What Size Generator Do I Need? A Watts Method That Works for Any Job

The right generator size is the total running watts of everything you need at the same time, plus the largest start-up surge of any one motor, plus a margin on top. On my farm the pump and the freezers set that number, never the big figure printed on the box. Read the nameplate of each load, add the numbers, and shop for a unit that covers the total. Below you will find the method, a worked example, the loads that push the size up and the safety rules that stay the same at every size.

What Size Generator Do I Need? The Short Answer

Start with three numbers. Your running watts are the sum of every load that runs together. Your peak is that sum plus the largest single start-up surge.

Two generator ratings must cover those numbers. The running rating must cover your running watts plus a margin, and the starting rating must cover your peak plus the same margin. I set the margin at about 20 percent.

Generator makers group their units into rough bands. According to Champion, the bands below match these jobs.

Treat a band as a starting point only. A 3,000 watt unit and a 7,000 watt unit both sit in the middle band, and your own loads decide which one you need.

Running Watts and Starting Watts in Plain Words

Most sizing mistakes start with these two numbers. Learn the difference and the rest of the method is addition.

Running Watts

Running watts are what a load draws for as long as it runs. The nameplate lists volts and amps, and volts times amps equals watts. A 120 volt load that pulls 5 amps uses about 600 watts.

Starting Watts

A motor pulls extra current for a moment when it starts, and that surge is the starting wattage. Champion says starting watts are typically 1 to 2 times the running wattage, while Generac says two to three times for many motors, so the nameplate or the manual settles it. I explain how the two ratings affect generator sizing in my breakdown of starting watts and running watts.

Refrigerator compressors, sump pumps and air conditioners surge. Lights, a router and a charger do not. I keep a photo of each nameplate on my phone, so I can add the numbers without crawling behind the freezer. My guide to a generator running a refrigerator puts real watts on that load.

How to Add Up Your Loads Step by Step

The method takes ten minutes and a pen. Do it for the loads you will really run during an outage, not for every outlet in the house.

1. List what must run at the same time, most important first.
2. Write down the running watts of each from its nameplate or manual, using volts times amps when only amps are listed.
3. Add the running watts into one total.
4. Find the largest starting wattage among the motors on the list.
5. Add that one surge figure to the running total. The result is your peak.
6. Add a margin of about 20 percent to both numbers.
7. Check the plug and voltage of every load, because a 240 volt load needs a unit with a 240 volt outlet.

A Worked Example

Here is a set of farm-yard loads, using the high end of Champion’s ranges so the answer errs on the safe side.

The running total is 2,200 watts. The pump has the largest surge at 2,150 watts, so the peak is 2,200 plus 2,150, or 4,350 watts. A 20 percent margin lifts those to about 2,640 running watts and about 5,220 starting watts. Shop for a unit rated for at least about 2,700 running watts and about 5,200 starting watts. For the full answer to how many watts does it take to run a house, see my guide on whole-house loads.

This method is cautious on purpose. It counts the pump’s running watts twice, once in the total and once inside its surge, and that cushion is cheap insurance against a stalled motor. I start the biggest motor first and add the small loads one at a time. A fridge or pump will restart on its own later, so the starting rating has to stay in reserve while the other loads run. The surge of that kind of load is in my write-up on a generator running a sump pump.

Which Loads Push the Size Up

Motors and heating elements decide the size, and some of them need more than the plug alone shows.

Motors and Compressors

A refrigerator, a freezer, a sump pump and an air conditioner all run on a motor or compressor, so each has a start-up surge. Champion lists a half horsepower sump pump at 800 to 1,050 running watts and 1,300 to 2,150 starting watts. A fridge compressor also cycles on and off, so over a day a refrigerator averages far less than its running figure.

Central Air and Well Pumps

These are the heavy loads. Champion puts a 3 ton central air conditioner at 3,000 to 3,500 running watts and 5,000 to 6,000 starting watts, while Generac says the same size can need up to 15 to 18 kilowatts at start-up. Makers disagree, so the unit’s own nameplate is the only number to trust. I answer can a generator run central air conditioning in a separate guide, with the nameplate numbers to check.

A deep-well pump often runs on 240 volts, and a generator with only 120 volt outlets cannot start it.

On my farm the irrigation pump is the load I size around, so I read its plate before I look at any generator. I answer can a generator run a well pump in a separate guide, starting with the pump’s nameplate.

