Battery Storage Sizing Calculator
Usable energy is the only honest unit — this sizes the bank from the loads you actually need, the days you want to cover, and what your chemistry really lets you use.
Backup sizing for the loads you list — a whole-home claim needs a real load analysis. Chemistry depth-of-discharge figures are planning values; the battery manufacturer’s ratings win.
How the math works
Nameplate capacity = (daily critical load × days of autonomy) ÷ depth of discharge ÷ system efficiency. Depth of discharge is chemistry: LiFePO₄ ~90%, lithium-ion ~80%, lead-acid/AGM ~50% — a lead-acid bank must be twice the nameplate size for the same usable energy, which is the whole ballgame in this math. Amp-hours = nameplate kWh × 1,000 ÷ system voltage.
Size for critical loads — fridge, lights, medical, communications, a fan — not the whole panel. Whole-home backup is a load-analysis project, not a bigger number in this form.
A worked example
8 kWh/day of critical loads for 2 days on LiFePO₄ (90% DoD) at 90% system efficiency: 16 ÷ 0.90 ÷ 0.90 = 19.75 kWh of nameplate storage — 411.5 amp-hours at 48 V. The same job on lead-acid at 50% DoD would need about 35.6 kWh of nameplate.
Questions from the field
How much battery do I need for two days off grid? Start with the critical-load kWh per day, multiply by days, then divide by your chemistry’s depth of discharge and the system efficiency. The example: 8 kWh × 2 days becomes a 19.75 kWh LiFePO₄ bank.
Why does chemistry change the size so much? Depth of discharge. Lithium chemistries let you use 80–90% of the nameplate; lead-acid is done at about 50% if you want it to live. Same usable energy, very different bank.
What voltage should I pick? Higher voltage means fewer amp-hours and smaller wire for the same power: 48 V is the working default for home storage, 12/24 V for small and mobile systems.
Will this run my whole house? Only if your “critical load” input honestly is the whole house — AC included. Most backup designs separate critical loads onto their own panel; this tool sizes whatever load figure you give it.
The other calculators
Want us to look at your shop?
The free tools cover the leaks most shops share. The free Snapshot is the one-on-one version: 15 free minutes with our team to pressure-test what you found and map your next step. No pressure either way — you leave with a short list you can act on yourself.
Book Your Free Snapshot — book your free 15-minute call first, then answer a few quick questions, so the call itself is spent on your shop, not on background.