Sizing a backup generator
4,000 running watts plus a 2,000 W surge means a peak of 6,000 W - a 6 kW generator or larger.
Generator Size Calculator
Generator Size - figures update as you type, nothing leaves your browser.
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Figures update as you type. Nothing you enter leaves this tab, and no values are stored.
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A generator size calculator works out the generator capacity you need from the total running watts of everything you want to power plus the surge that motors draw when they start. Undersize it and it stalls or trips when the fridge or pump kicks in; oversize it and you burn extra fuel idling. Getting the size right is about matching both the steady load and the brief but large starting spike. The tool adds your total running watts to the largest single motor surge - the key insight being that surges do not all happen at once, so you size for running load plus the biggest one starting, not the sum of every surge. Motors like fridges, pumps, air conditioners and power tools can draw two to three times their running watts for a moment at startup. The result is the peak demand in kilowatts; choose a generator rated above it, and leave headroom for anything you might add later.
Using Generator Size Calculator
- Add the running watts of the devices that may operate together.
- Enter the largest additional motor-start surge in watts.
- Compare the calculated peak demand with a generator's continuous and surge ratings before choosing equipment.
Where generator size calculator helps
Separate steady demand from startup demand
List the equipment that must operate at the same time, then check which devices have motors or compressors. Their starting requirement can exceed the running wattage. A total of nameplate running watts is only a planning input; compare both running and surge ratings with the generator manufacturer's guidance. Also check the required voltage and phase. A higher watt rating does not make an incompatible electrical connection suitable.
Before you use the result
- Starting surges and simultaneous loads vary by appliance. Check equipment nameplates and leave capacity for loads the simple model does not include.
- It calculates in your browser from the numbers you enter, so a wrong input gives a wrong result - sanity-check the figures.
- Nothing is saved between visits; note down or screenshot the result if you need to keep it.
Questions about Generator Size Calculator
Do I add up every appliance's surge?
No - surges rarely coincide. Size for total running watts plus the single largest motor surge, which is what the tool does.
Is my data saved or uploaded?
No. Generator Size Calculator calculates the result entirely in this browser; the values you enter are not stored or sent to a server.
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