Electrical & Electronics
Battery Runtime Calculator
Estimate battery life from Ah or Wh, device loads, reserve charge and conversion efficiency.
Your inputs
Your results
Estimated runtime
- Usable battery energy
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- Average device load
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- Battery power draw
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- Battery current
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t = E / PAssumes steady average loads and nominal voltage. Battery age, temperature and discharge rate can shorten runtime.
Estimate battery runtime from usable energy and loads
Estimate how long a battery can support average device loads after reserve capacity and conversion losses. The model uses a fixed nominal voltage, capacity and efficiency. It provides a planning estimate; it does not simulate changing battery voltage, temperature, aging, discharge-rate effects or starting surges.
Step by step
- Enter nominal battery voltage and capacity in Ah, Wh or kWh. Set the starting charge and the reserve charge at which operation should stop.
- Enter conversion efficiency and additional idle power drawn from the battery. Add each device's power, quantity and duty cycle; a continuously operating device uses 100% duty.
- Review usable energy, average load power, battery-side power, current and runtime. Compare alternative loads or reserves and print the estimate when useful.
Settings and limits
- Capacity and reserve
- Capacity in Ah is converted to Wh using the entered nominal voltage; kWh is converted to Wh. Usable energy equals full capacity × (starting charge − reserve) / 100. Reserve cannot exceed the starting charge; both percentages describe the full battery capacity.
- Efficiency and idle draw
- Battery-side power equals average load power / (efficiency / 100) + idle power. Efficiency must be positive and no greater than 100%. Idle draw is added on the battery side, so do not also include the same loss as a device load.
- Duty cycle
- Each device contributes watts × quantity × duty / 100 to the average load. Duty must be between 0% and 100%. Runtime assumes that average stays constant; brief high-power demands and device startup behavior are outside this model.
Worked example
Enter 12 V and 100 Ah, a starting charge of 90%, a reserve of 20%, 90% efficiency and 5 W idle draw. Add one 45 W device at 100% duty.
| E | Eᵤ | P | I | t |
|---|---|---|---|---|
| 1200 Wh | 840 Wh | 55 W | 4.58333 A | 15.2727 h |
Full energy is 1200 Wh and usable energy is 840 Wh. Battery-side power is 45 / 0.90 + 5 = 55 W, so runtime is about 15.2727 h, displayed as approximately 15 h 16 min. Estimated battery current is 4.58333 A.
Questions and troubleshooting
Why does the real battery stop sooner?
The calculation keeps nominal energy, voltage and efficiency fixed. Available energy and device behavior can vary in use, and equipment may stop at its own voltage threshold. Use measured capacity and realistic average loads, then allow for the operating conditions.
What happens when total battery draw is zero?
With no effective device load and zero idle draw, the model reports no discharge rather than a finite runtime. A positive idle draw still drains the battery even when every device load is zero.
Why are the results blank or zero?
Blank results indicate an invalid input, such as nonpositive capacity or voltage, efficiency outside its range, or reserve above starting charge. A valid reserve equal to starting charge leaves zero usable energy and gives zero runtime when there is positive draw.