Generator Size Calculator
Pick the circuits you want to keep on during an outage and see the generator class that covers them, in kilowatts.
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Data: typical running and starting wattages from U.S. DOE Energy Saver appliance guidance and manufacturer generator sizing charts; motor starting surge from National Electrical Code locked-rotor conventions described in words, 2026 · Updated September 2026
How generator sizing works
Running watts add up
Each appliance you check has a typical running wattage, the power it draws once it is going. Those are summed for the base load.
One motor starts at a time
Motors like a refrigerator compressor, sump pump, well pump, or air conditioner pull two to six times their running watts for a moment at start. The calculator adds only the largest single surge on top of the running total, the standard sizing approach, since motors rarely all start in the same second.
Headroom and the class
Twenty percent headroom is added so the unit is not running flat out, then the result lands in a class: portable, large portable, air-cooled standby, or whole-home. Big 240-volt loads like central air and a well pump are what push a house into standby territory.
The connection is the real job
Any generator that powers house circuits connects through a transfer switch or interlock at the panel. An electrician installs it, and standby units add gas or propane supply and an automatic switch. The sizing here gets you ready for that conversation.
Common questions
A typical house with central air, a well pump, and electric cooking or drying lands in the 20 to 26 kW range without load management. With an automatic load-shedding module that runs the big loads one at a time, a 14 to 22 kW standby unit often covers the same house.
A 2-ton unit can sometimes start on a large portable with a 240-volt outlet. Three tons and up usually needs 10 kW or more unless the air conditioner has a soft-start kit, which cuts the starting surge roughly in half.
Portable units cover the basics for a few hours at a time and need you home to start them and refuel. Standby units start on their own within seconds, run on natural gas or propane, and carry the whole house or a chosen set of circuits. Long or frequent outages, a sump pump, a well, or medical equipment tip the choice toward standby.
Feeding power backward through an outlet, called backfeeding, sends electricity out to the utility lines and can injure a line worker, and it bypasses every protection in the panel. A transfer switch or interlock, installed by an electrician, is the accepted connection.
A gasoline portable typically runs 8 to 12 hours on a tank at half load. Propane stores for years without going stale. Natural gas standby units run as long as the utility gas flows, which is the reason most permanent installs use it.
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