How it works
Generators carry two ratings. Running watts, also called rated watts, is what it can supply continuously. Starting watts, also called surge or peak watts, is the short burst it can give for a few seconds while a motor gets going. Refrigerators, pumps, furnace blowers and air conditioners can draw two to three times their running power when they start.
To size a generator, add up the running watts of everything you want on at once. Then add the extra starting watts of the single item with the biggest surge, because motors rarely start at the same moment. Both totals need to fit within the generator’s ratings.
Headroom keeps the generator from running at full load, which is louder, burns more fuel and shortens its life. Wattages in the example list are typical values; the label on each appliance or its manual gives the real number. Connecting a generator to house wiring needs a transfer switch or interlock installed by an electrician.
Formula
Running watts = sum of running watts. Starting watts = running watts + the largest (starting − running) of any one item.
Questions people ask
What size generator do I need to run a house during an outage?
For the essentials (refrigerator, sump pump, furnace fan, lights, a microwave and internet) most homes need about 4,000 to 5,000 running watts and 5,500 to 7,000 starting watts. Central air conditioning or electric heat, water heaters and well pumps push that much higher.
What is the difference between running and starting watts?
Running watts are the steady power an appliance uses. Starting watts are the brief extra power a motor needs to get moving, often two to three times the running watts for a few seconds. Items without motors, like lights and TVs, have no extra starting load.
Can a generator run a refrigerator?
Yes. A typical refrigerator uses about 700 running watts and needs about 2,200 watts to start, so even a 3,000-watt portable generator can run one. Check the label on yours for exact numbers.
How do I find an appliance’s wattage?
Look at the nameplate, usually inside the door or on the back. If it lists amps instead, multiply amps by volts (120 V for most outlets) to get watts. For starting watts on a motor, check the manual or use two to three times the running watts as a rough guide.