What size generator do I need to run a house?

A portable generator can run most of a house, but rarely all of it at once. The trick is to decide what you actually need during an outage, then size for that. Here's how, from a small list to central air.

Size by what you run, not by square feet

A 1,500 sq ft house and a 3,000 sq ft house use about the same fridge, lights and Wi-Fi. What changes the answer is the big stuff:

• Central air or a heat pump. This is the biggest load most homes have.
• A well pump. Big start-up surge, and usually 240 V.
• Electric heat, an electric water heater, dryer or range. Each can use 4,000 W or more.

Bigger homes tend to have bigger AC units, and that's the main way square footage matters. So look at what's in your home first.

Three common home setups

These use the figures in our calculator, from Champion's and Honda's charts. The method: add up running watts, then add the one biggest start-up surge, then add 20% headroom.

Setup	Running W	Starting W	With 20% headroom
Essentials: fridge, 1/2 HP sump pump, furnace fan, lights, Wi-Fi, chargers, TV	2,445	3,595	about 2,950 / 4,350
Essentials + 8,000 BTU window AC	3,195	4,445	about 3,850 / 5,350
Essentials + 1/2 HP well pump (240 V)	3,445	5,445	about 4,150 / 6,550
Essentials + 3-ton central AC (240 V)	5,945	8,445	about 7,150 / 10,150

The central AC row uses the top of Champion's range for a 3-ton unit: 3,500 running and 6,000 starting watts. The well pump row uses 3,000 starting watts, the motor maker Franklin Electric's minimum generator size for a 1/2 HP 2-wire pump.

LCEC, a Florida power co-op, says essentials in a typical home "average 5,000–7,500 watts". That's in line with the middle rows once you add a few more things, like a microwave or a second fridge.

What size generator for a 1,000, 1,500, 2,000 or 3,000 sq ft house?

People search for this a lot, so here's a rough guide. Treat it as a starting point, and check what's actually in your home.

Home	Often has	Plan on
1,000–1,500 sq ft, gas heat, no central AC	Essentials, maybe a window AC	3,500–5,000 W
1,500–2,000 sq ft, gas furnace, well water	Essentials + well pump	5,000–7,500 W, with 240 V
About 2,000 sq ft with 2–3 ton central AC	Essentials + central AC	7,500–10,000 W, with 240 V
2,500–3,000+ sq ft with 4-ton AC or well pump	Essentials + 4-ton AC + well pump	10,000 W or more, or a soft start on the AC

For the last row, the math is 2,445 (essentials) + 1,000 (well pump) + 5,000 (4-ton AC, the top of Champion's 4,000–5,000 W range) = 8,445 running watts. The AC's surge adds another 3,000, so about 11,445 starting. With headroom, that's about 10,150 running and 13,750 starting. That's the top of the portable range.

Worked example: a 1,500 sq ft home with a window AC

The calculator above is set up for this one:

• Running: 400 (fridge) + 1,000 (sump pump) + 700 (furnace fan) + 100 (lights) + 15 (Wi-Fi) + 80 (chargers) + 150 (TV) + 750 (window AC) = 3,195 W
• Biggest surge: the window AC, 2,000 − 750 = 1,250 W
• Starting: 3,195 + 1,250 = 4,445 W
• With 20% headroom: about 3,850 running and 5,350 starting watts

So a generator of about 4,000 running watts or more fits, like a 5,000 watt class generator. Change the list above to match your home.

Can a portable generator run a whole house?

Sometimes, but it's the exception. A whole house with central air, an electric range, dryer and water heater can need 20,000 W or more if they all run at once. Champion lists an electric dryer at 4,000–6,000 W and a water heater at 3,000–4,500 W on their own.

Most people take one of three routes:

1. Pick your circuits. Run the essentials and one comfort item. Skip the dryer and the oven during the outage.
2. Take turns. Run the water heater for an hour, then switch it off before the AC comes on.
3. Tame the AC's start. A soft start kit cuts the AC's start-up surge. Micro-Air says its home soft starter works on ACs and heat pumps up to 6 tons. See will a generator run central air.

If you want everything on with no juggling, that's a home standby generator, which an installer sizes for you.

Your panel size isn't the answer either

A 200 amp panel can carry up to 48,000 watts (200 A × 240 V). That's what the wiring can handle, not what you use. Size to your running load. More in what size generator for 200 amp service.

How the generator connects to the house

To run hard-wired things like a furnace, well pump or central AC, the generator needs:

• A 240 V outlet. An L14-30 outlet carries up to 7,200 W; a 14-50 carries up to 12,000 W.
• A transfer switch or interlock, installed by a licensed electrician. See interlock vs transfer switch and how to connect a generator to a house.

Never plug a generator into a wall outlet or dryer outlet with a "double male" cord. Generac warns this backfeeding can send power back down the utility lines and kill line workers.

Check your own home

• Fridge, freezer, pumps: look at the label. Amps × 120 = watts.
• Central AC: the outdoor unit's label lists its size in BTU or tons (12,000 BTU = 1 ton).
• Well pump: the control box or the driller's paperwork lists HP.

Then add them up in the calculator above.

Safety: Run it outdoors only, 20+ feet from the house, away from windows and doors. Read our carbon monoxide guide.