Generator Size Calculator – What Size Generator Do I Need? | CalcLoft
Select the appliances you want to power and get total running watts, the largest starting surge and a recommended generator size in watts and kW — instantly.
How generator sizing works
Every appliance draws running watts continuously while it is on. Anything with a motor or compressor — a refrigerator, pump, AC or furnace fan — also draws a short burst of extra starting watts the moment it kicks on. To size a generator:
Recommended size = total running watts + largest single starting surge
The biggest motor’s running watts are already counted in your total, so you only add the extra watts it needs to start. That is why the calculator adds the largest (starting − running) gap across your appliances, not every appliance’s full starting watts.
Example: a refrigerator (600 W running / 2200 W starting), a furnace fan (750 W / 2350 W) and lights (300 W) total 1650 W running. The furnace fan has the largest surge gap: 2350 − 750 = 1600 W. Recommended size: 1650 + 1600 = 3250 W, about 3.3 kW.
Typical home generator sizes
A rough guide by household — always run your own numbers through the calculator above.
| What you want to power | Typical running load | Usual generator class |
|---|---|---|
| Outage essentials — refrigerator, furnace fan, well or sump pump, a few lights, phone and router | ≈ 3–4 kW | 5–7.5 kW portable |
| Essentials plus kitchen — adds microwave, coffee maker, window AC and similar (run the big loads one at a time) | ≈ 5–7 kW | 7.5–10 kW portable or a small 10 kW standby |
| Whole-home comfort — central AC or heat pump, electric water heater, oven, several rooms at once | ≈ 10–22 kW | 10–22 kW whole-home standby |
Generator sizing tips
- Natural gas vs propane. A whole-home generator can run on either. Natural gas from the utility pipe means no refills during a long outage, but most units produce slightly less peak output on natural gas than on propane. If your load sits close to the rating, plan for propane or size up.
- Motors draw 2–3× their running watts to start. Compressors, pumps and fans pull a surge of current for a few seconds until they spin up. Ignore it and the generator can bog down, stall or trip when the largest appliance kicks on.
- Round up to a standard size. Pick a generator whose rated running watts comfortably exceed your total — running near the limit shortens generator life and causes nuisance trips.
Related guides
Generator calculator FAQ
Portable or whole-home generator — which do I need?
A portable generator is cheaper, starts by hand and powers whatever you plug into it — usually enough for essentials during an outage. A whole-home (standby) generator is wired into your panel with an automatic transfer switch, starts itself when the power fails, and can run hardwired loads such as central AC, a well pump or an electric water heater. Need automatic backup for big hardwired appliances? Choose whole-home. Occasional essentials only? A portable usually makes sense.
Why is the starting wattage higher than the running wattage?
Motors and compressors draw a surge of current — typically 2–3 times their running watts — for a few seconds when they start, until they spin up to speed. A refrigerator that uses 600 W running can briefly pull around 2200 W. The calculator adds the largest single starting surge to your running total so the generator doesn’t bog down or trip when the biggest motor kicks on.
How do I read a generator nameplate?
Generators list two numbers: running (rated/continuous) watts — what the unit delivers hour after hour — and starting (surge/maximum) watts — the brief peak it can handle when a motor starts. Your total running watts must stay under the running rating, and your running total plus the largest starting surge must stay under the starting rating. Ratings in kW are thousands of watts: 1 kW = 1000 W.
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