Generator Size Calculator

This free generator size calculator answers "what size generator do I need?" Choose the appliances you want to run during a power outage, on a job site or when camping — fridge, sump pump, well pump, furnace, window AC, microwave and more — and it adds up the running watts and the starting (surge) watts to recommend the right portable, inverter or home standby generator size.

How generator size is calculated

Running watts = sum of running watts of everything on at once
Peak watts = running watts + the largest single starting surge (starting − running)
Recommended generator = running watts × 1.2 (20% margin), and starting watts ≥ peak watts

Only one motor usually starts at a time, so the calculator adds the largest surge rather than all of them. Keeping the generator at around 80% of its rating or less makes it run cooler, quieter and longer.

Worked example

A fridge (700 W, 2,200 W starting), ten LED bulbs (100 W) and a TV with internet (200 W) need 1,000 running watts. The fridge compressor adds 1,500 W when it starts, so the peak is 2,500 W. A generator rated at least 1,200 running watts and 2,500 starting watts covers it — a typical 3,000 W inverter generator.

Generator sizes for common situations

Computed from the typical appliance values above. Use the calculator for your own appliances.

What you want to run	Running W	Peak starting W	Recommended generator
Fridge, lights, TV and internet	1,000	2,500	≥ 1,200 W running / 2,500 W starting
Outage essentials (+ sump pump, furnace blower)	2,925	4,425	≥ 3,600 W running / 4,500 W starting
Well-pump home (fridge, well pump, lights, microwave)	2,800	4,300	≥ 3,400 W running / 4,300 W starting
Summer essentials (fridge, window AC, lights, TV)	2,200	4,600	≥ 2,700 W running / 4,600 W starting
Essentials + water heater + washer	8,075	9,575	≥ 9,700 W running / 9,600 W starting

Typical running and starting watts

Appliance	Running watts	Starting watts
Refrigerator / freezer	700	2,200
Sump pump (1/2 HP)	1,050	2,150
Well pump (1/2 HP)	1,000	2,100
Furnace blower (1/2 HP)	875	2,350
Window AC (10,000 BTU)	1,200	3,600
LED lights (10 bulbs)	100	100
TV + internet router	200	200
Desktop computer + monitor	300	300
Microwave (1,000 W)	1,000	1,000
Coffee maker	1,000	1,000
Space heater	1,500	1,500
Washing machine	1,150	2,250
Electric clothes dryer	5,400	6,750
Electric water heater	4,000	4,000
Electric stove burner (8")	2,100	2,100
Garage door opener (1/2 HP)	875	2,350
CPAP machine	50	50

The appliance label or manual gives exact figures. Motors list running amps; multiply by the voltage (120 V or 240 V) for watts, and allow 2–3 times that for starting.

Related calculations

Prefer a quiet, fuel-free backup? See how long a power station lasts with the battery runtime calculator.
Keeping computers and routers up during short outages? Use the UPS runtime calculator.
See your whole home's daily energy use with the home energy usage calculator.

Frequently asked questions

What size generator do I need to run a house?

For outage essentials — a fridge, sump pump, furnace blower, lights, TV and internet — you need about 2,925 running watts and 4,425 starting watts, so a generator rated around 3,600 running watts covers it with margin. Running a whole house with central air, an electric range and a water heater usually needs a 15–22 kW home standby generator.

What size generator do I need for a refrigerator?

A typical fridge runs at about 700 W but needs around 2,200 W for a moment when its compressor starts. A generator with at least 2,200 starting watts runs it; in practice a 2,000–3,000 W inverter generator handles a fridge plus a few small loads.

What is the difference between running watts and starting watts?

Running (rated) watts are what an appliance draws continuously. Starting (surge) watts are the brief extra power a motor or compressor needs to get going, often 2–3 times its running watts for a second or two. A generator must cover all running watts plus the largest starting surge, because motors rarely start at exactly the same moment.

Can a portable generator run central air conditioning?

Usually not. A central AC compressor can draw several times its running power when it starts, so even a 3-ton unit may need a large portable or a home standby generator. A soft-start kit on the compressor cuts the starting surge substantially. Check the locked-rotor amps (LRA) on the outdoor unit nameplate and ask an HVAC technician.

Do I need an inverter generator for electronics?

Inverter generators produce clean, stable power (low harmonic distortion), which is safest for computers, TVs, CPAP machines and appliances with control boards. They are also quieter and more fuel-efficient at light loads. Conventional open-frame generators give more watts per dollar for heavy loads like well pumps and power tools.

How do I connect a generator to my house safely?

Never plug a generator into a wall outlet ("backfeeding") — it can electrocute utility workers and damage your wiring. Use extension cords rated for the load, or have an electrician install a transfer switch or interlock kit. Always run generators outdoors, at least 20 feet from windows and doors, because their exhaust contains deadly carbon monoxide.