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Power / Generator sizing

Generator Size Calculator

Check the loads you want to keep running. The calculator sums running watts, adds the single largest starting surge — the way sizing actually works — and stamps a recommendation with 20% headroom.

Select loads
Heating & cooling
Pumps & motors
Kitchen
Laundry & bath
Lights & electronics

Wattages are typical values from manufacturer sizing guides. Your unit's nameplate is the final word — see how we source numbers.

Rating · RecommendedNO. GEN-01
0 W
Select loads to size
Running load0 W
Largest starting surge—
Peak demand0 W
Headroom applied20%

Peak = running total + largest single surge. 240 V equipment (central AC, well pump, dryer) needs a 240 V generator and a transfer switch installed by a licensed electrician (NEC 702).

How this sizing works

Generator sizing fails in one specific way: people add up running watts and ignore motor starts. A compressor or pump pulls two to four times its running load for the first moment it spins up. So the number that matters is everything already running, plus the single biggest surge on top of it. That's the peak your generator has to survive, and it's the calculation this tool performs — the same method Generac, Honda and Champion use in their own sizing worksheets.

We then add 20% headroom. Generators run best loaded at 50–75% of rating: fuel burn stays sane, the engine isn't screaming, and you keep margin for the load you forgot about.

Which generator class you land in

Generator classes by peak demand
Peak demandClassTypically covers
≤ 2,000 WSmall inverterFridge OR sump + lights, phones, wifi
2,000–4,500 WMid inverter / small portableFridge, sump, furnace fan, lights, TV
4,500–8,500 WLarge portableEssentials + well pump or window AC
8,500–13,000 WXL portable / small standbyMost of the house incl. small central AC (soft-start helps)
13,000 W +Whole-home standbyEverything, automatic transfer
Safety line Never backfeed a generator through a dryer or wall outlet. Connecting a generator to house wiring requires a transfer switch or interlock installed by a licensed electrician — it's an NEC requirement and it protects line workers.

Frequently asked

What size generator do I need for a refrigerator and freezer?

A refrigerator typically runs at about 700 W but surges to roughly 2,200 W when the compressor starts. A 2,500–3,000 W generator handles a fridge plus lights and phone charging with headroom to spare.

Why do starting watts matter more than running watts?

Motors (compressors, pumps, blowers) draw 2–4x their running wattage for the first second at startup. Your generator must absorb the single largest surge on top of everything else already running — that peak, not the running total, is what trips an undersized unit.

Can a portable generator run central air conditioning?

Usually only large ones. A 3-ton central AC can surge to roughly 9,000 W at start, so you generally need a 10,000+ W portable or a standby unit — plus a 240 V connection through a transfer switch. A soft-start kit can cut the surge dramatically and let a smaller generator work; an HVAC tech can confirm your unit’s LRA.

Is it bad to oversize a generator?

Mildly. Oversizing wastes fuel and money, and running a big generator at very low load for long periods can cause carbon buildup in some engines. Aim for your typical load to sit around 50–75% of rated output.

Where do I find the exact surge watts for my appliance?

On the nameplate. Motors list LRA (locked-rotor amps): multiply LRA by voltage to get starting watts. For a fridge or AC, the plate is usually inside the door frame or on the outdoor condenser unit.

Sources