Generator Size Calculator

Safety: Never run a generator indoors or in a garage because of carbon monoxide. Connect to home circuits only through a transfer switch or interlock installed by a licensed electrician. Never back-feed through an outlet.

How to use this generator size calculator

1. Tick the appliances you want to run at the same time during an outage. Leave out anything you can do without.
2. Check the running and starting watts. If you only see volts and amps on the label, multiply them to get watts.
3. Add the quantity for items you have more than one of, and use the custom rows for anything not listed.
4. Choose how much headroom you want. Read the recommended generator size, plus the kVA and amps.

How do I calculate generator wattage?

1. Add the running watts of every item you will use together.
2. Find the starting watts of each motor-driven item and take the biggest extra surge, which is starting watts minus running watts. Only one motor is assumed to start at a time.
3. Add the two numbers. That is the minimum wattage the generator must deliver.
4. Divide by the load factor, such as 0.8, so the generator is not at its limit. Round up to the next available size.

Example: a refrigerator (700 W running, 2,200 W starting), a furnace blower (800/2,350), a sump pump (800/1,300), lights and chargers (300 W). Running total is 2,600 W. The largest surge is 1,550 W (the furnace blower). Needed: 4,150 W. At 80% load, choose about 5,200 W.

Typical appliance wattages

These are common figures for planning. Real values differ by model, age and size.

Appliance	Running watts	Starting watts
Refrigerator or freezer	700	2,200
Sump pump (1/3 HP)	800	1,300
Furnace blower (1/2 HP)	800	2,350
Well pump (1/2 HP)	1,000	2,100
Window air conditioner (10,000 BTU)	1,200	3,600
Washing machine	1,150	2,250
Microwave (1,000 W)	1,000	1,000
Coffee maker	1,000	1,000
Electric water heater	4,000	4,000
Space heater	1,500	1,500
LED TV	150	150
LED light bulbs (each)	10	10
Laptop, router and phone chargers	200	200

What size generator do I need for a house?

It depends entirely on the loads you choose. As a rough guide, a short list of essentials with no large motors often fits in 3,000 to 5,000 watts, while essentials plus a well pump, a window air conditioner or an electric water heater can need 7,500 watts or more. Running a whole home including central air conditioning is a job for a large standby generator, and the system should be sized by an electrician from a full load calculation.

Rated watts, surge watts, kVA and amps

• Rated (running) watts are what the generator can supply continuously.
• Surge (peak) watts are what it can supply for a few seconds to start motors.
• kVA is volt-amps ÷ 1,000. Divide watts by 0.8 as an estimate, which is what this calculator shows.
• Amps at 120 V or 240 V show what the circuit has to carry: amps = watts ÷ volts.

Which fuel and type?

Portable gasoline generators are the most common and affordable. Propane and dual-fuel units store fuel longer. Inverter generators produce cleaner power for electronics and run quieter, usually at a higher price per watt. Standby generators connect to your home's gas supply and start automatically. All of these are rated in watts, so the sizing works the same way.

Getting the list right

Most sizing mistakes come from the list, not the maths. Adding the starting watts of every appliance oversizes the generator, because motors rarely start at the same instant. Leaving surge out undersizes it, and the generator trips when the fridge kicks in. If two large motors really can start together, such as a well pump and an air conditioner, count both surges. Engines also lose power at high altitude and in extreme heat, so leave more headroom there. For computers and medical devices, an inverter generator gives cleaner power.