List what you must keep running, add the running watts of everything on at once, add the largest single starting surge, then multiply average draw by the hours to get watt-hours. A refrigerator, router, lights and phones average roughly 225 W — about 6.4 kWh over a day once losses are included.
Step 1 — Decide what really needs power
Separate needs from comforts. Most households choose a short list: refrigerator and freezer, internet, lighting, phone charging, a sump or well pump where they have one, medical equipment, and sometimes a fan, TV or furnace blower. Heat from electricity (space heaters, ovens, dryers) and large air conditioners are the loads that overwhelm small systems.
Medical equipment is its own category: use the device's label and ask its maker and the prescriber about backup power. A table of typical watts is not a safe basis for a medical decision.
Step 2 — Running watts and starting watts
Every appliance has a nameplate or label with watts, or volts and amps (watts = volts × amps). Motors and compressors — refrigerators, sump pumps, well pumps, air conditioners — draw a larger current for a moment when they start. That starting surge decides whether a battery's inverter or a generator copes.
| Appliance (typical) | Running watts | Starting watts | Duty cycle |
|---|---|---|---|
| Refrigerator, modern | 150 | 600 | about 40% |
| Chest freezer, 7 cu ft | 100 | 400 | about 35% |
| Sump pump, 1/3 hp | 800 | 1,500 | about 15% |
| Well pump, 1/2 hp | 1,000 | 2,100 | about 20% |
| Gas furnace blower | 750 | 1,500 | about 50% |
| Window air conditioner | 600 | 1,800 | about 60% |
| Wi-Fi router and modem | 25 | — | 100% |
| LED lighting, 10 bulbs | 100 | — | 100% |
| Space heater | 1,500 | — | 100% |
The appliance load and runtime calculator does this arithmetic for your list and shows which products cover it.
Step 3 — Watt-hours for the hours you need
Cooling appliances cycle, so average draw is lower than running watts. Multiply each appliance's running watts by its duty cycle to get an average, add them, and multiply by hours. Divide by 0.85 to allow for inverter losses when the source is a battery.
| Example load | Average draw | Energy for 12 h | Energy for 24 h | Energy for 72 h |
|---|---|---|---|---|
| Refrigerator + router + lights + phones | 225 W | 3.2 kWh | 6.4 kWh | 19.1 kWh |
| The same + sump pump | 345 W | 4.9 kWh | 9.7 kWh | 29.2 kWh |
| The same + furnace blower and TV | 820 W | 11.6 kWh | 23.2 kWh | 69.5 kWh |
Step 4 — Match the number to a product
- For a battery power station: capacity (Wh) above your energy figure, AC output (W) above your running watts, and a surge rating above your starting watts.
- For a generator: running watts above your total, starting watts above your starting requirement, and a fuel plan for the hours you need.
- For a mix (a battery for essentials, a generator for long outages), size the battery for the essentials and the generator for recharging it.
Browse power stations and generators, or see the appliance pages for refrigerators and sump pumps.
Questions readers ask
Why do you add only the largest starting surge?
How many watts does a refrigerator use?
Can I run a space heater or air conditioner from a power station?
Figures marked as Wattstanding calculations show their working; product figures come from the sources on each product page. How we work