10% above 80% taper vs CharIN DC Fast charging curve analysis
exact classification: SAE J1772 EV charging level standardAssumptions
- DC Fast charging taper begins at 80% SoC (industry-standard BMS behavior)
- Taper factor of 2× approximates the logarithmic slowdown above 80%
- 50 kW threshold distinguishes DC Fast from L2 charging (SAE J1772 classification)
- Charging time based on battery-side power; wall-to-battery efficiency not applied to duration
Edge Cases
- Cold-battery DC Fast (<50°F) without preconditioning: vehicles throttle peak power 30–60% until the pack warms — observed Tesla Supercharger sessions can stretch from 18 to 35 minutes.
- L1 (120V, 12A continuous) for plug-in hybrid (PHEV) ~10 kWh packs: 8–10 hour full overnight charge is feasible; for full BEVs (60+ kWh), L1 alone is insufficient for daily use.
- 350 kW DC Fast stations capped by the vehicle's onboard BMS: most BEVs peak at 150–250 kW even on a 350 kW dispenser — match expected time to the vehicle's published peak charge curve, not the station's nameplate.
Data Sources
- CharIN — DC Fast charging curve analysis
- SAE J1772 — EV charging level standard
- InsideEVs — real-world DC Fast charging data
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| CharIN DC Fast curve analysis | ±10% above 80% taper | 2026-05 |
| SAE J1772 charging level standard | exact (statutory) | 2026-05 |
EV Charger ROI Calculator
Formula
INPUT:
annual_miles (mi/yr)
public_charging_fraction (fraction 0–1, share of miles charged publicly)
public_rate ($/kWh, public DC Fast average)
home_rate ($/kWh, home L2 rate)
charger_cost ($, hardware purchase)
install_labor ($, electrician labor)
rebate ($, utility/state EVSE rebate)
CALCULATION:
annual_energy = annual_miles / 3.5 (default 3.5 mi/kWh efficiency)
public_energy = annual_energy × public_charging_fraction
rate_difference = max(0, public_rate − home_rate)
annual_savings = public_energy × rate_difference
net_install_cost = charger_cost + install_labor − rebate
payback_years = net_install_cost / annual_savings
net_5yr = annual_savings × 5 − net_install_cost
net_10yr = annual_savings × 10 − net_install_cost
OUTPUT:
annual_savings ($)
net_install_cost ($)
payback_years (yr)
net_5yr ($)
net_10yr ($)
BENCHMARK:
±10% of ChargePoint home charger hardware pricing
±15% of AFDC home charging ROI analysisAssumptions
- Default EV efficiency: 3.5 mi/kWh (EPA FuelEconomy.gov sedan/SUV average)
- Savings only accrue when home charging rate < public charging rate
- Rebates (EVSE utility programs, state incentives) reduce net install cost
- Charger maintenance costs not modeled
Edge Cases
- 100A or smaller residential service panel: a 40A L2 charger may require a 200A panel upgrade ($1,500–$3,500) — model does not include panel upgrade by default; add the upgrade cost to Net Install Cost. Consult a licensed electrician for NEC Article 220 load calculation.
- Federal 30C residential charger credit (30% up to $1,000) terminated after September 30, 2025 — installations placed in service from October 1, 2025 onward receive $0 federal credit. Some state and utility rebates remain.
- Smart L2 chargers ($600–$900) versus basic 40A units ($300–$450): the $300–$500 premium pays back via TOU optimization and load-balancing in households with 2+ EVs or panel-constrained service.
