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Generator Size Calculator

Pick the appliances you want to run in a power outage and see the running watts, starting watts and the generator size you need, with a 20% safety margin.

How to use the Generator Size Calculator

  1. Tick every appliance you want to run at the same time during an outage.
  2. Add anything else with its running and starting watts from the label.
  3. Read the running watts, the peak watts and the suggested generator size.

How the generator size is calculated

  1. Add the running watts of everything you will use at once.
  2. Find the biggest starting surge. Motors such as a refrigerator compressor or furnace blower need extra power for a moment. Motors rarely all start at the same instant, so the calculator adds only the single largest surge on top of the running total.
  3. Add headroom. A generator that runs at its limit all night wears out and may trip. The suggestion adds about 20%.

The wattage figures in the list are typical values from common generator charts. Your own label is more accurate, so use the "something else" boxes if yours differs.

Stay safe with a generator

  • Run it outdoors only, never in a garage, basement or near a window, even with the door open.
  • Install battery-powered carbon monoxide alarms in the house. See what to do in a power outage.
  • Never plug a generator into a wall outlet. That "backfeeds" the power lines and can hurt utility workers. Use a transfer switch fitted by an electrician, or plug appliances in with heavy-duty extension cords.
  • Let it cool before you refuel, and store fuel in approved containers away from the house.

Protect the food while you wait

A refrigerator holds food safely for about four hours without power if the door stays shut, and a full freezer for about 48 hours. If a garage freezer is on your list, read about why a garage fridge or freezer fails in winter. Keeping a space heater on a generator is rarely practical, so see what a plug-in heater costs to run before you plan on one.

Frequently asked questions

What size generator do I need to run a refrigerator and furnace?

A typical refrigerator needs about 700 running watts and a furnace blower about 800, with starting surges above 2,000 watts each. Together with a few lights and chargers, a 3,000 to 3,500 watt generator is a common starting point. Check the labels on your own appliances.

What are running watts and starting watts?

Running watts are what an appliance uses while it works. Starting watts are the short surge a motor needs to get going, often two to three times the running figure. A generator must cover both.

Can I run a central air conditioner on a portable generator?

Usually not. Central air conditioners, electric dryers, ranges and electric water heaters need much more power than most portable units give. A licensed electrician can advise on a larger standby generator and a transfer switch.

Where should I put a generator?

Outdoors, well away from the house, doors, windows and vents. The Consumer Product Safety Commission advises at least 20 feet. Generators make carbon monoxide, which you cannot see or smell.

Learn more