Lithium and lead-acid storage

Solar battery bank and runtime calculator

Estimate the nominal battery capacity required for days away from charging and the usable runtime of an existing bank.

Enter energy, autonomy, and battery assumptions

Use daily watt-hours from the RV power consumption calculator whenever possible.

Battery and voltage
Reserve and runtime

In chemistry-preset mode, Calculate uses 80% usable depth for LiFePO₄ or 50% for lead-acid; the custom entry is ignored. Choose custom to use your own manufacturer-supported percentage. These presets are planning assumptions, not universal discharge limits. Chemistry changes never overwrite your custom entry. Verify battery and BMS specifications; reserve and efficiency are applied separately.

Formula

Battery-bank sizing formula

nominal Wh = daily Wh × days ÷ usable fraction ÷ reserve fraction ÷ efficiency
Ah = nominal Wh ÷ system volts

Lead-acid performance also varies with discharge rate, temperature, condition, and Peukert effect. Lithium systems are limited by battery and BMS specifications.

Battery selection

Capacity and current are separate checks

Match nominal voltage and required capacity, then verify permitted series/parallel arrangements, BMS continuous and peak current, charge acceptance, and low-temperature behavior in the battery manual. Use the series/parallel tool to compare nominal bank configurations.

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Size storage and check existing-bank runtime: worked example

At 1,200 Wh/day for two days, 80% usable depth, 20% reserve of the usable portion, and 90% efficiency, nominal capacity is 4,166.7 Wh or 347.2 Ah at 12 V. An existing 200 Ah bank delivers 1,382.4 Wh after those assumptions, about 13.8 hours at a steady 100 W.

Frequently asked questions

How should I choose chemistry and depth of discharge?

Choose the chemistry preset to apply 80% for LiFePO₄ or 50% for lead-acid when you calculate. Choose custom to use a manufacturer-supported percentage instead. Presets are example planning values, not universal limits; account for age, temperature, and discharge rate. Older saved results reopen in custom mode to preserve their original assumptions.

Is reserve the same as depth of discharge?

No. Here, depth of discharge is the usable fraction of nominal capacity. A 20% reserve then keeps 20% of that usable portion unused. With 80% usable depth, 64% of nominal energy remains before efficiency losses.

Why might real runtime be shorter?

The estimate assumes a steady average load and constant nominal voltage. Lead-acid Peukert effects, cold weather, battery aging, inverter idle power, and BMS cutoffs can reduce runtime. It does not test peak discharge-current capability.