Data Sources
- EPA FuelEconomy.gov — EV efficiency reference data
- ChargePoint — home charger hardware cost data
- AFDC — home charging ROI analysis
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| ChargePoint home charger pricing | ±10% | 2026-05 |
| AFDC home charging ROI analysis | ±15% | 2026-05 |
Electric Bill with EV Calculator
Formula
INPUT:
annual_miles (mi/yr)
ev_efficiency (mi/kWh, EPA-rated)
charging_efficiency (fraction, default 0.90 for L2)
electricity_rate ($/kWh)
current_monthly_kwh (kWh/mo, household electricity baseline)
CALCULATION:
monthly_miles = annual_miles / 12
monthly_ev_kwh = monthly_miles / ev_efficiency / charging_efficiency
monthly_increase = monthly_ev_kwh × electricity_rate
new_total_bill = (current_monthly_kwh × electricity_rate) + monthly_increase
annual_increase = monthly_increase × 12
OUTPUT:
monthly_ev_kwh (kWh/mo added by EV charging)
monthly_increase ($)
new_total_bill ($/mo)
annual_increase ($)
BENCHMARK:
±2% of EIA residential monthly electricity consumption survey
±10% of DOE EV home charging analysisAssumptions
- Charging efficiency: 90% (L2 default — wall-to-battery energy loss)
- EV efficiency entered by user; typical range 2.5 mi/kWh (trucks) to 4.5 mi/kWh (sedans)
- Miles distributed evenly across all 12 months
- Flat electricity rate applied — no time-of-use pricing modeled
Edge Cases
- 100A service-panel limit: a single 40A L2 circuit (32A continuous, 7.7 kW) can saturate available capacity when AC + dryer + EV run simultaneously — a 200A panel upgrade ($1,500–$3,500) is typically required. Consult a licensed electrician for NEC Article 220 calculation.
- Two-EV household: monthly kWh roughly doubles. Households should verify utility EV-time-of-use plan eligibility (PG&E EV2-A, ComEd Hourly) which can offset the cost increase 30–50%.
- Solar offset: midday solar production can charge the EV directly when home, reducing utility-bill impact — but only with daytime charging and either NEM credit or battery-coupled self-consumption.
Data Sources
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| EIA residential consumption (monthly) | ±2% | 2026-05 |
| DOE EV home charging analysis | ±10% | 2026-05 |
Heat Pump Savings Calculator
Formula
INPUT:
annual_heating_btu (BTU/yr, from load calculation or utility bill estimate)
current_fuel (gas | oil | propane | electric)
fuel_price ($/therm for gas, $/gal for oil/propane, $/kWh for electric)
heat_pump_hspf (HSPF rating, 8–15 typical ENERGY STAR range)
electricity_rate ($/kWh, for heat pump operating cost)
install_cost ($, total installed cost of heat pump system)
CALCULATION:
current_annual_cost per fuel type:
gas: therms = annual_btu / (0.80 AFUE × 100,000) → cost = therms × fuel_price
oil: gallons = annual_btu / (0.83 AFUE × 138,700) → cost = gallons × fuel_price
propane: gallons = annual_btu / (0.80 AFUE × 91,500) → cost = gallons × fuel_price
electric: kwh = annual_btu / 3,412 → cost = kwh × electricity_rate
heat_pump_kwh = annual_btu / (hspf × 1,000)
heat_pump_cost = heat_pump_kwh × electricity_rate
annual_savings = current_annual_cost − heat_pump_cost
payback_years = install_cost / annual_savings
OUTPUT:
current_annual_cost ($)
heat_pump_cost ($)
annual_savings ($)
payback_years (yr)
10yr_net_savings ($)
BENCHMARK:
±15% of ENERGY STAR ASHP Savings Calculator
code-baseline: DOE Building Energy Codes ProgramAssumptions
- Gas furnace AFUE: 0.80 (typical older unit; ENERGY STAR minimum is 0.80)
- Oil boiler AFUE: 0.83; propane furnace AFUE: 0.80 (industry defaults)
- HSPF entered by user; ENERGY STAR typical values: Zone 1–2 ≈ 12, Zone 5 ≈ 9, Zone 7 ≈ 8
- Fuel prices held constant — 10-year projection uses the same annual savings figure
- 25C heat pump tax credit: $0 for 2026+ (expired per OBBBA, Pub.L. 119-21)
Edge Cases
- Cold-climate operation below −15°F: standard HSPF ratings degrade sharply — specify a cold-climate ASHP (HSPF2 ≥ 8.5, NEEP ccASHP-listed) or plan for resistance backup. Consult a licensed HVAC contractor for a Manual J sized to your design temperature.
