What Size Generator Do I Need for My House?

For most houses, the answer comes from two numbers: the running watts of everything you want on at the same time, and the one extra surge from the biggest motor when it starts. A fridge, a freezer, the furnace blower, lights and Wi-Fi come to about 2,300 running watts and 3,100 watts at the start, which a 3,000- to 4,000-watt generator covers. Add a 240-volt well pump and you need roughly 6,000 to 7,500 watts with a 240-volt outlet. Central air is what pushes a house into standby territory, and for that one you should read the nameplate instead of trusting any chart, including ours.

Below is how to work it out for your own house, with the numbers shown so you can redo them.

The short answer by household

These are our worked examples from further down this page, using the high end of manufacturer appliance ranges. Your numbers will differ; the method won’t.

For the whole-house case, Generac lands in the same place: about 20 or 22 kW for a whole house of roughly 2,000 square feet, and 10 to 18 kW for most homes of about 1,550 square feet, depending on whether the heat and water are gas and city or a heat pump and a well. That’s from Generac’s own sizing guide (August 2026), and Generac sells the bigger units, so treat it as the upper end.

If you’re still deciding between a generator, a battery or your car, start with how to choose backup power for your home. This page assumes you want a generator and need to know how big.

Running watts vs starting watts, and why the charts disagree

Running watts are what an appliance draws while it runs. Starting watts (also called surge or peak) are the short spike when a motor or compressor kicks on. Motors can pull three to six times their running watts for that moment. Lights, TVs, chargers, microwaves and resistance heaters don’t have that spike.

Generators are rated the same way: a running (rated) figure you can hold, and a higher peak figure for a moment. You need both of yours to fit under both of theirs.

The charts online disagree for two reasons.

1. Some list starting watts as the total, some as the extra. Generac’s buyer’s guide lists a fridge at 700 running watts plus 2,200 additional starting watts, so a peak of 2,900 W. Champion’s chart has columns headed “Running Watts” and “Starting Watts” and doesn’t say which one it means; we read its starting figure as the total peak. Mix the two conventions and you either undersize by a whole running load or count it twice.

2. The ranges are wide, and some are simply far apart. Here’s the same equipment from four sources:

Generac doesn’t even agree with itself. Its 2024 buyer’s guide, which defines starting watts as “extra watts needed for 2-3 seconds,” gives a 2-ton (24,000 BTU) central air unit a peak of about 8,750 W (3,800 running plus 4,950 extra) and a 40,000 BTU unit about 12,700 W (6,000 plus 6,700). Both are well below the 15,000 to 18,000 W its 2026 web guide gives a 3-ton unit. The same 2024 table puts a 1/2 hp furnace fan at 800 W plus 2,350 W extra, a 3,150 W peak, against 1,600 W in Champion’s chart.

The fridge spread is annoying but survivable. The air conditioner spread is a factor of three, and that changes which generator you buy. The nameplate settles it; see reading the nameplate below.

One more number that confuses people: the Department of Energy’s rating procedure for efficient new homes assumes a fridge uses about 450 kWh a year, which works out to about 50 watts on average. That’s the right number for working out how long your fuel lasts. It’s the wrong number for sizing, because the generator has to handle the compressor when it’s actually running and starting.

Appliance watts: the table we use

Running and starting watts from Champion’s chart, the only manufacturer chart we found that gives ranges for both running and starting watts, with voltage added. Use the high end unless you have the nameplate. “Starting” is the total peak.

Medical equipment is its own question. For a CPAP or oxygen concentrator, ask your doctor and the equipment supplier what backup they recommend, and register with your utility’s medical program if it has one.

Step by step: list, add running watts, add the biggest start

Short versionAdd up the running watts of everything that will be on at once, then add the extra surge of the single biggest motor (its starting watts minus its running watts).

1. List what has to stay on at the same time. Not everything you own; what you’d actually run together during an outage. Be honest about the microwave and the coffee maker.
2. Write down running and starting watts for each. Use the nameplate where you can, the high end of the table above where you can’t.
3. Add up the running watts. That total has to fit within about 90 percent of the generator’s running rating.
4. Find the biggest extra surge. For each motor, subtract running from starting. Take the largest.
5. Add that extra to the running total. That’s your peak. It has to fit under the generator’s peak rating.
6. Check what happens if two motors start together. Add the second-largest extra too. If that pushes you over the peak rating, either size up or plan to start the big motor first and let it settle before switching on the rest.
7. Check voltage. Any 240-volt load means the generator needs a 240-volt outlet and a hookup that carries 240 volts into your panel.

Why only the biggest start? Motors rarely start at the exact same moment, so Generac, Honda and most sizing guides add only the largest surge. The exception is the moment the generator comes on after an outage, when the fridge, freezer and pump may all want to start at once. That’s why step 6 is there.

