How to use this calculator
Battery nameplate capacity is not the same as energy available to appliances. Reserve settings and conversion losses reduce usable energy, while cycling loads make actual runtime vary.
- Add running watts of essential loads.
- Enter capacity and reserve.
- Use average—not startup—load for runtime.
FORMULA
The math behind the estimate
capacity × (1 − reserve) × inverter efficiency ÷ load in kW = runtime hours.
Worked example
A 13.5 kWh battery with 10% reserve and 90% efficiency supplies 10.94 kWh, or 13.7 hours at an 800 W load.
Change the defaults to model your home. The result is an estimate, not a guaranteed bill or savings promise.
How to read the result
Use the estimate to compare scenarios and find the inputs worth measuring more carefully. Utility bills can include fixed charges, taxes, delivery fees, minimums, tiers, and demand charges that are not fully represented by a simple energy rate.
Assumptions and limitations
- Load is constant and below inverter limits.
- Startup surge and temperature are excluded.
- Capacity can decline with age.
Climate, equipment condition, installation, controls, and household behavior can materially change results. Verify consequential installation and purchasing decisions with qualified providers.
Practical savings context
Make an essential-load inventory before buying. A plug meter can improve average-load estimates.
SOURCE
Data and methodology
National Renewable Energy Laboratory — Energy Storage
Research overview accessed August 2026. Geographic scope: United States. Last reviewed: August 6, 2026.
WattMeasure uses the source to frame the method and assumptions; your editable inputs determine the displayed result.
QUESTIONS
Battery runtime FAQ
How accurate is this battery runtime estimate?
It is a planning estimate based on the values you enter. Actual power draw, cycling, utility charges, climate, and equipment condition can change the result.
Should I use the default electricity rate?
Your own marginal or all-in supply rate is better. Check a recent bill or your utility's published rate schedule.
Does this include taxes and utility fees?
Only when they are reflected in the rate you enter. Fixed monthly charges are not included because they usually do not change with one appliance's usage.
Last reviewed August 6, 2026 · Data update policy