What size battery or generator do I need?
You need two numbers: watts, meaning how much power runs at once, and kWh, meaning how much energy you use over the outage.
Watts = size
Add up the running watts of everything on at once, then add the biggest start-up surge. Fridges, pumps, and AC compressors briefly pull 2–3× their running watts when they start.
kWh = how long
Watts × hours ÷ 1,000. A 400 W load for 5 hours uses 2 kWh. Batteries are rated in kWh. Generators run as long as they have fuel.
Worked example 1: fridge + furnace fan + Wi-Fi, 24 hours
- Fridge: 275 W typical × 8 h Assumption: compressor on 1/3 of the time = 2.2 kWh
- Furnace fan: 550 W typical × 8 h Assumption = 4.4 kWh
- Wi-Fi: 15 W × 24 h = 0.36 kWh
- Total 6.96 kWh ÷ 0.85 = 8.2 kWh of battery
- Peak: 400 + 800 + 20 = 1,220 W running, + 800 W largest surge = 2,020 W
Worked example 2: CPAP with heated humidifier + Wi-Fi + phone charging, 1 night and day
About 1.48 kWh and 240 W peak. Look for a portable station with at least that much capacity; a 1 kWh unit would fall short of a full 24 hours. Check your CPAP's label for its exact watts.
Appliance wattage table
| Appliance | Running W | Starting W | Daily kWh note |
|---|---|---|---|
| Refrigerator | 150–400 | 800–1200 | ~1.0-2.0 (EST: 1/3 duty cycle) Estimate |
| Freezer | 100–500 | 500–1000 | ~0.8-1.5 (EST) Estimate |
| Furnace fan 1/2 HP | 300–800 | 800–1600 | EST: running W x heating hours Estimate |
| Sump pump 1/3 HP | 500–800 | 1000–1600 | EST ~3.8 at 20% runtime wet weather (outagemath.com) Estimate |
| Sump pump 1/2 HP | 800–1050 | 1300–2150 | EST ~5 at 20% runtime Estimate |
| Well pump 1/2 HP | 750–1000 | 1500–2000 | EST ~0.8 at ~1 hr/day (outagemath.com; 240V surge can reach 5,300W) Estimate |
| Well pump 1 HP | 1000–2000 | 2000–4000 | EST Estimate |
| Window AC 5,000 BTU | 450–600 | 900–1200 | EST running W x hours Estimate |
| Window AC 10,000 BTU | 900–1200 | 1800–2400 | EST Estimate |
| Central AC 3 ton | 3000–3500 | 5000–6000 | EST Estimate |
| Electric range (one burner) | 1200–2400 | — | EST Estimate |
| Microwave | 600–1200 | — | EST Estimate |
| Space heater | 750–1500 | — | EST Estimate |
| Electric water heater | 3000–4500 | — | EST Estimate |
| Washing machine | 500–1000 | 1000–2300 | EST Estimate |
| TV 32-55in | 80–400 | — | EST Estimate |
| LED bulb (each) | 8–15 | — | EST Estimate |
| Phone/laptop charger | 20–100 | — | EST Estimate |
| CPAP no heat | 30–60 | — | EST ~0.24-0.48 for 8 h Estimate |
| CPAP heated humidifier | 60–120 | — | EST ~0.5-1.0 for 8 h Estimate |
| Router/modem (Wi-Fi) | 10–20 | — | EST ~0.24-0.48 (24 h) Estimate |
| Security system | 15–40 | — | EST Estimate |
| Ceiling fan | 15–75 | — | EST Estimate |
| EV Level 2 charging | no verified figure | — | ESTIMATE - needs source (charger nameplate e.g. 7.2-11.5 kW) Estimate |
| Medical devices (oxygen concentrator etc.) | no verified figure | — | use device nameplate Estimate |
Running and starting watts: Champion Power Equipment generator wattage chart, as of 2026-10-25. Daily kWh notes are our computed estimates. Some sump and well pump figures cite outagemath.com. A 240 V well pump can surge to 5,300 W, so check its nameplate.
Generator vs. battery
For a generator, match its running watts to your running total and its starting watts to your peak. Fuel takes care of the kWh. For a battery, you need both the watts and the kWh. We haven't verified generator prices yet, so we don't list any.
FAQ
What size generator do I need to run a refrigerator and furnace?
Using the Champion wattage chart, a fridge (400 W max running, 1,200 W start) plus a 1/2 HP furnace fan (800 W max running, 1,600 W start) and Wi-Fi need about 1,220 running watts and 2,020 peak watts, so a 2,500 W-class generator or power station covers it.
How many kWh do I need for a 24-hour outage?
For a fridge, a furnace fan running 8 hours, and Wi-Fi, our estimate is about 8.2 kWh, including the 0.85 usable-capacity assumption.
Backup power, before the next outage
A short email when it matters: outage-season prep checklists before summer storms and winter freezes, state and utility rebates that open, change, or run out of money, and real price drops on the power stations in our data.