120V or 240V: why a well pump or central air changes the answer

Short answerIf you need a 240-volt appliance, wattage alone isn’t enough. The generator has to put out 240 volts, and the connection to your house has to carry it.

Most American homes run on split-phase power: two 120-volt legs that together make 240 volts. Lights, outlets, the fridge and the furnace blower are 120V. The big loads are usually 240V: central air, many well pumps, electric water heaters, electric ranges and dryers. In your panel, a 240V circuit sits on a double-pole breaker (two handles tied together). Your pump’s control box or label will also say.

What it means in practice:

• Small inverter generators are often 120V only. The Honda EU2200i, for example, is a 120-volt, 15-amp unit. It can’t run a 240V well pump at any wattage.
• Many larger portables have a 120/240V outlet. The Generac GP7500E, for example, is rated 7,500 running and 9,375 peak watts at 120/240V. On propane, the dual-fuel version drops to 6,800 running and 8,500 peak.
• 120V loads share the generator between two legs. On a 120/240V portable, each 120V leg carries roughly half the capacity. A 7,500-watt unit isn’t 7,500 watts on one outlet; ask your electrician to split your 120V circuits across both legs.
• The hookup must carry 240V. Extension cords from the generator’s 120V outlets won’t feed a well pump. You need an inlet and a transfer switch or interlock wired for 240V. See 30 amp or 50 amp: matching your generator, cord, inlet and breaker and how to connect a generator to your house. Your electrician and local inspector decide what’s allowed where you live.

Well pumps get their own page: what size generator runs a well pump.

Read the nameplate: LRA, RLA, and how to convert amps to watts

Short answerMultiply amps by volts. The LRA (locked rotor amps) times 240 gives you the worst-case starting surge of a 240V compressor. The RLA (rated load amps) times 240 gives a slightly high, safe estimate of its running draw.

Every central AC condenser, heat pump and many pump motors have a label with these numbers. On an outdoor AC unit it’s usually on the side panel. You’ll see:

• RLA (or RL Amps): what the compressor draws while running at its rated load.
• LRA: what the compressor draws if the motor is energized but not yet turning. That’s the moment of starting.
• MCA and MOCP (or Max Fuse): minimum circuit ampacity and maximum breaker size. Those are for your electrician, not for generator sizing.

Strictly speaking, amps times volts gives volt-amps, not watts. For generator sizing, treat it as the peak number to compare against the generator’s surge rating, and you’ll be on the safe side.

Here is one real manufacturer table, from the installation manual of the Payne PA3Z central air conditioner (208/230V), with our multiplication added:

A second 3-ton unit, the Goodman GSXN4 in the 36,000 BTU size, lists RLA 16 and LRA 91.9 on its spec sheet. So for the two standard 3-ton units above, without help, the starting spike is about 21,000 to 22,000 volt-amps for a moment. Generac’s 15,000 to 18,000 watts is much closer to that than Champion’s 5,000 to 6,000.

Where does a number like Champion’s come from? We don’t know how Champion arrived at it. What we do know: a soft starter ramps the compressor up instead of slamming it on. Micro-Air, which makes soft starters for home and RV air conditioners, lists average reductions of 65 to 70 percent of LRA on its spec sheets. Applied to the Payne 3-ton unit, that’s 88 amps down to about 26 to 31 amps, or roughly 6,300 to 7,400 volt-amps. Inverter-driven (variable-speed) compressors also start gently. So a chart figure of 5,000 to 6,000 watts is plausible for a soft-started or inverter unit and badly low for a standard one.

What to do:

1. Find the LRA on your condenser’s nameplate.
2. Multiply by 240. That’s your no-help peak.
3. If that’s out of reach, ask an HVAC contractor about a soft starter for your specific compressor, and check its maker’s rated reduction for your model.

Batteries publish their surge in amps, which makes this comparison easy. A Tesla Powerwall 3 can start a motor with up to 185 amps LRA, and so can Tesla’s Powershare home backup from a Cybertruck or a newer Model 3 or Model Y (ordered from October 2026; check your trim) paired with a Powerwall 3. A Cybertruck without a Powerwall is rated 110 amps. Both clear the 88 to 92 amps of the 3-ton units above. More on that in home battery or standby generator and the full central air guide, can a generator run your central air.

Three worked examples

Same method each time, with the high end of Champion’s ranges unless the nameplate says otherwise.

1. Essentials on 120V: gas heat, city water

A 3,000-watt inverter such as the Generac GP3500iO (3,000 running, 3,500 peak) carries the running load at about 76 percent and covers one start. If the fridge and furnace fan kick on at the same moment, 3,870 W is over its 3,500 W peak, and it may stall or trip. A 4,000-watt-class unit gives you that margin.

Add a 1,200-watt microwave and the running total is 3,470 W with a 4,270 W peak. That’s beyond a 3,000-watt unit, unless you agree in your house that nobody microwaves while the furnace runs.

