Residential Calculators
Backup Runtime Calculator
A battery's runtime isn't just its kWh rating divided by average draw — it depends on exactly which loads you're running, your climate, and the season. Pick a battery and the loads you want backed up, and we'll estimate how long it actually lasts.
Your location
Enter your ZIP and we'll pull in your climate zone, solar resource, utility, current electricity rate, TOU availability, and battery incentives automatically.
Enter a 5-digit ZIP to auto-fill everything below.
Your battery
Pick a common model or enter your own capacity and power specs. Sizing a battery from scratch instead of checking one you already have? Try the Battery Size Calculator.
Tesla Powerwall 2 — 13.5 kWh, 5.0 kW continuous, 7.0 kW peak (manufacturer datasheet).
What's running during the outage?
Check everything you want backed up. Each item uses a sourced running-watts / startup-watts / duty-cycle default — see the assumptions panel below for exact figures and sources.
Heating & cooling system
Heating and cooling are usually the largest single loads in a home — and usually the reason runtime is much shorter than the "capacity ÷ average draw" napkin math. The "Central AC" and "Heat pump" checkboxes above pull their wattage from what you select here.
Defaulted to 3 tons (typical for a US home) — edit if you know your actual system size.
For multiple units, enter the combined (aggregate) system size.
Season
Lithium batteries lose some usable capacity in cold weather. This applies a cold-weather derate when relevant to your climate zone above — it doesn't change which loads are checked, so make sure the heating or cooling loads you actually expect for this season are checked above.
No cold-weather derate applied in mild conditions.
Load breakdown
| Load | Running W | Startup W | Daily kWh |
|---|
Assumptions in this estimate
- Usable capacity / round-trip loss95%
- Cold-weather capacity derate15%, applied only in Winter + Cold/Very Cold climate zones
- Runtime formulaUsable capacity ÷ average continuous draw (does not model discharge-curve tapering near empty)
- Multi-unit power scalingAssumed linear per unit (approximation)
Wattage, duty-cycle, and runtime defaults come from EnergySage, Generac, Honda, and manufacturer datasheets — see each load's tooltip for its specific source. Battery power specs come from manufacturer datasheets — see the Battery Size Calculator's assumptions panel for links. Actual runtime varies with battery age/state of health, temperature, and how closely real appliance draw matches these defaults. This is a planning estimate, not an engineering design — for panel-level circuit backup or code compliance, work with a licensed electrician.
A battery is capacity-limited — once its stored energy is used, it's done until recharged. A generator is fuel-limited instead — it keeps running as long as fuel is available and resupplied, typically every 8–24 hours for portable units, or indefinitely on a natural gas line. That structural difference matters more than either number alone.
Battery vs. Generator CalculatorEnter your ZIP above to see your area's solar resource and how much daytime solar could extend this runtime — potentially indefinitely for essential loads.
Solar + Battery Calculator