Heaters and Other Plain Loads

A space heater draws 750 to 1,500 watts and a microwave 600 to 1,200, according to Champion. They have no motor surge, but they add straight to the running total and fill a small unit fast. Leave them off the list unless you must run them.

What Size Portable Generator Do I Need for Common Jobs?

Jobs fall into three groups, and each points to a different size.

• Camping and light use: A small unit of up to about 2,000 watts suits a fridge, some lights and a phone charger. Add up the plates before you add anything more. If you camp with it, my picks of generators for camping stay light and quiet enough for a tent site.
• Workshop and farm tools: A midsize unit of 2,000 to about 3,500 watts suits 120 volt loads, such as a fridge plus a window air conditioner or a space heater, if their running and starting watts fit. A 240 volt well pump or central air needs a 240 volt outlet and its own nameplate check.
• Home essentials during an outage: Champion places this in the 3,000 to 7,000 watt band. A large portable of up to about 8,500 watts can run hardwired gear, such as a well pump, through a transfer switch, and it burns plenty of gasoline.

A portable power station suits light loads such as a router and lights. It makes no exhaust, but it stores a fixed amount of energy and stops when that is used up, so it is no substitute for a fuel generator at the same size. For my yard the workshop chargers and lights come first and the freezers second, which keeps the load list short.

How Fuel Type Changes the Number

A dual-fuel generator lists two outputs. According to Champion, propane gives about 10 percent less wattage than gasoline, and natural gas about 10 to 15 percent less. One Champion dual-fuel model lists 3,500 running watts on gasoline and 3,150 on propane, which is that 10 percent drop in practice. Size on the lower figure for the fuel you will really have.

I size on the propane figure, because propane is the fuel I can store for a storm. For the full answer to how much propane does a generator use, see my guide on cylinder burn rates.

Why a Whole House Is a Different Sizing Job

Square footage does not set the size. Generac says to match your peak electrical load rather than your floor area, and it advises a professional load analysis for a standby unit. Feeding a house from a portable generator needs a transfer switch or an interlock installed by a licensed electrician; I explain what is a generator transfer switch in a separate guide.

Safety Rules That Do Not Change With Size

A bigger generator is not a safer one. Every rule below holds at 2,000 watts and at 12,000.

Carbon Monoxide

According to the CPSC, carbon monoxide can kill in minutes, and a portable generator belongs outside, at least 20 feet from the house. The CDC adds that the 20 feet applies to doors, windows and vents too. Never run it in a garage, carport or any enclosed or partly enclosed space, and keep the exhaust pointed away from the home and other buildings. A carbon monoxide shutoff on the unit is a backup and never a reason to run it closer. I stand the unit on the gravel pad in the farm yard, exhaust pointed away from the house and the sheds. My answer to how far should a generator be from the house covers where I set the unit and why.

The studies agree on placement. After a Maine ice storm, the strongest risk factor for poisoning was a generator in a basement or an attached structure such as a garage (1). In Kentucky in 2009, ten deaths were attributed to carbon monoxide poisoning and eight of them were related to inappropriate generator location (2). After four hurricanes in Florida in 2004, misplaced generators, indoors, in garages or outdoors near windows, caused nearly all of the exposures in a CDC analysis (3).

Garage tests show how fast it goes wrong. In NIST tests with a generator in an attached garage, peak house carbon monoxide ranged across seven door and ventilation settings from under 10 parts per million with a low-emission prototype generator to over 10,000 with an unmodified one (4). Alarms matter as well: after a North Carolina ice storm, 96.2 percent of the severe poisonings happened in homes with no reported working carbon monoxide alarm (5).

Fuel, Fire and Wet Weather

Shut the engine off and let it cool before you refuel, as OSHA advises, because fuel spilled on hot parts can ignite. Store fuel in approved containers outside the home. I refuel outdoors on a cool engine and wipe up any spill before I start it. I check the oil on level ground before every run.

Keep the generator dry. OSHA says not to use one in rain or wet conditions, and a canopy is the answer only where the manual allows one and keeps the exhaust open. Never touch it with wet hands or while standing in water. I cover safe setup considerations in my guide on running a generator in the rain.

Connecting It Safely

Never plug a generator into a wall outlet. OSHA warns that power can flow back through the house wiring to the utility lines and energize them, and that is how line workers and neighbors get hurt. For the workshop I run one heavy, grounded outdoor cord from the generator and lay it where nobody trips on it. Take the cord gauge and length from the manual’s chart.

Sizing Mistakes I See Most

The same few errors cost people money or a tripped breaker in the middle of an outage.

