Generator Sizing Calculator
Size a generator from load and surge inputs.
Calculate generator sizing calculator from clear inputs
Generator Sizing Calculator gives a focused worksheet for home & diy questions where a quick deterministic estimate is more useful than a spreadsheet. The generator sizing calculator fields mirror the variables in its formula, and the output separates the main result from supporting values so you can sanity-check this calculation before reusing it.
4200 running watts plus 1800 surge with 20 percent reserve recommends 7200 watts.
Keep the estimate private in your browser
For Generator Sizing Calculator, every calculation runs in your browser with no upload, account, cookie, live lookup, or server-side storage. The generator sizing calculator result is best treated as a planning number because the real-world constraints behind this estimate can change the answer. Adjust the generator sizing calculator assumptions and confirm important decisions against the relevant source of truth for that exact situation.
Frequently Asked Questions
How is the recommended size calculated?
(Running watts + the single largest starting surge) × (1 + reserve percent). 4,200 running watts plus a 1,800 W surge with 20% reserve recommends 7,200 W.
Why add only the largest surge?
Motors like fridges and well pumps draw extra power only for a few seconds when starting, and they rarely start at the same moment. Sizing for the biggest one covers the worst single start.
Where do I find running and starting watts?
On the appliance label or manual. If only amps are listed, multiply by the voltage (120 or 240) for watts; starting watts for motors are often 2 to 3 times running watts.
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