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Generator Sizing Calculator

Estimate generator wattage from running watts, starting surge, power factor, and safety margin for home backup planning.

Generator Sizing Calculator

Loads to power at the same time

Set how many of each you need to run together. Motor loads add a startup surge; only the single biggest surge is counted on top of the running total.

700 W running, +2,200 W start

1,200 W running, +1,800 W start

3,500 W running, +4,500 W start

800 W running, +1,300 W start

800 W running, +1,300 W start

2,000 W running, +2,000 W start

1,500 W running, no surge

4,000 W running, no surge

1,000 W running, no surge

100 W running, no surge

200 W running, no surge

1,000 W running, +2,000 W start

Headroom so the generator is not run at 100%. 20 to 25% is typical.

Recommended generator size

5,280 W

About 5.3 kW to cover 8 units across 4 loads.

Running watts

2,200 W

Largest startup surge

+2,200 W

Peak (starting) watts

4,400 W

Generators are rated by running (rated) watts and a higher starting (surge) watts. Match running watts to 2,200 W and starting watts to at least 4,400 W.

Includes a 20% reserve (880 W headroom) so the unit is not run at full load.

Estimate only. Check each appliance nameplate for its actual running and starting watts, and have a licensed electrician size and connect any whole-home or transfer-switch setup.

Frequently Asked Questions about the Generator Sizing Calculator

How do I size a generator for my appliances?
Add up the running (rated) watts of everything you want to power at the same time, then add the single largest starting (surge) watts from your motor-driven loads. That total is the starting watts your generator must supply, while the running total is the continuous load it must hold. This calculator does both sums for you and lets you add a reserve margin so you are not running the unit at full capacity.
Why do you only add the largest starting surge instead of every appliance's surge?
Motors draw a brief inrush surge only for the moment they start, and in practice your appliances do not all start at the exact same instant. The standard manufacturer worksheet method (used by Champion, Generac, and Honda) assumes everything is already running and then the single biggest motor kicks on. Adding only the largest surge avoids massively oversizing the generator while still covering the worst realistic spike.
What is the difference between running watts and starting watts?
Running (rated) watts are the steady power an appliance draws while it operates. Starting (surge) watts are the extra power a motor needs for a few seconds to overcome inertia at startup, often two to three times its running watts. Resistive loads like heaters, lights, and microwaves have no surge, so their starting watts equal their running watts.
What reserve margin should I use?
Manufacturers generally advise against running a generator continuously at 100% of its rating, so a 20 to 25% reserve is common. The reserve scales up the peak watts to give headroom for an extra appliance or for derating in heat and altitude. Set it to 0 if you want the raw required figure with no padding.
Where do I find the exact watts for my appliances?
Check the nameplate or sticker on each appliance, usually on the back or base, which lists watts or amps and volts (watts equal amps times volts). The values built into this tool are typical figures for common household and jobsite items, so use your own nameplate numbers when you can. Tools and pumps vary widely by horsepower, so verify the surge rating before you buy.
Is this calculator a substitute for an electrician?
No. This is an estimate to help you compare generator sizes, not a wiring or installation plan. Any whole-home connection, transfer switch, or interlock must be sized and installed by a licensed electrician to meet local code and keep you and utility workers safe.

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