Battery Backup Calculator

Pick the loads you want to keep running, and see how many hours your battery can cover them.

This calculates backup runtime hours during an outage. Want dollar savings from time-of-use rate arbitrage instead? See the Battery ROI Calculator.

This tool provides a planning estimate based on the assumptions shown below the results — it is not a quote, and actual costs and savings will vary by installer, local rates, and your specific home.

Select your loads and battery specs to estimate backup runtime.

How this calculator works

Runtime = (usable battery capacity × round-trip efficiency) ÷ total continuous load. A 90% round-trip efficiency is applied by default, typical for current lithium home batteries. This estimates continuous runtime for loads running the whole time — cycling loads (like a fridge compressor) in practice draw their rated wattage only part of each hour, so real-world runtime for a fridge-only backup is often longer than this simple estimate.

The inverter check compares your selected loads' combined wattage against the inverter's continuous power rating — not its brief surge rating, which is typically higher and only sustained for a few seconds (relevant for motor start-up loads like well pumps).

Source: ENERGY STAR Home Battery Storage; NREL battery performance benchmarks, last updated 2026-07-01.