Solar Battery Calculator

A battery must cover the selected loads for the desired time without being drained beyond its usable limit. This solar battery calculator separates usable energy from nameplate energy and converts the result to amp-hours for low-voltage battery-bank planning.

Daily backup energy should include only circuits expected to run during an outage. Multiply average watts by runtime and sum in kWh; large startup surges affect inverter power even when their energy is small.

Nominal capacity divides usable energy by allowed depth of discharge. A battery with 80% usable depth needs 12.5 kWh of nameplate capacity to deliver 10 kWh.

Ideal charge time divides energy to replace by charger power. Real systems take longer because power tapers, batteries have charge limits, and solar varies through the day.

Assumptions and edge cases

The sizing assumes the daily load is an average across the whole backup window, that depth of discharge is the manufacturer's usable fraction rather than a hard floor, and that charge power stays constant. Real banks depart from all three. Lithium iron phosphate systems publish 80 to 100 percent usable capacity and already hold a reserve internally, so a second derate double-counts it, while flooded lead-acid is normally limited to 50 percent to protect cycle life. Capacity is rated near 25 °C and falls in cold weather, and the amp-hour conversion uses nominal bank voltage. Zero or negative inputs are rejected rather than returning an infinite bank.

Worked example

A 10 kWh daily critical load for two days needs 20 usable kWh. At 80% depth of discharge, the bank is 25 kWh nominal, or about 521 Ah at 48 V.

Second worked example

A second case contrasts two chemistries at identical delivered energy. A cabin uses 3.5 kWh a day and wants three days of autonomy, so usable energy is 10.5 kWh. Held to 50 percent depth of discharge, a flooded lead-acid bank must be 21 kWh nominal, or 21,000 ÷ 24 = 875 Ah at 24 V. The same 10.5 kWh from a lithium system rated 90 percent usable needs 11.7 kWh nominal, about 487 Ah at 24 V — roughly half the amp-hours for the same result.

How to read the result

Check continuous inverter kW, surge rating, minimum temperature, and reserve policy in addition to energy capacity. Whole-home air conditioning can dominate a backup design.

Three bands make the result actionable. Below about 10 kWh nominal, one wall-mounted unit carries a refrigerator, lighting, networking, and a well pump through a night. Between 10 and 30 kWh the design needs two or three units, and the inverter's continuous kilowatt rating rather than stored energy becomes the binding constraint. Above 30 kWh the design is effectively whole-home, and for long outages a standby generator often costs less per hour of coverage. If ideal charge time exceeds the daylight window in your design month, resize the array or charge controller, not the battery.

Planning inputs are deliberately editable because two homes in the same state can have different tariffs, roof conditions, equipment, and operating schedules. Save a scenario with “Copy link,” then change one assumption at a time. This makes the result useful for comparing decisions without implying false precision.

Formulas and defaults are documented in our data method. The principal references for this page are:

1. NREL — PVWatts Calculator documentation (2025) — Production modeling, array orientation, and system-loss context.
2. EIA — Electric Power Monthly, Table 5.6.A (2024 annual) — Residential electricity price benchmarks by state.
3. U.S. Department of Energy — Homeowner’s Guide to the Federal Tax Credit for Solar Photovoltaics (accessed methodology, 2025) — Background only; 2026 eligibility is never assumed.
4. NREL — U.S. Solar Photovoltaic System and Energy Storage Cost Benchmarks (2024) — Residential storage capacity, chemistry, and installed-cost context.
5. NFPA 70, National Electrical Code (2023 edition) — Article 706, Energy Storage Systems — Disconnect, location, and installation requirements for residential storage.

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