Generator Size Calculator

Please fill in the required fields with values greater than 0.
–running watts
–starting watts needed
–recommended generator (watts)
–recommended generator (kW)

Typical running and starting watts are used. Check the labels on your appliances and have an electrician connect the generator safely.

How to use this calculator

Enter how many of each item you want to power during an outage. Press Calculate to see the running watts, the extra power needed when motors start and the recommended generator size.

The formula

Running watts = sum of the running watts of all items
Starting watts = running watts + the largest extra start-up surge
Recommended size = starting watts × 1.2, rounded up to the next 500 W

Example: One refrigerator, one window air conditioner, one TV and 10 LED bulbs need 2,150 running watts. The air conditioner adds a 2,400 W surge, so you need 4,550 starting watts and a generator of about 5,500 W.

Choosing the right generator size

The calculator gives three numbers. Running watts is the steady load once everything is on. Starting watts adds the largest motor surge, because a fridge or air conditioner draws extra power for a few seconds as it starts. The recommended size adds a 20% margin and rounds up to the next 500 watts, so the generator is not running flat out.

A worked example

A refrigerator, a window air conditioner, a TV and 10 LED bulbs total about 2,150 running watts. The air conditioner’s start-up surge brings the peak to 4,550 watts. Adding 20% gives about 5,460 watts, so the suggestion is a generator of about 5,500 watts, or 5.5 kW.

What changes the size you need

  • Motor loads. Pumps, compressors and air conditioners have the biggest surges. One large motor can decide the generator size.
  • What runs at the same time. If you can stagger loads, such as running the sump pump and the AC at different times, you may get by with a smaller unit.
  • Your actual appliances. The calculator uses typical wattages. Check the data plate on your own appliances, which lists watts or volts and amps.
  • Fuel and altitude. Many generators produce less power on natural gas or propane than on gasoline, and output drops at high elevations.

Safety and common mistakes

  • Run the generator outdoors, well away from doors, windows and vents, and install battery carbon monoxide alarms inside the home.
  • Never plug a generator into a wall outlet to backfeed the house. It can energize utility lines and injure line workers.
  • Use heavy-duty outdoor extension cords rated for the load, and keep connections dry.
  • Do not refuel a hot generator. Let it cool first.

Buying and planning

Portable generators cost less and can power essentials through cords. Standby generators connect to your home’s wiring and start on their own, but they cost more and need professional installation and permits. Think about fuel too. A generator running all day uses a lot of fuel, so store it safely in approved containers and rotate it so it stays fresh. Have a licensed electrician handle any connection to your home’s wiring, and check local codes and noise rules before you buy or install.

Frequently asked questions

What size generator do I need for my house?
Add up the running watts of everything you want to power, add the largest start-up surge and add a 20 percent margin.
What are starting watts?
Motors such as refrigerators and air conditioners need extra power for a moment when they start. This is the starting or surge wattage.
Can a portable generator run my whole house?
A whole house usually needs 7,500 to 20,000 watts. Many people power only the essentials with a portable unit.
Can I run a generator inside the house?
Never. Generators produce carbon monoxide. Run them outdoors, far from windows and doors.
What is an inverter generator?
An inverter generator gives cleaner power for electronics and is quieter, but is usually more expensive.
How long can a generator run continuously?
Many portable generators run anywhere from several hours to about half a day on a full tank, depending on tank size and load. Check the run time listed by the maker, which is usually given at a 25% or 50% load. Shut the unit down to refuel and check the oil. Follow the maintenance schedule in your owner’s manual.
What size generator do I need to run central air conditioning?
Central air units have large starting surges, so they often need a large portable unit or a standby generator. The exact size depends on the AC’s tonnage and starting current. A soft-start kit can reduce the surge and may allow a smaller generator. Check the AC’s nameplate and ask a licensed electrician or HVAC pro to size it.
Do I need a transfer switch for a generator?
If you want to power circuits through your home’s wiring, yes. A transfer switch or interlock keeps the house disconnected from the utility while the generator runs, which prevents dangerous backfeed. It also lets you power hardwired items like a furnace or well pump. Installation usually needs a licensed electrician and a permit.