Solar Battery Bank Size Calculator

How to use the solar battery bank size calculator

Enter your daily electricity usage in kilowatt-hours, the days of autonomy you want, the system voltage your bank runs at (12, 24, or 48 V), and the usable depth of discharge, and you get the battery bank size you need — in amp-hours first, then the total capacity in watt-hours and kilowatt-hours. Amp-hours is the headline number because that’s how batteries are sold and wired; the watt-hours figure is the honest measure of how much energy the bank actually holds.

Start with daily usage. This is the energy the bank has to carry, not what your solar produces — so use the loads that run off the battery, whether that’s a whole off-grid cabin at 10 kWh a day or just the fridge and a few lights you want to keep alive during an outage. If you don’t have a figure yet, the battery-watt-hours-calculator and your appliance wattages will get you to a daily kWh you can trust.

Days of autonomy is the part people underestimate. It’s how long the bank must run the loads on battery alone — through a cloudy stretch when the panels barely charge, or a grid outage you have to ride out — without any fresh charge coming in. One day is the common default for grid-tied backup; off-grid systems in cloudy climates often size for two or three so a run of grey days doesn’t leave them dark. Every extra day multiplies the bank.

Depth of discharge is the other big lever, and it’s set by chemistry. Lead-acid batteries should only be drained to about 50% usable before they wear out fast, while lithium and LiFePO4 happily give up around 80%. That difference is huge: the same energy needs a far bigger lead-acid bank than a lithium one, because you’re sizing around the slice you can safely use, not the nameplate capacity. Set the depth to match the batteries you’re actually buying.

System voltage doesn’t change the energy, only the amp-hours. A bank at 48 V carries the same watt-hours as one at 12 V but needs a quarter of the amp-hours to do it, which means thinner cables and smaller fuses for the same power. That’s why larger off-grid systems run at 48 V. Remember this sizes the battery only — pair it with the solar-panel-output-calculator for how much production you need to refill the bank, and the battery-watt-hours-calculator to check any single battery’s energy.

The formula

The bank has to hold a day’s energy times your days of autonomy, then get scaled up so you’re only ever using the safe slice of it, then converted to amp-hours at your system voltage. Daily kWh is multiplied by 1,000 to become watt-hours so the units line up with the amp-hours and volts batteries are rated in:

needed Wh = daily kWh × 1000 × days of autonomy
bank Wh = needed Wh ÷ usable depth of discharge
bank Ah = bank Wh ÷ system voltage

Worked example with the defaults — 10 kWh of daily usage, 1 day of autonomy, 80% usable, at 48 V: 10 × 1000 × 1 = 10,000 Wh of energy needed; ÷ 0.80 usable = 12,500 Wh (12.5 kWh) of actual bank capacity, because you only safely use 80% of it; ÷ 48 V = about 260 Ah. So a 48 V bank of roughly 260 Ah covers one cloudy day of a 10 kWh load.

The two levers move the number fast. At 24 V the same energy needs about 520 Ah — half the voltage means double the amp-hours for identical watt-hours. And 2 days of autonomy instead of 1 doubles the bank to about 520 Ah as well, since you’re asking it to carry twice the energy through to the next good charge.

Frequently asked questions