How the watt total is built
Peak watts = sum of running watts + the largest (starting minus running). Recommended running watts = peak x 1.2, rounded up to the next 100.
Example with the default checks: 700 + 1,050 + 100 + 250 = 2,100 running watts. The fridge needs 1,500 extra watts to start and the sump pump needs 1,100 extra, so only 1,500 is added. Peak = 3,600 watts. 3,600 x 1.2 = 4,320, which rounds up to 4,400 running watts.
What this leaves out
It does not know your voltage, whether two motors can start together, or the altitude derate on the generator. A well pump and a central air compressor should not be assumed to start on a small portable unit just because the running watts fit. If you are connecting to a panel, an electrician has to separate the circuits and install an interlock so the generator cannot backfeed the utility lines.
Safety
Never run a portable generator indoors, in a garage, or under a window. Carbon monoxide has no smell. Keep the unit dry, and do not refuel it while it is hot.
Frequently asked questions
What is the difference between running and starting watts?
Running watts are what an appliance draws while it is on. Starting watts are the brief extra a motor needs to get moving. This calculator adds every running watt, then only the single largest extra starting watt, because one motor usually starts at a time.
Why add 20 percent headroom?
A generator that sits at its rated maximum runs hot and has no room for a small extra load. The recommended size is the peak watts times 1.2, rounded up to the next 100 watts.
What size generator runs a fridge and a sump pump?
Using the preset values, a fridge (700 running / 2,200 starting), a 1/2 HP sump pump (1,050 / 2,150), 100 watts of lights, and 250 watts for a TV and router total 2,100 running watts. The largest extra surge is 1,500 watts, so the peak is 3,600 watts and the 20 percent recommendation is 4,400 running watts.
Can I run a portable generator in the garage?
No. Carbon monoxide from a portable generator kills, including in a garage with the door open. Keep it outdoors, away from windows, and connect it to house wiring only through a transfer switch or interlock installed by an electrician.