How Many Watts Do I Need a Generator For: A Simple Method

The simple method in three steps

Sizing a generator comes down to one question: what is the heaviest load it must carry at the moment the hardest-starting appliance kicks on? Power needs have two parts. Running watts are what a device draws in normal operation. Starting watts are the extra surge a motor draws for a second or two as it spins up, and that surge is usually several times the running figure. Your generator has to cover both.

The method takes three steps. First, list the appliances you will run together. Second, record the running and starting watts for each one from the nameplate or the maker's own specs. Third, total the running watts, add the single largest starting surge, and match the result to a generator class. The difference between running and starting watts is a useful primer before you start.

Step 1: List what must run

Divide everything you might plug in into must-run and nice-to-run. Must-run loads are the refrigerator, a well pump if you have one, heating or cooling, lights, and any medical device. Nice-to-run loads are entertainment, small kitchen appliances, or a phone charger. You cannot run everything at once on a portable generator, so you need to decide which circuits matter most when the grid goes down.

Write down the nameplate wattage for each item. The nameplate is the metal or plastic sticker on the back or bottom of the appliance. It lists volts and amps, and sometimes watts. If watts are not shown, multiply volts by amps to get running watts. For example, a device rated at 120 volts and 5 amps draws 600 running watts. You can also look up the specification on the maker's official page.

Step 2: Note the starting surge for motor-driven items

Motors draw extra power when they start. A refrigerator, sump pump, well pump, furnace blower, and air conditioner all have a starting surge. The nameplate sometimes lists the locked-rotor amps or a separate starting wattage. If not, a common rule of thumb is to use three times the running watts for a rough surge estimate, but the exact value comes from the manufacturer. When you have the surge for each motor-driven item, record it.

For everything else, like lights and resistive heaters, the starting watts equal the running watts because there is no motor. You only add the one largest surge, not all of them, because not every motor will start at the same instant. If you assume the worst case, you will end up with a generator that is larger than you actually need.

Step 3: Add the largest surge to the total running watts

Add all the running watts together. Then add the largest starting surge from the items you listed. That number is the minimum power your generator must deliver, in watts, at the moment of heaviest demand. For a little safety margin, you can add 10% on top. This method works for both conventional and inverter generators.

Once you have that figure, match it to a size class. A household with a few essentials often lands around 2,000 to 3,500 watts. A whole house with a well pump, furnace, and refrigerator can reach 5,000 watts or more. If you need to power a central air conditioner, a larger standby or high-output portable unit is usually required. You can compare the best portable generators or narrow down to under 3000 watts if that fits your load.

Match the total to a generator class

The result tells you the class of generator to look for. If your total is around 2,000 to 3,000 watts, a small inverter generator is quiet and fuel efficient. In the 3,000 to 5,000 watt range, you get enough power for a refrigerator, a few lights, and a window air conditioner. For more demanding loads, see the best 3000 to 5000 watt generators or best 5001 to 9000 watt generators.

Remember that the generator’s rated output is the continuous running watts, and its maximum output is the starting watts it can handle for a short period. Do not run a generator at its absolute maximum for more than half an hour. Overloading a generator can damage both the generator and the connected equipment. For a deeper look at how watts are measured, read what does generator watts mean.

Placement and carbon monoxide safety

Once you have sized your generator, place it correctly. The U.S. Consumer Product Safety Commission says to never operate a portable generator indoors or in garages, basements, or sheds. Always run it outside, well away from windows, doors, and vents. The CDC recommends at least 20 feet of distance from the house, with the exhaust facing away. This is not optional: generator exhaust contains carbon monoxide, an odorless gas that can be deadly in minutes.

Install battery-operated or battery-backup CO alarms on every level of your home and outside sleeping areas. The symptoms of CO poisoning include headache, dizziness, weakness, nausea, and confusion. If you suspect exposure, get to fresh air and call emergency services. For more, see the generator safety checklist and the carbon monoxide safety guide.

What to pick for your use

Questions

What is the difference between running watts and starting watts?

Running watts, also called rated watts, are the continuous power an appliance needs to operate. Starting watts are the extra power a motor draws for a few seconds when it turns on. That surge is often two to three times the running watts, so you need to account for it in your total.

Do I add starting watts for every appliance?

No. You add all the running watts, then add the starting surge only for the appliance with the highest surge. That is because not every motor starts at the same moment. Adding all surges would overestimate the demand and make the generator larger than necessary.

How do I find the wattage of an appliance?

Look at the nameplate on the appliance, which usually lists volts and amps. Multiply volts by amps to get running watts. If the nameplate also gives a locked-rotor value or a starting wattage, use that for the surge. The maker’s official specification page is another reliable source.

What generator size do I need for a refrigerator and lights?

A typical refrigerator draws about 600 to 800 running watts, and a surge of 1,200 to 2,000 watts. Adding a few lights and a phone charger brings the total to around 1,500 to 2,500 watts. A 2,000 to 3,000 watt inverter generator is often enough for that load.

Can I run my house entirely on a portable generator?

It depends on the load. If you have electric heat, a large central air conditioner, or an electric water heater, you likely need a high-output portable or a standby generator. For a modest set of essentials, a portable generator in the 3,000 to 5,000 watt class can work with a transfer switch.

Is it safe to run a generator in my garage with the door open?

No. Carbon monoxide can build up even with the door open. The CDC recommends using generators outdoors, more than 20 feet from windows, doors, and vents. Never operate a generator inside a home, garage, basement, or shed.