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What size generator do you need?
Updated
Every generator has two numbers. Starting watts, also called peak or surge watts, is a short burst for motors that need a kick to get going. Running watts, also called rated watts, is what it can hold all day. Sellers put the starting number in the name. Size by the running number.
Generator wattage calculator
Tick what you want to run at the same time.
Typical figures. Your appliance may differ, so check its label. Starting watts here means all running watts plus the single largest starting surge, because motors rarely start at the same moment. The size tip keeps you at 80 percent of running watts or less.
A worked example
Say you want a refrigerator, a gas furnace blower, LED lights, and the TV and router. That is 700 + 800 + 100 + 200 = 1,800 running watts. The biggest single surge is 1,500 watts, so you need 3,300 starting watts. With some headroom, look for about 2,250 running watts or more.
Add a 10,000 BTU window air conditioner and it jumps to 3,000 running and 5,400 starting watts. Now you want about 3,750 running watts. That is past the Champion 4000-Watt, which has 3,000 running watts on gas, and it fits the Westinghouse iGen5000DF at 3,900 running and 5,000 peak.
How far the name is from the running watts
| Generator | Number in the name | Running watts (gas) | Running as a share of the name |
|---|---|---|---|
| Westinghouse WGen9500DF | 12,500 | 9,500 | 76% |
| Westinghouse WGen5300DFcv | 6,500 | 5,300 | 82% |
| Westinghouse iGen5000DF | 5,000 | 3,900 | 78% |
| Champion 4000-Watt Dual Fuel | 4,000 | 3,000 | 75% |
| WEN 56235i | 2,350 | 1,900 | 81% |
Three sizing mistakes
- Sizing by the name. A "4,000 watt" unit with 3,000 running watts will trip its overload if you ask it for 3,500 watts for more than a moment.
- Forgetting 240 volts. Most well pumps, electric dryers, electric water heaters and central air units need 240 volts. The inverter picks on this site are 120 volts only. For 240 volt loads you need a unit like the WGen9500DF or WGen5300DFcv, and an electrician.
- Running flat out. A generator held at its limit runs hot, loud and thirsty. Planning to use 80 percent or less gives you room for the next thing you plug in.
The watt figures in the calculator are typical values. The label on your own appliance is the real answer. Look for watts, or multiply volts by amps.
Safety first, every time
- Carbon monoxide kills. Never run a generator indoors, in a garage, in a shed or on a porch, even with the door open. Keep it at least 20 feet from the house. Point the exhaust away from windows, doors and vents. Put battery CO alarms inside your home.
- Never backfeed. Do not plug a generator into a wall outlet or dryer outlet. It can kill a utility worker or a neighbor and can start a fire. To power home circuits you need a transfer switch or an interlock kit, installed by a licensed electrician.
- Without that, run heavy-duty outdoor extension cords from the generator straight to each appliance.
- Let the engine cool before you refuel. Store fuel outside the house.