Battery & energy
Battery Runtime Calculator
Battery runtime equals usable stored watt-hours divided by load watts. Stored watt-hours equal battery voltage multiplied by amp-hour capacity.
Estimate ideal and adjusted runtime from battery energy, load, usable depth, and conversion efficiency.
Calculated result
Estimated runtime
Calculating…
Calculated locally in your browser
Planning math only. Verify equipment specifications and installation requirements separately.
What this calculator returns
Treat the answer as an energy-budget estimate and compare it with the battery and equipment documentation.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
Nominal Wh = V × Ah; adjusted runtime = Wh × usable depth × efficiency / load W.
- V
- Nominal battery or bank voltage.
- Ah
- Charge capacity in amp-hours.
- Wh
- Nominal or usable stored energy in watt-hours.
- W
- Average load power in watts.
- η
- Entered conversion efficiency.
Usable depth and efficiency are entered as percentages.
Worked example
| Input | Value |
|---|---|
| Battery voltage | 12 V |
| Capacity | 100 Ah |
| Load | 120 W |
12 V × 100 Ah = 1,200 Wh; at 120 W the ideal runtime is 10 hours before adjustment factors.
Reference table
| Appliance | Planning watts | Use note |
|---|---|---|
| LED lamp | 10 W | Nominal example |
| Laptop computer | 60–120 W | Typical planning range |
| Refrigerator while running | 100–250 W | Cycling load; startup can be higher |
| Microwave oven | 900–1,500 W | Input varies by model |
| Portable space heater | 1,500 W | Common high setting |
| Electric kettle | 1,200–1,800 W | Varies by market and model |
Frequently asked questions
Why is actual battery runtime often shorter?
Usable depth, conversion loss, discharge rate, temperature, aging, and standby draw can all reduce delivered energy.
How do amp-hours become watt-hours?
Nominal watt-hours equal battery voltage multiplied by amp-hours before usable-depth and efficiency adjustments.
Assumptions and limitations
- No battery chemistry, aging, temperature, Peukert, BMS, or discharge-current prediction.
- Usable depth and efficiency are user assumptions.
Method and sources
Review the electrical formulas, calculation methodology, and standards status for the references most relevant to this calculation. The broader technical sources index records source scope and verification. Last reviewed .