• Buying on starting watts: The big number on the box is the short peak. Compare running watts first.
• Adding running watts only: Motors then trip the unit at start-up. I once added only the running watts for a pump, and the unit tripped on start-up.
• Ignoring voltage: A 240 volt pump or air conditioner will not start on a 120 volt only unit.
• Oversizing: A far larger unit costs more to buy and, as Generac puts it, more every time it runs.
• Trusting a table over the plate: Makers disagree on air conditioner surge, so the nameplate wins.

Write Down These Three Numbers Before You Shop

Write down three numbers: your running total, your peak with the largest surge, and the voltage of your biggest motor. Add a margin, check every plate, and keep the unit outdoors, far from the house, every time it runs. Match both ratings, running and starting, to the fuel you will burn, since propane gives less than gasoline. Confirm that the outlets fit your plugs, and leave any house connection to a licensed electrician. Before storm season I start mine once with a light load, so the first run of the year is not the one I need in an outage. Get those three numbers right and the right generator size follows from simple addition. Once you have your load list, I compare nine options in my roundup of the top generators for home backup.

Frequently Asked Questions

How big of a generator do you need to power a 2000 sq ft house?

The size of a generator for a 2000 sq ft house depends on the loads you run, not the floor area. Generac says to match your peak electrical load rather than your square footage, and a standby unit needs a professional load analysis. A portable unit feeds a house only through a transfer switch or interlock installed by a licensed electrician.

What will a 3000 watt generator run?

A 3000 watt generator runs the loads whose combined running watts fit its running rating, while the running total plus the largest surge also fits its starting rating. Using the top of Champion’s ranges, a refrigerator at 400 watts, a freezer at 500, ten LED bulbs at 150 and a router and chargers at 100 total 1,150 running watts, which fits with room to spare when 3,000 is the running figure. Check first whether the 3,000 is the model’s running or its starting number.

What will a 7500 watt generator run in a house?

A 7500 watt generator runs the loads whose running watts add up to its rated output, so first check whether the 7,500 is the model’s running or its starting output. Champion places 3,000 to 7,000 watts in the home essentials band: refrigerator, freezer, window air conditioner and TV. Central air or a well pump needs a check against its nameplate and a 240 volt outlet.

What will a 12000 watt generator run in a house?

A 12000 watt generator sits in Champion’s 7,000 to 12,500 watt band, which it lists for central air, an electric water heater, a well pump and a sump pump. The loads still add up, so total the running watts and the largest surge as in the worked example. Connect it to a house only through a transfer switch or interlock installed by a licensed electrician.

What size generator do I need to run a refrigerator?

The generator size for a refrigerator comes from its nameplate, and Champion lists 150 to 400 running watts and 800 to 1,200 starting watts. A unit of about 2,000 watts covers one fridge with room for lights and a router, if its starting rating is above the fridge’s surge. Add a freezer or a pump and run the full method.

References

1. Daley WR, Smith A, Paz-Argandona E, Malilay J, McGeehin M. An outbreak of carbon monoxide poisoning after a major ice storm in Maine. J Emerg Med. 2000;18(1):87-93.
2. Lutterloh EC, Iqbal S, Clower JH, et al. Carbon monoxide poisoning after an ice storm in Kentucky, 2009. Public Health Rep. 2011;126(Suppl 1):108-115.
3. Centers for Disease Control and Prevention. Carbon monoxide poisoning from hurricane-associated use of portable generators – Florida, 2004. JAMA. 2005;294(12):1482.
4. Emmerich SJ, Wang L, Persily AK. Measured carbon monoxide concentrations from stock and reduced-emission prototype portable generators operated in an attached garage. J Air Waste Manag Assoc. 2017;67(8):889-898.
5. Centers for Disease Control and Prevention. Use of carbon monoxide alarms to prevent poisonings during a power outage – North Carolina, December 2002. JAMA. 2004;291(14):1691.

Safety Note: Generator exhaust contains carbon monoxide, which can kill in minutes. Run a fuel generator outdoors only, never in a home, garage, basement, shed or other enclosed or partly enclosed space, even with doors or windows open. The CPSC says to place it at least 20 feet from your home and other buildings, with the exhaust pointed away from doors, windows and vents. Install battery-operated or battery-backup carbon monoxide alarms on every level of the home and outside sleeping areas. Never plug a generator into a wall outlet: connect it to a house only through a transfer switch or interlock installed by a licensed electrician. Refuel only with the engine off and cool, and read the manufacturer’s manual, including for a portable power station.