Smaller still: a Honda EU2200i (1,800 rated, 2,200 maximum) runs just the fridge, freezer, lights and router: 1,070 running, plus 800 when the fridge starts, makes 1,870 W. If the fridge and freezer start together, that’s 2,370 W, over the 2,200 limit. It works most of the time and fails when both compressors want to start at once, for example right after you restart the generator. Plug in one, wait a minute, then the other.

2. Rural house with a 1 hp well pump (240V)

A Generac GP7500E fits with room to spare on gasoline (7,500 running, 9,375 peak), and the dual-fuel version (GP7500E DF) covers the three-motor worst case even on propane (6,800 running, 8,500 peak). The running load is 57 percent of that propane rating, well inside the 90 percent margin. Both have the 240V outlet the pump needs.

If you read Champion’s starting figures as extra instead of total, the pump alone comes to 7,870 W and the pump-plus-furnace case to 9,470 W, just above the GP7500E’s 9,375 W peak. On a 7,500-watt unit, start the pump first and let it settle, or check the starting figure on your pump’s label.

With a 1/2 hp pump (1,000 running, 2,000 starting), the running total drops to 2,870 W and the peak to 3,870 W. But the pump is often still 240V, which rules out a 120V-only inverter however many watts it has.

Can a power station do this job instead? Sometimes. See can a portable power station run your well pump, furnace or fridge.

3. Whole house: about 2,000 sq ft, 3-ton central air, electric water heater

Here we use the Payne 3-ton nameplate: RLA 17.5 A and LRA 88 A at 240V.

A Generac 22 kW standby is rated 22,000 watts on propane and 21,000 on natural gas. The running load is about 56 to 59 percent of that, which is comfortable. The bare locked-rotor start on top of everything else is not, on paper. That’s the gap a soft starter, an inverter compressor, or load management (a controller that holds back big loads like the water heater while the AC starts) is meant to close. Ask your installer which they plan to use and what motor-starting rating the specific generator has. Two 22 kW quotes can handle this very differently.

Whole-house sizing also decides fuel use: a 22 kW unit at half load burns about 2.5 gallons of propane an hour. See how long a generator runs on each fuel.

Common sizing mistakes

Applying the 90 percent rule to the peak. The common advice is to keep your generator at or below 90 percent of its capacity to avoid overheating and shutdowns. That margin is for running watts, the load it carries for hours. The starting peak lasts a moment and is checked against the peak rating. Treating 90 percent of the peak rating as your running limit overloads the generator; ignoring the peak altogether is how pumps fail to start.

Forgetting the furnace blower. A gas furnace still needs electricity for its blower and controls. In the examples above it’s 800 running watts and the second-largest surge in the house, and Generac’s 2024 table puts its start at 3,150 W, nearly twice Champion’s figure. Leave it off the list and the winter outage plan doesn’t work.

Adding every starting watt together. If you sum all the starting columns, a fridge, freezer, furnace fan and well pump come to 7,800 W of “surge” on top of the running load. Only the biggest extra matters, plus the check for two motors at once. Adding everything buys a generator twice as big as you need.

Mixing “extra” and “total” starting watts. Generac’s 2,200 W for a fridge is extra; Champion’s 1,200 W reads as a total. Check which kind your chart uses before you add.

Using average watts for sizing. A fridge averages about 50 watts over a year. Size for 400 running and 1,200 starting; use the 50-watt average to plan fuel.

Ignoring voltage. A 9,000-watt total doesn’t help a 240V pump if the generator or the hookup only delivers 120V.

Trusting a chart over the nameplate for central air. For a 3-ton unit, the charts disagree by a factor of three. The LRA on your condenser is the number that counts.

Once you know your size, the next step is getting that power into the house safely: how to connect a generator to your house.

Questions people ask

How do I convert LRA to watts?

Multiply the locked rotor amps on the nameplate by the voltage. A 240-volt compressor with an LRA of 88 amps draws about 88 x 240 = 21,120 volt-amps for a moment when it starts.

What size generator runs a fridge, a freezer, the furnace blower and the lights?

With 10 LED bulbs, the modem and router and a TV included, about 2,300 running watts and 3,100 watts at the moment the biggest motor starts, using the high end of Champion's appliance ranges. If two of those motors start at the same moment, the peak climbs to about 3,900 watts.

What size generator do I need for a 1 hp well pump?

A 1 hp pump needs 1,000 to 2,000 running watts and 2,000 to 4,000 starting watts, per Champion. Many well pumps run on 240 volts, so the generator needs a 240-volt outlet as well as enough watts.

How big a generator runs a whole 2,000-square-foot house?

Generac says a home of about 2,000 square feet usually needs a 20 or 22 kW standby generator for the whole house. Central air decides most of that, so check the LRA on your condenser's nameplate.

Should a generator run at 100 percent of its rating?

No. Common advice is to keep the steady load at or below 90 percent of the running-watt rating. That margin applies to running watts, not to the starting peak.