Solar Battery Storage Calculator

The formula is: Required Capacity (kWh) = Daily Usage × (Backup Hours ÷ 24) ÷ DoD. Then divide by each battery's usable capacity and round up. Example: 30 kWh/day, 24-hour backup, 90% DoD → 30 × 1 ÷ 0.9 = 33.3 kWh required. A Tesla Powerwall at 13.5 kWh usable needs 3 units. A Franklin WH at 13.6 kWh also needs 3 units. Choosing a higher-capacity model like the LG RESU Prime (16 kWh) could reduce the count to 3 as well, but at a different price point.

Both use LFP chemistry and carry 10-year warranties, but they differ in architecture. The Powerwall 3 is a monolithic 13.5 kWh unit that integrates a 97.5% efficient inverter, making it the simplest retrofit for most homes. Enphase IQ batteries are modular at 5 kWh per unit (stacked in pairs for 10 kWh), which lets you start small and expand. Enphase also integrates tightly with its microinverter ecosystem. If you want simplicity, Powerwall; if you want modularity or already have Enphase solar, IQ Battery.

It depends heavily on your state's electricity rate structure. In California (NEM 3.0, peak rates $0.40–0.55/kWh), New York (peak $0.35+), and Massachusetts, payback can be 5–8 years, which is financially compelling. In Texas and Florida, where TOU spreads are narrower, payback often exceeds 12–15 years — making the financial case weak. The calculator shows your specific payback based on your state's TOU rates. If you're primarily buying a battery for backup power rather than savings, the economics are separate from TOU arbitrage.

Most residential batteries can power the whole home for several hours, but not indefinitely. The critical factor is hardwiring: your battery must be connected through a transfer switch or a whole-home gateway (like the Powerwall 3's Gateway) so it properly isolates from the grid during an outage. Without that hardware, the battery cannot legally discharge during grid outages (anti-islanding protection). Beyond the hardware, you need enough capacity for your actual loads — a home with electric heat or a well pump can drain even a large battery quickly.

Most residential lithium batteries carry a 10-year warranty covering at least 70–80% capacity retention (the specific floor varies by manufacturer). Tesla warrants 70% at 10 years unlimited cycles. Enphase warrants 70% at 4,000 cycles or 10 years. LG RESU warrants 80% at 4,000 cycles or 10 years. The Franklin WH warrants 70% at 6,000 cycles, which is among the highest cycle warranties available. Always verify current warranty terms with the manufacturer — these figures can change with new firmware versions or product updates.

Yes. Under the Inflation Reduction Act (IRA), standalone battery systems (not paired with solar) became eligible for the 30% federal Investment Tax Credit starting January 1, 2023 — provided the battery has at least 3 kWh of capacity. If paired with a new solar installation, the entire system including the battery qualifies for the 30% credit. Several states (California SGIP, New York, Oregon) offer additional incentives on top of the federal credit. These incentives directly reduce your effective cost and payback period.

It depends on your goals. If your priority is the lowest cost of electricity, a grid-tied system without batteries usually wins in states with full retail net metering — you effectively use the grid as a free battery. If you want backup power during outages, you need a battery: standard grid-tied solar shuts off during blackouts (anti-islanding protection required by code). If your state has time-of-use rates with a large peak/off-peak spread (California NEM 3.0, parts of New York and Massachusetts), a battery can pay for itself through arbitrage. Use this calculator to model your specific state — payback in flat-rate states often exceeds 12 years, while high-TOU states can hit 5–8 years.

Lithium-iron-phosphate (LFP) batteries — the dominant residential chemistry in 2026 — are designed for 6,000–10,000 charge cycles. At one full cycle per day for daily TOU arbitrage, that translates to 15–25 years of usable life. NMC chemistry typically rates 3,000–5,000 cycles or 8–14 years under the same usage. Real-world lifespan also depends on ambient temperature (sustained heat above 95°F shortens life), depth-of-discharge habits (deeper cycles age the cell faster), and firmware management. Most warranties guarantee 70–80% capacity retention at year 10 — meaning the battery may still operate at year 15, just at reduced capacity. Plan for an inverter replacement at year 10–12 even if the battery cells are still healthy.

LFP (Lithium Iron Phosphate) and NMC (Nickel Manganese Cobalt) are the two dominant chemistries in residential storage. LFP is thermally stable (much lower fire risk), tolerates higher cycle counts, and degrades slowly — Tesla Powerwall 3, Enphase IQ Battery, and Franklin WH all use LFP in 2026. NMC offers higher energy density (smaller physical size for the same kWh), but it runs hotter, degrades faster in heat, and carries a higher thermal-runaway risk — early LG RESU models used NMC. For residential use, LFP is now the safer and longer-lasting choice; NMC's energy-density advantage matters more in EVs and space-constrained installations. Always check the chemistry on the spec sheet before signing — manufacturers do not always lead with it.

California's Self-Generation Incentive Program (SGIP) is a state-level rebate administered by major utilities (PG&E, SCE, SoCalGas, SDG&E) that pays homeowners up to $1,000/kWh of installed battery capacity for qualifying systems. Standard residential customers receive a base rate around $150–$200/kWh; equity-tier and equity-resiliency customers (low-income, medically vulnerable, or in high-fire-threat districts) can receive the full $1,000/kWh. The rebate is paid to your installer and credited toward your installed cost. Combined with the 30% federal ITC, SGIP can cut a $15,000 battery to under $7,000 net for qualifying households. Application deadlines and budget tranches change yearly — verify current availability with your utility before signing a contract.

Yes, retrofitting a battery onto an existing solar system is technically straightforward but adds cost compared to installing both at once. Two common architectures: (1) AC-coupled — the battery uses its own inverter and connects on the AC side of your existing solar inverter; works with any solar inverter brand but adds ~10% efficiency loss from double conversion. Tesla Powerwall, Enphase IQ Battery, and Franklin WH all support AC-coupled retrofits. (2) DC-coupled — the battery shares the solar inverter's DC bus; more efficient but requires either swapping your existing inverter or installing a hybrid inverter. Plan for $1,500–$3,500 in extra labor and electrical work versus a co-installed system. Battery-readiness wiring at original installation time can cut this cost significantly.