Energy use
Battery Runtime Calculator
Runtime is usable energy divided by load. Enter the kilowatt-hours you can actually draw and the watts or kilowatts the load uses. This is not a home-battery installation price.
Numbers stay in this browser. EnergyToolMage does not upload your inputs or require an account.
Formula
hours = usable_kWh × 1000 ÷ load_W or hours = usable_kWh ÷ load_kW
Assumptions
- Load is constant.
- Usable kWh already includes the depth of discharge and inverter efficiency you care about.
- No temperature derate unless you lowered usable kWh yourself.
How it works
Energy and power stay in SI units. Hours = usable kWh ÷ load kW, or usable kWh × 1000 ÷ load W. Zero load is rejected.
Inputs and results
Inputs: usable kWh, load (W or kW). Output: hours.
Worked examples
10 kWh usable, 500 W load
10 × 1000 ÷ 500 = 20 hours. A 1.2 kW load on 7.2 kWh usable is 6 hours.
How to use this calculator
- Enter usable battery energy in kWh.
- Enter the load and choose W or kW.
- Read hours of runtime at that constant load.
- If the load cycles, this is an upper-bound estimate.
Limitations
- Does not size a generator or certify a UPS.
- Does not price battery hardware or installation.
- Does not model Peukert, inverter idle, or surge watts.
Sources and freshness
- Every rate, wattage, hour, and loss percentage is a user input or a labeled example. Nothing here is a live utility tariff.
- The $0.16/kWh figure that appears in examples is an illustrative default, not a U.S. average and not current.
- Annualization uses 365.25 days per year and 365.25 ÷ 12 days per month. Your billing cycle will differ.
- No external battery-spec dataset is used. Usable kWh is a user input.
Questions
Is this nameplate kWh or usable kWh?
Enter usable energy. Nameplate kWh minus reserved capacity and inverter losses is a number you supply. This page does not invent a depth-of-discharge factor.
What if the load is zero?
Runtime is undefined. The calculator rejects a zero load so it never divides by zero.
Does this price a home battery install?
No. Install and replacement cost belong to a home-project cost tool, not this runtime identity.
Related energy tools
- kWh from wattsKilowatt-hours are power times time. Enter watts or kilowatts and how long the load runs. Optionally price the energy with your own rate.
- Electricity costMultiply energy you already know in kilowatt-hours by the dollars-per-kWh on your bill. Use this when a meter, EnergyGuide label, or another calculator already gave you kWh.
- Appliance costUse this when you know an appliance’s watts and how often you run it. The page annualizes with a disclosed calendar convention and your bill rate.