How to use it
- Find the capacity. Look for watt-hours (Wh) on the label or spec sheet. A unit sold as 2 kWh holds about 2,000 Wh.
- Enter your average load. Add the watts of everything that will run. For anything that cycles on and off, use its average across the whole day.
- Set the usable share. 85 to 90% is a sensible planning figure for running AC appliances through the built-in inverter.
- Set a target. The bar shows how much of your target outage this battery covers.
The formula
runtime (hours) = capacity (Wh) × usable share ÷ average load (W)
With the starting numbers, 2,048 Wh × 0.90 ÷ 150 W gives 12.3 hours. To last a full 24 hours at 150 W you would need 150 × 24 ÷ 0.90 = 4,000 Wh.
Reading the result
| What you see | What it means | What to do |
|---|---|---|
| Runtime is longer than the target | The battery has spare energy for this load. | Decide whether to add another appliance or keep the spare as a buffer. |
| Runtime is 50 to 99% of the target | You will run out before the outage ends. | Trim the load, or plan a recharge from solar, a vehicle or a generator. |
| Runtime is under half the target | The battery is too small for this job. | Use the capacity figure shown as your shopping target. |
Why real runtime differs
- Cold. Lithium batteries deliver less energy when cold, and many will not accept a charge below freezing. Check the temperature range in the manual.
- Small loads. The inverter uses some power just by being switched on, which matters most when the load is only a few watts.
- Age. Capacity fades slowly with use, so an older unit holds less than its label.
- Surge. This calculator covers energy only. A battery with plenty of watt-hours can still fail to start a motor if its output rating is too low.
Common questions
What is the difference between watts and watt-hours?
Watts measure how fast power is used at one moment. Watt-hours measure the total energy stored or used over time. A 100 W load running for 5 hours uses 500 Wh.
Should I use 100% for usable capacity?
No. Converting battery power to household AC loses some energy, and most people keep a small reserve. Use 100% only if you are comparing against a manufacturer's own claim.
Can I run a space heater from a power station?
Only briefly. A 1,500 W heater would empty a 2,000 Wh battery in a little over an hour. Batteries suit low, steady loads such as a fridge, a router and lights.
How do I find my fridge's average load?
Measure it with a plug-in energy meter for 24 hours, or use the yearly figure on its energy label. The measuring guide below walks through both.
Related
Reviewed October 2026. Results are planning estimates. Check them against your equipment's specifications. Report a problem.