Solar battery calculator: how much storage you need
Home batteries are rated in usable kWh — the energy you can actually draw. The calculator works from what you need to keep running:
Usable kWh = daily kWh × days of backup ÷ 0.9
The 0.9 covers inverter and battery losses; Enphase rates its IQ Battery 5P at about 90% round-trip efficiency. Then it converts to the number of 13.5 kWh batteries — the median size installed in 2025 according to Lawrence Berkeley National Laboratory, and the usable capacity of a Tesla Powerwall 3.
| Daily load | 1 day | 2 days | 3 days |
|---|---|---|---|
| 5 kWh (fridge, lights, internet) | 5.6 kWh · 1 battery | 11.1 kWh · 1 | 16.7 kWh · 2 |
| 10 kWh (essentials + furnace fan, sump) | 11.1 kWh · 1 | 22.2 kWh · 2 | 33.3 kWh · 3 |
| 20 kWh (most of the house, no AC) | 22.2 kWh · 2 | 44.4 kWh · 4 | 66.7 kWh · 5 |
| 29 kWh (U.S. average use) | 32.2 kWh · 3 | 64.4 kWh · 5 | 96.7 kWh · 8 |
With solar panels refilling the battery during the day, one day of storage usually rides through a multi-day outage outside of winter.
Solar battery bank calculator: amp-hours and voltage
Off-grid and RV systems are sized in amp-hours at a bank voltage: kWh × 1,000 ÷ volts. Lithium iron phosphate (LFP) banks can use nearly their full rating; lead-acid banks should only be drained to about 50%, so they need twice the nominal capacity for the same usable energy.
- 10 kWh usable at 48 V: about 230 Ah of lithium, or about 460 Ah of lead-acid.
- The same at 12 V: about 930 Ah of lithium — which is why bigger systems move to 24 or 48 V and thinner cables.
Switch the calculator to Check a battery's runtime to go the other way: enter amp-hours, voltage and a load in watts.
What a home battery costs in 2026
EnergySage's H1 2026 data put the national median at about $1,130 per kWh installed, before incentives — roughly $15,000 for a 13.5 kWh battery. The 30% federal residential credit (25D) no longer applies to systems installed after 2025 under P.L. 119-21, so the price you're quoted is close to what you'll pay unless your state or utility offers a rebate.
Paying for solar itself? Solar lease vs buy explains what the 2026 tax changes mean. Our view: a battery pays for itself mainly where exported solar earns little — under net-billing tariffs like California's — or where backup power is worth a lot to you. Two numbers matter besides kWh: the battery's continuous and peak kW, which decide whether it can start a well pump or central air, and its warranty. For backup on a budget, compare a portable unit with the generator size calculator; to size the panels that charge it, use the solar panel calculator.
