Generator Size Calculator

Pick what you need to keep running during an outage. We work out the running and starting watts, the size of portable or standby generator that covers it, and what each option costs up front and per year.

Start from

Fuel at your home Outage hours a year Power must come back on its own, even when I'm away

Water & flooding

Sump pump (1/3 HP)800 W running · 1,300 W starting

Sump pump (1/2 HP)1,050 W running · 2,150 W starting

Well pump (1/2 HP)1,000 W running · 2,100 W starting · 240 V

Well pump (3/4 HP)1,500 W running · 3,000 W starting · 240 V

Well pump (1 HP)2,000 W running · 5,000 W starting · 240 V

Kitchen

Refrigerator300 W running · 1,500 W starting

Freezer (chest or upright)200 W running · 1,000 W starting

Microwave1,000 W running · 1,000 W starting

Coffee maker900 W running · 900 W starting

Electric cooktop (2 burners)3,000 W running · 3,000 W starting · 240 V

Dishwasher1,500 W running · 1,500 W starting

Heating & cooling

Gas furnace blower (1/3 HP)700 W running · 1,400 W starting

Gas furnace blower (1/2 HP)875 W running · 2,350 W starting

Boiler, gas or oil (burner + circulator)300 W running · 700 W starting

Central AC or heat pump (2 ton)2,000 W running · 6,000 W starting · 240 V

Central AC or heat pump (3 ton)3,200 W running · 9,500 W starting · 240 V

Central AC or heat pump (4 ton)4,300 W running · 12,500 W starting · 240 V

Central AC or heat pump (5 ton)5,300 W running · 15,500 W starting · 240 V

Window AC (10,000 BTU)900 W running · 2,700 W starting

Space heater1,500 W running · 1,500 W starting

Dehumidifier600 W running · 900 W starting

Lights, internet & devices

LED lighting (whole house)200 W running · 200 W starting

Wi-Fi router + modem30 W running · 30 W starting

TV150 W running · 150 W starting

Computer + monitor150 W running · 150 W starting

Phone & laptop chargers60 W running · 60 W starting

Garage door opener (1/2 HP)875 W running · 2,350 W starting

Laundry & large electric loads

Washing machine1,150 W running · 2,250 W starting

Gas dryer700 W running · 1,800 W starting

Electric dryer5,400 W running · 6,750 W starting · 240 V

Electric water heater4,500 W running · 4,500 W starting · 240 V

EV charger (Level 2)7,200 W running · 7,200 W starting · 240 V

Medical

CPAP machine60 W running · 60 W starting

Oxygen concentrator350 W running · 700 W starting

How the calculator sizes your generator

Running watts: we add up the running watts of every appliance you select, assuming they all run at once.
Starting peak: motors (pumps, compressors, blowers) need a burst of extra power to start. We add the largest single burst on top of everything else running, which is the moment a generator struggles most.
Headroom: a generator shouldn't run flat out for hours, so its running rating must be about 20% above your total.
Fuel: standby generators produce about 10% less power on natural gas than on propane, and we account for that.

Portable generators are labelled by their starting watts in most stores (a "5,000-watt" generator usually runs about 4,000 watts continuously), so we use the same convention. Appliance wattages are typical values from manufacturer charts; the exact numbers are on your appliance's label.

Frequently asked questions

How do I calculate what size generator I need?

Add up the running watts of everything you want to power at the same time. Then add the extra starting surge of the single biggest motor, such as a well pump, sump pump or AC compressor, because motors need two to three times their running watts for a second or two when they start. Pick a generator whose starting watts cover that peak and whose running watts stay about 20% above your total, so it isn't working flat out.

What is the difference between running watts and starting watts?

Running watts are what an appliance draws while it runs. Starting watts (or surge watts) are the brief spike when a motor kicks on. Generators list both, and you need to cover both: your total running load and the biggest single surge on top of it.

Is a 22 kW generator enough for a whole house?

For most homes with gas heat and up to a 4-ton central AC, yes. Homes with electric heat, an electric water heater, an electric range and a large AC can need more, or a load manager that switches big loads off in turn. Many homes need far less than 22 kW if they only back up essentials: run the calculator with your own list.

Can I plug a generator into a wall outlet?

No. Feeding power into a wall outlet ("backfeeding") can electrocute utility workers and start fires. Connect through a transfer switch or a breaker interlock kit installed by an electrician, or run extension cords to individual appliances.

Look up a specific case

What can a 3,500, 5,000, 7,500 or 9,500-watt generator run? Is a 22 kW standby enough? See every generator size, or the size you need for a sump pump, well pump, refrigerator or central AC.