What Size Generator Do I Need?

Most homes that work this through honestly land between 3,000 and 7,500 running watts. The 9,500- and 13,000-watt machines that dominate search results are the right answer for houses with central air, electric heat or a large well pump — and the wrong answer, expensively, for everyone else.

Step 1: list what runs at the same time

The phrase doing the work is at the same time. You do not need to power your microwave, your coffee maker, your toaster and your air fryer simultaneously — you need to power them one at a time, so only the largest of them counts. Same with the washing machine and the dryer.

For a realistic outage plan, the simultaneous list is usually short:

Step 2: add the largest surge, not every surge

This is where most DIY sizing goes wrong in the expensive direction. If you add every start-up surge in the table above — 2,200 plus 1,400 plus 1,300 plus 2,000 — you get 6,900 watts of surge on top of 3,500 running watts, and you conclude you need an 11,000-watt generator.

You do not, because those motors do not all start in the same instant. The method published by both charts we cite is to add only the single largest additional surge. Here that is the refrigerator's 2,200 watts.

Step 3: derate for continuous running

A generator's running-watt rating is what it can sustain, but holding any engine at 100% of its rating for three days makes it run hot and wear fast. We divide the running total by 0.8, which leaves 20% headroom.

3,500 divided by 0.8 is 4,375, so round to 4,400 running watts minimum, with at least 5,700 starting watts. That derating is our adjustment rather than the charts', and we state it every time a result appears so you can disregard it if you disagree.

The three loads that change everything

Work the method and you will find your number is modest — unless one of these is on the list:

• Central air conditioning. Large surge, large sustained load. This is the single most common reason a sizing exercise ends above 11,000 watts. Neither chart we cite publishes central AC figures, so read the nameplate on your condenser and see generator size for air conditioner.
• A well pump bigger than 1/3 HP. EWEB's 1/3 HP figure is 1,000 running and 2,000 additional starting. Larger pumps surge much harder — one soft-starter manufacturer states deep well pumps typically draw 65-75 amps to start. Generator size for well pump covers it.
• Electric heat of any kind. No surge at all, but the full rating continuously. EWEB puts an electric water heater at 4,500 watts flat; a space heater at 1,500.

If one of these pushes your number up, check the soft-starter route before you buy a bigger machine. Halving a compressor's or pump's start-up draw can move you down a whole wattage class, and that saving is usually larger than the soft starter costs.

Then size the connection, which can cap everything

A 30 amp transfer switch passes roughly 7,200 watts at 240 V, full stop. Buy a 9,500 running-watt generator and feed it through a 30 amp switch and over 2,000 watts of what you paid for cannot reach the house. If your sizing lands above about 7,500 running watts, budget for a 50 amp transfer switch or an interlock kit at the same time.

Where each of the three common answers lands

Essential circuits, dual fuel, lowest cost

WEN DF8000X 8,000W Dual Fuel

$623.17·View on Amazon

Quiet backup at a sane price

Champion 4,500W RV Ready Inverter

$674.10·View on Amazon10% off

The default whole-house pick

Westinghouse WGen9500DF 12,500W Dual Fuel

$999.00·View on Amazon

Left to right: the 6,500 running-watt dual fuel answer most essential-circuit houses need; the 3,500-watt quiet inverter answer for a house without a well pump; the 9,500-watt answer when a well pump and a window AC are both on the list.

Do it with your own numbers

The load calculator runs this method on your actual list, including a custom-watts row for anything the published charts do not cover, and gives you a wattage class with a current price on the matching pick. It takes about two minutes and it is the only part of this page you really need.