This battery storage calculator sizes a battery bank for solar or backup: usable capacity needed from your daily load, days of autonomy, battery chemistry’s depth of discharge, and system voltage.
Nameplate ≠ usable: the bank must be oversized by 1/(DoD × efficiency) so the energy you can actually spend survives the round trip. Cold derates lead-acid further (up to −30% near freezing); lithium resists cold discharge but refuses charging below 0 °C without heating. Size solar to refill it on our Solar Panel Calculator.
Capacity = Load × Days ÷ (DoD × Efficiency)
Every storage technology hides part of its nameplate: lead-acid surrenders half to longevity, lithium a fifth. Loads get itemized on our Appliance Energy Calculator; generation sizing sits on the Solar Panel Calculator. Part of our Ecology Calculators collection.
Worked Example (Verified)
- 1,500 Wh/day, 1 day autonomy, LFP at 48 V (defaults): 1,500 ÷ (0.8 × 0.9) = 2,083 Wh nameplate ≈ 43 Ah @ 48 V — one standard 50 Ah rack battery covers it
- Same load on lead-acid: 1,500 ÷ (0.5 × 0.9) = 3,333 Wh — 60% more nameplate for the same usable energy, the chemistry tax in one line
- Cabin math: 3,000 Wh/day × 2 cloudy days on LFP = 8.3 kWh — autonomy days are the most expensive input; shave loads before buying days
- Critical-loads realism: a fridge + lights + router + phones ≈ 1,200–1,800 Wh/day — whole-home backup (AC, dryer, oven) is a 10–20 kWh conversation and a different budget
Sizing Judgment
- List loads in watt-hours honestly (nameplate watts × actual run-hours — duty cycles again).
- Surge is separate: inverters must start motors (fridge compressors surge 3–7× running watts) — capacity here is energy, not power.
- Round up to purchasable units and one increment beyond — degradation guarantees you’ll use the margin.
- Critical-loads panels beat whole-home banks for most outage goals: a tenth of the cost, all of the dignity.
Related Tools
- Solar Panel Calculator — array size to refill the bank
- Appliance Energy Calculator — build the load list
- Ecology Calculators — the full collection
Frequently Asked Questions
How do I size a battery bank?
Nameplate Wh = daily Wh × autonomy days ÷ (depth of discharge × inverter efficiency). For 1,500 Wh/day, 1 day, lithium (80% DoD, 90% efficient): 1,500 ÷ 0.72 = 2,083 Wh ≈ 2.1 kWh, or ~43 Ah at 48 V. This calculator handles the divisions for any chemistry and voltage.
What is depth of discharge and why does it matter?
DoD is the fraction of nameplate capacity you may use without wrecking lifespan: lead-acid ~50%, lithium iron phosphate ~80–90%. A “10 kWh” lead-acid bank is a 5 kWh bank in practice — comparing chemistries on nameplate alone is how systems end up undersized by half.
How many kWh does a house need for backup?
Depends on the loads you refuse to lose: critical circuits (fridge, lights, internet, devices) run 1.2–1.8 kWh/day → a 2–3 kWh usable bank per outage day. Whole-home with HVAC starts at 10–20 kWh usable. The defaults’ 1,500 Wh/day models the critical-loads strategy most homes actually deploy.
12V, 24V or 48V — which voltage?
Higher voltage = lower current for the same power = thinner cables, less loss, happier inverters. 12 V suits small/RV systems (≤ ~1.5 kW), 24 V mid, 48 V is the home-storage standard (the defaults) and the input of most hybrid inverters. Amp-hours shrink as voltage rises for identical energy.
How long will the battery last (lifespan)?
Cycle life at rated DoD: lead-acid ~500–1,000 cycles, LFP ~3,000–6,000. Daily cycling: LFP gives a decade-plus; lead-acid 2–3 years — the purchase price ratio and the lifespan ratio roughly cancel, which is why the market argues about both so cheerfully.
Is this battery storage calculator free?
Yes — free, instant, no sign-up. Final system design belongs with an installer who can see your panel and loads.