The Battery Discharge Time @ Fixed Watt Load Calculator estimates how long a battery can run when powering a device that consumes a constant watt load. You can enter capacity in Wh or Ah and the calculator automatically converts to Wh when needed.
How Battery Discharge Time Works
Battery capacity represents stored energy. The device or appliance consumes power (watts) continuously. When you divide total energy by power consumption, you get the expected runtime.
Formula:
Runtime (hours) = Battery Wh ÷ Load W
- Enter Wh directly
- Or enter Ah + Voltage
- Calculator auto-converts Ah × V = Wh
- Works for any fixed load device
Examples
- 48V 10Ah @ 100W ≈ 4.8 hours
- 12V 100Ah @ 300W ≈ 4 hours
- Power bank 80Wh @ 20W ≈ 4 hours
Where To Use This Calculator
- eBike motors
- Solar backup systems
- Camping batteries
- Inverters and UPS
- Portable power stations
- Emergency lights and tools
Frequently Asked Questions
Q: How do I calculate battery discharge time?
Convert capacity to Watt-hours and divide by the watt load. For Ah values, multiply by voltage first (Ah × V = Wh).
Q: Do I need voltage?
Only if capacity is entered in Ah. When Wh is entered, voltage is not required.
Q: Does this calculator work for LiFePO₄?
Yes. It works for LiFePO₄, Lithium-ion, Lead-acid, AGM, and most battery chemistries.
Q: Will discharge efficiency reduce runtime?
Yes. Real runtime is usually 5–15% lower due to inverter and conversion losses.
Q: Is discharge time the same as charging time?
No. Discharge time tells how long the battery can run a load, not how long it takes to recharge.
