Generator Size Calculator

What size generator do I need?

Generator sizing is two sums, not one. The first is the running load: everything that will be drawing power at the same time, added up in watts. The second is the startup peak: that running load plus the biggest inrush any single motor pulls when it kicks in. Compressors and pumps briefly demand two to three times their running figure, and a generator that cannot supply that spike will sag, trip its breaker or stall — even though the running numbers looked fine on paper.

This calculator uses nameplate-typical values for the appliances most households actually back up, adds your running total, then adds only the largest single surge. That last decision is the whole argument, and it is worth understanding before you spend money.

Worked example: the classic outage kit

Fridge (700 W), sump pump (800 W), furnace blower (800 W), a lighting circuit (300 W) and the TV, router and phone chargers (500 W). Running load: 3,100 W. The largest surge belongs to the fridge and the furnace blower, both adding about 1,500 W over their running draw, so the startup peak is 3,100 + 1,500 = 4,600 W. Add 10% headroom and you need 5,060 W, which lands on a 5,500 W portable — the size that shows up in every "what size generator do I need" thread for a reason. Headroom comes out at about 20%, enough to absorb a cold-start compressor without running the engine flat out.

Worked example: add a well pump and a window air conditioner

Same list, plus a 1/2 HP well pump (1,000 W running, 3,000 W start) and a window AC (1,200 W running, 3,600 W start). Running load rises to 5,300 W, and the biggest single surge is now the air conditioner at 2,400 W above its running figure. Peak: 7,700 W. With headroom that is 8,470 W, so a 9,500 W portable. Notice what happened — two appliances worth 2,200 running watts moved you up two whole size classes, because they brought their surges with them.

Worked example: central air pushes you into standby territory

A 3 ton central air conditioner runs at roughly 3,500 W and starts at 8,000 W. Pair it with just a fridge and lights and the running load is 4,500 W, but the peak is 9,000 W and the recommendation jumps to 12,000 W. That is the crossover where portables stop being practical and a permanently installed standby unit — or a soft-start kit on the compressor — becomes the cheaper answer.

Why not size for everything starting at once?

Because it would cost you roughly double for a scenario that does not happen. In the second example above, if the fridge, furnace, sump and AC all inrushed in the same instant you would need over 12,000 W instead of 9,500 W. In reality thermostats and float switches fire independently, and on a portable you control the order by hand: start the generator, let it stabilise, then switch on loads one at a time, biggest motor first. Home standby installations do the same thing automatically with load management modules that shed the air conditioner while the well pump starts.

Fuel burn is the hidden cost of oversizing

An engine sized far above your load still burns fuel to spin at 3,600 rpm. A conventional 7.5 kW open-frame unit running at a quarter load can drink 0.6 to 0.8 gallons an hour, which is a 5 gallon tank every seven or eight hours — plus a midnight refuelling trip in the rain. Inverter generators throttle the engine to match demand and can halve that at light load, which matters a lot in a three-day outage. Buy for your peak, not for a fantasy peak, and consider running a small inverter overnight for just the fridge and freezer.

Transfer switches: the safety part you cannot skip

Never plug a generator into a wall socket with a double-ended cord. Backfeeding energises the utility line outside your house, can kill a lineworker restoring power, and destroys the generator the moment the grid comes back. The legal options are extension cords straight to appliances, an interlock kit on your panel, or a manual transfer switch wired by an electrician. Run the generator outdoors, at least 20 feet from windows and doors with the exhaust pointed away — carbon monoxide from generators kills people every storm season. Bond-neutral requirements differ between a portable and a transfer switch installation, so ask the electrician which configuration your unit needs.

Limitations of these numbers

The wattages here are typical nameplate figures, not your appliances. Check the label on the back of each unit; a 1 HP well pump doubles the pump figures used here, and older fridges surge harder than new inverter-compressor models. Engine output derates with altitude (roughly 3% per 1,000 feet) and propane delivers around 10% less power than gasoline. Soft-start kits can cut an air conditioner's inrush by 60 to 70%, which sometimes keeps you on a portable instead of a standby. Treat the result as the size to shop for, and confirm anything load-critical with an electrician before you wire it into the house.