How to size a home generator
The right generator is big enough to carry everything you'll run at once — including the brief power spike when a motor kicks on — without being so oversized that you overpay and waste fuel. The trick is to add up the loads correctly, and that means understanding two different kinds of watts.
Running watts vs starting (surge) watts
Every appliance has two numbers that matter:
- Running watts — the steady power a device draws while it operates. A refrigerator might run on a few hundred watts.
- Starting (surge) watts — the brief spike when a device first turns on.
Anything with an electric motor — a fridge compressor, a well pump, a furnace blower, an air conditioner — needs a jolt of extra power to get the motor spinning, often 2–3 times its running watts for a fraction of a second. A pump that runs on 1,000 watts might surge to 2,500–3,000 watts at startup. A generator that can't supply that surge will stall, trip, or brown out the motor. Resistive loads like lights, a coffee maker, or a space heater don't surge — their starting and running watts are the same.
The sizing method
You don't add up every surge — only the worst one. Motors rarely start at the exact same instant, so the standard approach sizes for everything running plus the single largest startup spike:
- List what you'll run at the same time. Decide which loads matter during an outage — usually the fridge, some lights, a few outlets, and your heating or cooling source.
- Add up the running watts of everything on that list. This is your continuous load.
- Add the extra startup surge of the single biggest motor. Find the device with the largest difference between its starting and running watts, and add only that gap on top of the running total.
- Add about 20% headroom. This keeps the generator off its redline, covers estimation error, and leaves room to add a load later.
- Round up to a standard size. Generators come in set ratings (e.g., 3,500 / 5,000 / 7,500 / 10,000 watts) — pick the next size at or above your number.
Example: 1,800 running watts of essentials, a well pump that surges 1,500 watts above its running
draw, plus 20% headroom → (1800 + 1500) × 1.2 ≈ 3,960 watts → a
5,000-watt generator.
The usual culprits
A handful of loads drive most generator sizing because they combine high running watts with big surges:
- Well pump and sump pump — motor-driven, high surge, and often non-negotiable during a storm.
- Furnace blower — even a gas furnace needs electricity to run its blower and controls.
- Air conditioner / heat pump — large running load and a large compressor surge; central AC often pushes you to a bigger generator.
- Refrigerator and freezer — modest running watts but a real startup surge, and you'll want them on the list to protect food.
Transfer switches and hookup
How the generator connects to your home matters as much as its size. A transfer switch safely connects a generator to your home's circuits and isolates your house from the utility grid so power can't flow backward onto the lines. For a portable generator, you can also simply run heavy-duty extension cords to individual appliances — no panel connection required.
Portable vs inverter vs standby
- Portable. The most watts per dollar. Loud, runs on gas, and needs manual setup and refueling — good for occasional outages.
- Inverter. Quieter, more fuel-efficient, and produces cleaner power that's safer for sensitive electronics. Usually smaller and pricier per watt — good for electronics and quieter operation.
- Standby. Permanently installed, wired to your panel, and starts automatically when the power drops. The most convenient and most expensive, and a professional installation — sized to run much or all of the house.