Home Battery Size Calculator

State selection here does not add a programme payment. State programme figures are a TODO — fill them only from an official utility / commission / programme page, with a check date, before publishing.

Whole-home backup: use your full daily use. Essential-loads-only backup: use only the daily use of the loads you want backed up (fridge, lights, internet, medical loads, etc.). Find daily use from your bill: monthly kWh ÷ 30 is a rough starting point — your bill, not this calculator, is the source.

Energy needed (kWh) = Daily use for the backup scope (kWh/day) × Outage hours ÷ 24

Example units needed = Energy needed ÷ 13.5 kWh, rounded UP to the next whole unit

13.5 kWh is used here only as the example unit size reported in the research (a single 13.5 kWh battery). It is not a brand recommendation and not a claim that every battery is 13.5 kWh.

What this formula leaves out — read before you rely on it: usable capacity vs nameplate capacity, inverter limits, surge/startup loads, solar recharging during an outage, degradation over time, and your panel / utility rules. Those are TODOs to check against the manufacturer's spec sheet and a licensed installer — do not fill them with guesses.

Whole-home example: 30 kWh/day × 12 hours ÷ 24 = 15.0 kWh needed → 15.0 ÷ 13.5 = 1.11 → rounded up = 2 example units (27.0 kWh nameplate).

Essential-loads example: 12 kWh/day for essential loads × 12 hours ÷ 24 = 6.0 kWh needed → 6.0 ÷ 13.5 = 0.44 → rounded up = 1 example unit.

Rounding up matters: a battery cannot be bought in fractions in this simplified model, so the nameplate total will usually be larger than the exact energy needed. That extra capacity is not wasted in real life — it covers the items listed above — but this calculator does not quantify them.

Last checked for this prototype: Research reports dated 4 November 2026. This prototype has NOT been re-checked against live utility / commission pages. Do not publish until the TODOs below are filled.

• Example unit size: 13.5 kWh — research report, citing GreenEnergyCalc and NuWatt battery-cost pages.
• Installed cost: NOT used in this size calculator. See the cost & payback calculator for the reported range.
• State programme payment: TODO — no rate is filled in by state in this prototype. Do not copy a rate from a blog or directory. Check: the official utility programme page, the state commission / DSIRE listing, and the programme terms for your exact utility and address, then record the rate, the eligibility rule, and the check date here.
• Federal credit: $0 for homeowner-owned systems installed in 2026 — see the 2026 rule box at the top. Lease / PPA treatment under §48E is different and depends on the contract price — do not apply it in this calculator.

Sources in the research reports: https://greenenergycalc.com/calculators/battery-storage/ ; https://nuwattenergy.com/en/battery-storage-cost-2026 ; https://thesolarbook.com/federal-solar-tax-credit-2026/

How is battery size calculated here?

Energy needed (kWh) = daily use for the backup scope × outage hours ÷ 24, then divided by 13.5 kWh and rounded up to whole example units. Usable capacity, inverter limits, surge loads, and panel rules are not included — confirm with a licensed installer.

What is the difference between whole-home and essential-loads backup?

Whole-home uses your full daily use; essential-loads uses only the loads you choose (fridge, lights, internet, medical loads). See the guide on whole-home vs essential loads.