Four numbers to write down
1. Daily energy (Wh)
Add up the energy each essential device uses in a day. A 10 W router running for 24 hours needs 240 Wh. A fridge is different because its compressor cycles on and off, so use a measured daily figure or the energy label, not running watts multiplied by 24.
2. Outage length
Multiply daily energy by the number of days you want to cover without recharging. Then divide by the usable share, typically 0.85 to 0.90, to allow for inverter losses.
3. Continuous output (W)
Add the running watts of everything that might be on at the same moment. The power station's AC output rating must be higher than that total.
4. Start-up surge (W)
Motors and compressors draw a brief burst when they start. Check the power station's surge or peak rating against the largest motor you will run.
A worked example
A fridge and a separate freezer are measured at 1.9 kWh a day together. A modem and router add 20 W around the clock, which is 480 Wh.
- Daily energy: 1,900 + 480 = 2,380 Wh.
- One day at 85% usable: 2,380 ÷ 0.85 = 2,800 Wh.
- Two days without recharging: 5,600 Wh.
- Running watts with both compressors on: about 180 + 150 + 20 = 350 W.
- Surge, if the fridge starts while the rest is running: 350 + 500 = 850 W.
So a unit of around 3,000 Wh with at least 1,000 W of output covers one day comfortably. Two days needs an expansion battery or a way to recharge.
What capacity classes are good for
| Capacity | Realistic job |
|---|---|
| 250 to 500 Wh | Phones, a laptop, a modem and router, a light. Internet for a day. |
| 1,000 to 1,500 Wh | The above plus a CPAP machine overnight, or a fridge for part of a day. |
| 2,000 to 3,000 Wh | A fridge and freezer for about a day. |
| 4,000 Wh and up | Multi-day fridge and freezer backup, or a furnace blower for some hours. |
What to compare before buying
- Battery chemistry. Lithium iron phosphate (LiFePO4) cells last for more charge cycles than older lithium-ion types, at the cost of extra weight.
- Expansion. Can you add a second battery later?
- Solar input. Note the maximum watts, the voltage range and the connector type.
- Recharge time from a wall outlet, which matters when power returns briefly.
- Operating temperature. Many units will not charge below freezing.
- Pass-through or UPS mode if you want it to take over automatically.
- Weight, warranty and local service. A 3,000 Wh unit can weigh as much as a small adult.
Common sizing mistakes
- Shopping by the model number. The number in the name may be watts, watt-hours or neither. Read the spec sheet.
- Ignoring surge. A unit with plenty of capacity can still refuse to start a compressor.
- Planning around heat. Electric heaters, kettles and hair dryers empty a battery in an hour or so.
- Counting on winter solar. Size the battery for the outage first and treat solar as a bonus.
Related
Reviewed October 2026 by Lee Palmer. This guide is general information, not professional advice. Suggest a correction.