- Dual-fuel hybrid (heat pump + gas furnace, switchover ~25°F): captures heat-pump efficiency above 25°F and gas economics below — model the heat-pump portion at full load above switchover only.
- Ductless mini-split single zone versus central ducted: mini-splits avoid duct losses (10–25% in unconditioned attics) but cover only the conditioned zone — sum multiple heads when comparing to a whole-house central system.
Data Sources
- ENERGY STAR ASHP Savings Calculator — ±15% benchmark
- ACCA Manual J 8th Edition — heating load methodology
- DOE FEMP — heat pump payback validation
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| ENERGY STAR ASHP Savings Calculator | ±15% | 2026-05 |
| DOE Building Energy Codes Program | code-baseline (no numeric margin) | 2026-05 |
Insulation ROI Calculator
Formula
INPUT:
current_r_value (R-value, 0–60)
target_r_value (R-value, 0–60)
annual_heating_cooling_cost ($, combined annual HVAC spend)
installation_cost ($, material + labor)
CALCULATION:
if current_r = 0: savings_fraction = 1.00
elif target_r ≤ current_r: savings_fraction = 0.00
else: savings_fraction = 1 − (current_r / target_r)
annual_savings = annual_heating_cooling_cost × savings_fraction
payback_years = installation_cost / annual_savings
10yr_net_savings = annual_savings × 10 − installation_cost
OUTPUT:
savings_fraction (fraction)
annual_savings ($)
payback_years (yr)
10yr_net_savings ($)
BENCHMARK:
code-baseline: DOE Insulation R-Value guide by climate zone
±15% of ENERGY STAR insulation savings guidanceAssumptions
- R-value ratio model: thermal conductivity ∝ 1/R-value (standard building physics)
- Applies to attic insulation; wall and floor assemblies use the same math but different baseline R-values
- Heating and cooling cost entered by user as a combined annual spend
- Moisture control, air sealing, and thermal bridging effects are not modeled
Edge Cases
- Climate-zone-specific attic targets: DOE recommends R-30 in Zone 1 (Miami) versus R-49–R-60 in Zone 7 (Duluth) — use the appropriate target R-value for your IECC climate zone, not a national default.
- Air-sealing before insulation: blower-door testing identifies leaks that insulation alone cannot fix — uncontrolled air infiltration can negate 30–50% of insulation gain. Consult a certified home energy auditor (BPI or RESNET).
- Retrofit phasing: attic insulation typically returns 30–50% versus 10–20% for wall retrofits — sequence attic first, then air sealing, then walls only if the budget remains.
Data Sources
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| DOE Insulation R-Value guide | code-baseline (no numeric margin) | 2026-05 |
| ENERGY STAR insulation guidance | ±15% | 2026-05 |
Carbon Footprint Calculator
Formula
INPUT:
monthly_kwh (kWh/mo, household electricity)
grid_intensity_lbs_per_kwh (lbs CO₂/kWh; US avg 0.92, state values vary widely)
annual_miles (mi/yr)
vehicle_mpg (mpg; 0 or omit if EV — transportation CO₂ = 0)
annual_therms (therms/yr, natural gas consumption)
solar_production_kwh (kWh/yr, optional solar offset — default 0)
CALCULATION:
electricity_co2 = monthly_kwh × 12 × grid_intensity
transport_co2 = annual_miles / vehicle_mpg × 19.6 (0 if EV — electricity row captures EV emissions)
gas_co2 = annual_therms × 11.7
solar_offset = solar_production_kwh × grid_intensity
net_co2 = electricity_co2 + transport_co2 + gas_co2 − solar_offset
tree_equivalent = net_co2 / 48 (EPA: 1 tree sequesters ~48 lbs CO₂/yr)
OUTPUT:
electricity_co2 (lbs)
transport_co2 (lbs)
gas_co2 (lbs)
solar_offset (lbs)
net_co2 (lbs)
tree_equivalent (trees)
BENCHMARK:
±5% of EPA eGRID state emission factors (utility-reported)
±10% of EPA Greenhouse Gas Equivalencies calculatorAssumptions
- US average grid intensity: ~0.92 lbs CO₂/kWh (EPA eGRID national average; state values vary significantly)
- EV transportation CO₂ = 0 in the transportation category — electricity use is already captured in the electricity row
- Gasoline combustion: 19.6 lbs CO₂/gal (EPA standard for regular gasoline)
- Natural gas combustion: 11.7 lbs CO₂/therm (EPA eGRID standard)
- 1 tree sequesters approximately 48 lbs CO₂/yr (EPA Greenhouse Gas Equivalencies)
Edge Cases
- Regional grid-factor extremes: Washington and Idaho (hydro-dominant) average ~0.10 lbs CO₂/kWh while West Virginia and Kentucky (coal-dominant) exceed 1.90 lbs CO₂/kWh — using a national average masks a 20× spread.
