Starting Watts and Running Watts, on One Sheet

Generators list two numbers: running watts and starting watts (sometimes called surge or peak). If you only look at one, you'll buy too small or trip the breaker the first time the fridge kicks on.

01What they mean

• Running watts: what the generator can supply all the time.
• Starting watts: a short burst it can supply for a few seconds.

Anything with a motor (fridge, freezer, sump pump, furnace blower, well pump) needs a big burst to start, often two or three times its running watts, for a second or two.

02Typical numbers

These are rough examples. Your appliance's label or manual has the real numbers.

• Refrigerator: around 700 running, 2,000+ starting
• Chest freezer: around 500 running, 1,500 starting
• Sump pump (1/2 hp): around 1,000 running, 2,000+ starting
• Gas furnace blower: a few hundred to 800 running, more to start
• Well pump: often 1,000 to 2,000 running and a big start surge. Many are 240-volt.
• Lights (LED): 10 each
• Phone chargers, router: 10 to 20 each
• Space heater: 1,500. It runs at that all the time. It will eat a small generator.

03How to add them up

1. Add the running watts of everything you want on at the same time.
2. Take the biggest starting watts of any one motor.
3. Add that surge to the running total of everything else.

The generator's running rating has to cover step 1. Its starting rating has to cover step 3.

Example

Fridge (700) + sump pump (1,000) + furnace blower (600) + lights and router (100) = 2,400 running watts. Biggest surge: sump pump at 2,000 starting. 2,400 − 1,000 + 2,000 = 3,400 watts at the moment it kicks on.

A generator rated about 3,500 starting / 3,000 running would just make it. Bigger gives you margin.

04Stagger the starts

Plug things in one at a time, biggest motor first. If two motors start at the same moment, the surge adds up.

See how to size a backup generator for the full walk-through.

Photo by rgaudet17 on Pixabay.

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