Battery Runtime Calculator
Estimate runtime from stored energy, reserve level, efficiency and load power.
Result
Estimated runtime in hours: 3.6Estimate
Real capacity and efficiency change with battery chemistry, age, temperature and discharge rate. Use measured average load and the manufacturer’s usable-capacity guidance where available.
- Estimated runtime in minutes
- 216Estimate
- Usable energy delivered to the load
- 720 Wh (0.72 kWh)Estimate
- Energy left in reserve
- 200 WhEstimate
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Results are estimates based on the ratings, usage and prices you enter. Actual consumption, losses, tariffs and equipment performance vary. Check your bill, meter and manufacturer information before relying on the result. Read the full disclaimer
How to use the Battery Runtime Calculator
- Enter battery energy capacity in Wh or kWh and the starting charge.
- Enter the charge you want left in reserve, system efficiency and average load power.
- Read estimated hours and minutes together with the usable delivered energy.
Runtime is usable energy divided by power
Battery capacity is energy. Load watts are power. Dividing the energy actually available to the load by average power produces time.
usable Wh = capacity × (starting charge − reserve) × efficiencyruntime hours = usable Wh ÷ load wattsThe result is a planning estimate. A battery may shut down earlier under a heavy load, in low temperatures or as it ages. Check both usable energy and maximum output power in the manufacturer specification.
The U.S. Department of Energy overview of energy storage describes storage capacity and power as distinct system characteristics.
With your numbers
- Usable energy = 1,000 Wh × (100% − 20%) × 90%
- Usable energy = 720 Wh
- Runtime = usable energy ÷ 200 W = 3.6 hours
Quick reference
Real capacity and efficiency change with battery chemistry, age, temperature and discharge rate. Use measured average load and the manufacturer’s usable-capacity guidance where available.
| Example | Estimated runtime in hours |
|---|---|
| 1,000 Wh battery at 200 W | 3.6 |
| 500 Wh battery at 50 W | 7.65 |
| 2 kWh battery at 500 W | 2.58 |
| 750 Wh battery at 100 W | 4.39 |
| 5 kWh storage at 1 kW | 3.8 |
| Small 100 Wh pack at 10 W | 8 |
Common questions about the Battery Runtime Calculator
- Why not divide the label capacity directly by watts?
- That assumes the battery starts full, is drained completely and loses nothing. Reserve and efficiency make those assumptions visible.
- Should I use peak or average load?
- Use average power for runtime. Peak power still matters for checking whether the battery and inverter can start and support the equipment.
- Will a 1,000 Wh battery always deliver 1,000 Wh?
- No. Usable capacity depends on product limits, temperature, age and discharge rate, and an inverter consumes some energy.