Battery Capacity Calculator: Size Your Backup Power in Wh and kWh
Estimate daily electricity use, backup battery size, and realistic runtime from appliance watts, hours, inverter efficiency and usable battery depth. Enter your own loads below.
Enter each appliance’s measured running watts and estimated daily operating hours. The result is a planning estimate, not a laboratory test. Battery chemistry, inverter losses, idle draw, startup surge and ambient temperature affect real results.
Multiply watts by hours, then account for losses and reserve
Daily consumption in watt-hours (Wh) is the sum of each appliance’s watts multiplied by hours used. Divide that by expected conversion efficiency and usable battery fraction to estimate nominal battery capacity. Size the inverter separately for simultaneous watts and startup surges.
Interactive Battery Capacity Calculator
Enter up to six appliance loads. Use measured watts where possible. For cycling appliances, enter equivalent average hours at rated wattage.
| Appliance | Running watts (W) | Hours/day | Daily Wh |
|---|---|---|---|
| Refrigerator | 560 | ||
| Lights | 100 | ||
| Laptop | 180 | ||
| Fan | 240 | ||
| Other load 1 | 0 | ||
| Other load 2 | 0 |
For 1 day, 85% efficiency and 90% usable battery: approximately 1,412 Wh (1.41 kWh) nominal capacity.
The calculator runs in your browser. If scripts are disabled, use the manual formula in the next section.
Example Appliance Energy Consumption
| Example load | Watts | Daily equivalent hours | Daily energy |
|---|---|---|---|
| Refrigerator (cycling) | 70W | 8h | 560Wh |
| LED lighting | 20W | 5h | 100Wh |
| Laptop | 60W | 3h | 180Wh |
| Fan | 40W | 6h | 240Wh |
| Total | — | — | 1,080Wh |
Battery Capacity Formula: Wh to kWh
A 1,000Wh battery stores 1kWh nominal energy. That does not mean every connected AC device receives the full 1kWh. Inverter conversion and usable battery reserve reduce delivered energy. For a multi-day outage, multiply daily demand by days before dividing by these factors.
| Daily energy | Days | Efficiency | Usable fraction | Required nominal battery |
|---|---|---|---|---|
| 500Wh | 1 | 85% | 90% | 654Wh |
| 1,080Wh | 1 | 85% | 90% | 1,412Wh |
| 1,080Wh | 2 | 85% | 90% | 2,824Wh |
| 2,000Wh | 3 | 85% | 90% | 7,843Wh |
A 1,000Wh battery stores 1kWh nominal energy. That does not mean every connected AC device receives the full 1kWh. Inverter conversion and usable battery reserve reduce delivered energy. For a multi-day outage, multiply daily demand by days before dividing by these factors.
Calculate Runtime for One Appliance
Runtime estimates are most useful for relatively steady loads. Startup surges, idle draw and temperature can shorten real operation. For a cycling fridge, use measured average watts over a full day rather than its compressor nameplate rating.
Example: 1,080Wh × 1 ÷ (0.85 × 0.90) = 1,412Wh. For one steady 100W AC load on a 1,024Wh battery at 85% efficiency, estimated runtime is 1,024 × 0.85 ÷ 100 = 8.7 hours before reserve and idle losses.
| Battery | 50W AC load | 100W AC load | 500W AC load |
|---|---|---|---|
| 512Wh | 8.7h | 4.4h | 0.9h |
| 1,024Wh | 17.4h | 8.7h | 1.7h |
| 2,048Wh | 34.8h | 17.4h | 3.5h |
Runtime estimates are most useful for relatively steady loads. Startup surges, idle draw and temperature can shorten real operation. For a cycling fridge, use measured average watts over a full day rather than its compressor nameplate rating.
What about motor startup surges?
Runtime estimates are most useful for relatively steady loads. Startup surges, idle draw and temperature can shorten real operation. For a cycling fridge, use measured average watts over a full day rather than its compressor nameplate rating.
What about motor startup surges?
Runtime estimates are most useful for relatively steady loads. Startup surges, idle draw and temperature can shorten real operation. For a cycling fridge, use measured average watts over a full day rather than its compressor nameplate rating.
Battery Sizing and Electrical Safety Checklist
Use the electrical label and include any other simultaneous AC loads.
Multiply watts by heating time in hours, then allow for inverter losses.
Keep the station dry, ventilated and away from spilled water or steam.
- Check battery, charger, inverter and transfer equipment instructions; use qualified electrical help for home wiring.
- Check battery, charger, inverter and transfer equipment instructions; use qualified electrical help for home wiring.
- Check battery, charger, inverter and transfer equipment instructions; use qualified electrical help for home wiring.
- Check battery, charger, inverter and transfer equipment instructions; use qualified electrical help for home wiring.
- Check battery, charger, inverter and transfer equipment instructions; use qualified electrical help for home wiring.
- Check battery, charger, inverter and transfer equipment instructions; use qualified electrical help for home wiring.
- Check battery, charger, inverter and transfer equipment instructions; use qualified electrical help for home wiring.
- Check battery, charger, inverter and transfer equipment instructions; use qualified electrical help for home wiring.
Final Verdict: Size Energy and Inverter Output Separately
Start with measured daily Wh, choose how many days you need, and allow for conversion losses and reserve. Then verify that the selected system can supply the maximum simultaneous running watts and startup surge. The calculator provides a planning target, not a guarantee of runtime.
Frequently Asked Questions
How do I calculate battery capacity in Wh?
Multiply each appliance’s watts by daily hours, add the results, multiply by backup days, and divide by expected efficiency and usable battery fraction.
How many kWh is a 1,000Wh battery?
One kilowatt-hour is 1,000 watt-hours, so 1,000Wh equals 1kWh of nominal stored energy.
Is a 1kWh battery enough for a refrigerator?
It depends on the refrigerator’s measured daily consumption, inverter efficiency, reserve and whether the battery can handle compressor startup.
Why does a 1,024Wh battery not deliver 1,024Wh of AC power?
Inverter losses, idle consumption, temperature and reserved capacity reduce usable AC energy.
Does a larger battery mean higher watt output?
No. Battery energy capacity in Wh and inverter output in W are separate specifications.
How do I estimate solar panel size?
Divide daily energy that must be replenished by realistic equivalent sun-hours and system efficiency; account for local weather and panel orientation.
Can this calculator size a whole-house backup system?
It can provide a preliminary energy estimate, but hardwired home systems require load studies, surge checks, transfer equipment and qualified design.
Sources and Editorial Methodology
Formulas use the definition of electrical energy (Wh = W × hours) and explicitly stated planning assumptions. No hands-on testing is claimed. Confirm inverter and battery specifications in the manuals for the exact product purchased.