What size generator do I need to run a house?

Updated 2026-10-31 · Sources linked inline

It depends on what "run a house" means. A portable generator won't power everything at once the way the grid does. Sizing by tier works better: pick which loads must run, add their watts and choose the generator that covers the total. Using the Honda and Champion charts, three realistic targets are summarized below.

How this guide was made: this is a research guide built on manufacturer specifications and federal guidance, with sources linked throughout. We haven't bench-tested any of the equipment mentioned, and we don't claim to.

The three targets:

Add 20% headroom if you want a margin (optional): Tier 1 becomes about 2,300 running and 3,500 starting; Tier 2 about 4,600 and 5,800; Tier 3 about 6,400 and 7,600. Rule of thumb from those numbers: about 3,500–4,000 W for essentials, 5,500–6,000 W for comfortable, 8,000 W with an AC. The sections below show every number.

The method

Our generator size calculator uses the same two methods:

• Standard: add running watts of everything that will run at once, plus the single largest start surge (starting minus running watts).
• Conservative worksheet: add running watts plus the largest listed starting figure.

Starting surge matters because a motor draws far more for a moment when it starts than it does running, and you size so the generator can survive that moment. Honda notes you shouldn't simply add every watt together: staggering the starts ("power management") lets a smaller generator do more (Honda).

Where the numbers come from

Honda and Champion publish typical figures. These are approximations, so check the nameplate on each of your own appliances.

Tier 1 worked example: essentials

Running: 192 + 700 + 800 + 100 + 20 + 100 = 1,912 W.

Start extras: fridge 1,200 − 192 = 1,008; furnace fan 1,400 − 700 = 700; sump pump 1,300 − 800 = 500. The largest is the fridge's 1,008 W.

• Standard minimum starting watts: 1,912 + 1,008 = 2,920 W.
• Conservative: 1,912 + 1,400 (the largest start figure) = 3,312 W.
• 20% headroom: about 2,294 W running and 3,504 W starting.

A generator rated for about 3,500 W starting covers both the standard (2,920 W) and conservative (3,312 W) figures. If you want the 20% headroom (3,504 W starting), look at a 4,000 W class. See also generator for a refrigerator, gas furnace and sump pump sizing.

Tier 2 worked example: comfortable

Add a freezer (500 W running, start extra 500), a microwave (1,200 W running, upper end of Champion's range) and a TV (190 W).

• Running: 1,912 + 500 + 1,200 + 190 = 3,802 W.
• The largest start extra is still the fridge at 1,008 W. Standard minimum: 4,810 W. Conservative: 3,802 + 1,400 = 5,202 W.
• 20% headroom: about 4,562 W running and 5,772 W starting.

A realistic option here is to skip the microwave while the furnace runs and use power management: don't run the microwave and the furnace blower at the same instant. Doing that moves you closer to Tier 1.

Tier 3 worked example: one room of cooling

A 10,000 BTU window air conditioner is 1,500 W running and 2,200 W starting (Honda), so its start extra is 700 W.

• Running: 3,802 + 1,500 = 5,302 W.
• Largest start extra stays at the fridge's 1,008 W, so standard minimum: 6,310 W. Conservative: 5,302 + 2,200 = 7,502 W.
• 20% headroom: about 6,362 W running and 7,572 W starting.

In practice, cooling is the load most people choose to drop. If you must run it, shed others while it runs.

The big 240-volt loads

Some appliances are too big for a portable generator to run alongside everything else. These are published running watts:

Example: Tier 1 plus a 3-ton central AC at the upper figures (3,500 W running, 6,000 W starting, start extra 2,500 W) is running 1,912 + 3,500 = 5,412 W. Standard starting is 5,412 + 2,500 = 7,912 W. Conservative: 5,412 + 6,000 = 11,412 W. That is beyond most portable generators sold for homeowners, which is why most people lock out whole-house cooling, electric water heating and electric dryers during an outage.

If you have a well, add the pump. A 1/2 hp pump at the upper figures adds 1,000 W running and 1,000 W of start extra. It can fill a pressure tank in short runs, so you can often run it before the furnace blower and microwave.

Load shedding: how to use a smaller generator

The point of the tiers is that you decide, ahead of time, what runs together:

1. Always on: fridge, sump pump, router, a few lights, chargers.
2. Cycle: furnace blower (it cycles with the thermostat), freezer (if the fridge and freezer aren't on one cord and the generator is borderline).
3. Occasional: microwave, coffee maker (Honda 600 W), well pump. Run one at a time.
4. Off: dryer, water heater, range, central AC.

Write that list down and tape it to the generator. A cord for each group makes it easy.

Getting power into the house

You have two safe ways to bring generator power inside:

• Extension cords to individual appliances. This is simplest and works for Tier 1 without any electrical work.
• A transfer switch or interlock installed by a licensed electrician, so a generator can power selected circuits through the panel. For example, Generac's portable generator brochure lists 120/240 V, 30 A L14-30R outlets on its larger models, the kind of outlet an electrician pairs with a transfer switch or inlet (Generac). A 30 A, 240 V outlet is limited to 30 × 240 = 7,200 W by arithmetic, regardless of the generator's marketing rating.

Never backfeed by plugging a generator into a wall outlet. CPSC says it is extremely dangerous and can electrocute utility workers and neighbors (CPSC). Ask an electrician about permits, a proper inlet and a circuit list that fits your generator.

Safety and fuel

• Run the generator outdoors, more than 20 feet from any opening, exhaust pointed away. Read generator carbon monoxide safety before you buy a cord.
• Plan fuel: bigger loads burn more per hour, and gas stored in approved containers ages. Check your generator's manual for run times and refueling steps.
• Don't refuel a hot generator.

Generator or power station?

If your essentials list is small and short-term, a battery power station may be quieter and simpler. For multi-day outages, a generator wins on energy. Generator vs power station compares them with real specs.

Summary

Run your own list through the generator size calculator, check your real appliances' nameplates, then use the Alabama outage checklist to finish the plan.