- Operational versus lifecycle emissions: this model captures operational only — solar panel and EV battery manufacturing contribute a one-time embodied carbon cost (typically paid back within 1–3 years of clean operation).
- Result accuracy: household-level estimates carry ±20–30% uncertainty because behavior (driving, thermostat, plug load) varies more than the input model can capture. For audit-grade analysis, consult a certified climate consultant.
Data Sources
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| EPA eGRID state factors | ±5% (utility-reported) | 2026-05 |
| EPA Greenhouse Gas Equivalencies | ±10% | 2026-05 |
Appliance Energy Cost Calculator
Formula
INPUT:
watts_current (W, active power draw — existing appliance)
watts_new (W, active power draw — replacement appliance)
hours_per_day (hr/day, daily operating hours)
electricity_rate ($/kWh)
purchase_cost ($, cost of new replacement appliance)
CALCULATION:
annual_kwh_current = watts_current / 1,000 × hours_per_day × 365
annual_kwh_new = watts_new / 1,000 × hours_per_day × 365
annual_cost = annual_kwh_current × electricity_rate
annual_savings = (watts_current − watts_new) / 1,000 × hours_per_day × 365 × electricity_rate
payback_years = purchase_cost / annual_savings
10yr_net_savings = annual_savings × 10 − purchase_cost
energy_vampire = hours_per_day ≥ 20 AND watts_current ≤ 25
OUTPUT:
annual_kwh_current (kWh)
annual_cost ($)
annual_savings ($)
payback_years (yr)
10yr_net_savings ($)
energy_vampire (boolean flag)
BENCHMARK:
±10% of ENERGY STAR Appliance Calculator energy estimates
±5% (survey-level) of EIA Residential Energy Consumption Survey (RECS)Assumptions
- Watt rating represents active power draw (as shown on ENERGY STAR yellow EnergyGuide labels)
- 365 operating days per year; seasonal appliances should use adjusted daily hours
- Energy vampire flag identifies always-on low-draw devices (routers, modems, standby chargers)
- Flat electricity rate applied — no demand charges or time-of-use pricing
Edge Cases
- ENERGY STAR upgrade worth-it threshold: replacing a working high-efficiency unit rarely pays back; replacing a broken or pre-2000 unit nearly always does — payback below 5 years is the practical 'go' line.
- Standby and phantom load: always-on devices (routers, set-top boxes, smart speakers, instant-on TVs) typically represent 5–10% of a household's monthly kWh — flag all >20 hr/day items.
- Time-of-use peak-window heavy use (cooktop, oven, EV charger, AC during 4–9 PM) can run 3–5× a flat rate — TOU savings require shifting these loads, not just upgrading them.
Data Sources
- ENERGY STAR Appliance Calculator — energy use benchmarks
- EIA RECS — Residential Energy Consumption Survey
Benchmark References
| Benchmark | Acceptance Margin | Last Checked |
|---|---|---|
| ENERGY STAR Appliance Calculator | ±10% | 2026-05 |
| EIA Residential Energy Consumption Survey (RECS) | ±5% (survey-level) | 2026-05 |