Generator Size Calculator

A full-size refrigerator draws about 150 W running but 1,000 to 1,200 W for a second when the compressor starts. A 2,000 to 2,200 W inverter generator starts and runs it with room for a few lights and a router. If a chest freezer and a furnace blower are on the same generator, plan on 2,500 W starting capacity so the biggest motor can start while the others are running.

For the essentials most people want (fridge, freezer, furnace blower, well or sump pump, lights, Wi-Fi, a microwave) a 5,000 to 7,500 W running unit with a 240V outlet covers it through an interlock or transfer switch. Adding an electric water heater (4,500 W), electric dryer (5,000 W) or central AC (3,500 W running, 10,000 W starting) pushes you to 10,000 W or more, which is where a permanently installed standby generator is usually the better buy. Run your actual list through the calculator above. The total is almost always smaller than people guess.

A 1/2 HP submersible well pump draws about 1,000 W running and 2,500 to 3,000 W to start, and a 1 HP pump about 2,000 W running and up to 6,000 W to start. Nearly all are 240V, so the generator needs an L14-30R or 14-50R outlet and must be connected to the panel through an interlock kit or transfer switch. For a 1/2 HP pump plus fridge, furnace and lights, a 5,000 W running / 6,500 W starting unit is a comfortable size.

An inverter generator makes clean sine-wave power (under 3 percent THD), throttles its engine to match the load so it is quieter and uses less fuel at part load, and is safe for laptops, TVs and furnace control boards. A conventional open-frame generator runs at a fixed 3,600 rpm, is louder, burns more fuel when lightly loaded and can have 10 to 25 percent THD, but costs far less per watt. Below about 4,500 W buy an inverter. Above that, open frame is fine if you keep electronics on a UPS, and large inverter units exist if you can pay for them.

Usually yes, for about $50 to $100 more. Propane stores indefinitely, does not gum up the carburetor, and a few 20 lb grill tanks are easier to keep than gas cans. There are trade-offs. Propane cuts output by about 10 percent, a 20 lb tank only runs a 7,000 W unit for 4 to 8 hours, and gasoline is cheaper per kWh. Tri-fuel units that also run on natural gas remove the fuel problem entirely if you have a gas meter.

That comes down to engine size and load more than rated watts. A 2,000 W inverter at a light load burns about 0.1 gal an hour, so 5 gallons is roughly 40 to 50 hours. A 7,500 W open-frame unit at half load burns about 0.6 gal an hour, so 5 gallons is 8 to 9 hours. A 12,000 W unit at half load burns over a gallon an hour. The calculator scales the manufacturer's rated run time to your actual average load.

No. A double-male cord into a dryer or range outlet backfeeds your panel and, through the meter, the utility lines. It can electrocute a line worker, it will destroy the generator when the utility power returns, it bypasses the breaker that should protect the cord, and it is illegal in every US jurisdiction. The only safe ways to feed house wiring are an interlock kit or a transfer switch installed by an electrician.

You need a transfer switch or an interlock kit if you want to power anything hardwired: a furnace, a well pump, a sump pump on a hardwired circuit, ceiling lights or any 240V appliance. If your essentials all have plugs and are 120V, heavy extension cords from the generator work without any panel changes. If the only hardwired load is a gas furnace blower, a furnace transfer switch mounted beside the furnace lets a cord run it, which is far less work than a panel interlock. An interlock kit is the cheaper panel method, while a manual transfer switch is simpler to operate and keeps you from overloading the generator.

At least 20 feet, with the exhaust pointed away from doors, windows and vents. Never in a garage, carport, basement or under an open window, even with doors open. Carbon monoxide from a generator can reach deadly levels indoors in minutes. Buy a unit with a CO shutoff sensor, put a battery-powered CO alarm on each sleeping level, and use a 25 ft or longer cord so the distance is easy to keep.

For loads under about 3,000 W with no 240V circuits, for short outages, or if you want silence and no fuel, a home battery or large power station is often the better buy, and it switches over without you doing anything. A generator wins for long outages, big loads, well pumps and central AC, because fuel is cheaper per kWh than battery capacity. Many people end up with both, a battery for the first night and a small dual-fuel generator to recharge it for an hour or two a day.