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Generator Size Calculator: How Many Watts Do You Need?

Check off what you want running during an outage. You get running watts, starting watts, and a straight answer about whether a portable will do it or you are looking at a standby unit. The math runs in your browser and nothing is sent anywhere.

Pick what you want to keep running

The basics

  • Refrigerator / freezer

    700 W running · +2,200 W to start

    0

  • Second refrigerator or chest freezer

    700 W running · +2,200 W to start

    0

  • Lights (about ten 60 W bulbs)

    600 W running

    0

  • Television

    500 W running

    0

  • Box fan (20")

    200 W running

    0

  • Phones, laptop, Wi-Fi router

    150 W running · our estimate

    0

Cooling (this is what decides the size)

  • Central AC (about 2 tons / 24,000 BTU)

    3,800 W running · +4,950 W to start

    0

  • Central AC (about 3+ tons / 40,000 BTU)

    6,000 W running · +6,700 W to start

    0

  • Heat pump

    4,700 W running · +4,500 W to start

    0

Kitchen and laundry

  • Microwave

    1,000 W running · our estimate

    0

  • Coffee maker

    1,000 W running · our estimate

    0

  • Washing machine

    1,150 W running · +2,250 W to start

    0

  • Electric clothes dryer

    5,400 W running · +1,350 W to start

    0

  • Electric water heater

    4,000 W running

    0

Pumps and air handling

  • Sump pump (1/2 HP)

    1,050 W running · +2,200 W to start

    0

  • Well pump (1/3 HP)

    1,000 W running · +2,200 W to start

    0

  • Furnace / air handler fan (1/2 HP)

    800 W running · +2,350 W to start

    0

  • Window AC (10,000 BTU)

    1,200 W running · +1,800 W to start

    0

Something else

Check the data plate on the appliance. A motor needs a surge to start, so we add roughly double its running watts for that moment.

Pick a few things to see a number.

Start with the fridge, some lights and whatever keeps the house livable. Add the air conditioning if you want it running during an outage. That one changes the answer most.

How the math works

  1. Running watts are added up for everything you checked, times the quantity.
  2. Starting watts are the running total plus the single largest starting surge on your list, because motors do not all start at the same instant.
  3. The size suggestion picks the smallest standard standby size where running load sits at or under about 80% of the rating and the surge still fits.

Watt figures: 15 rows come from the Homelite wattage worksheet Source: Homelite wattage worksheet (opens in new tab). The other 3 are marked as our estimates in the list. The category thresholds are common rules of thumb, our judgment, not code.

Before you run a portable: carbon monoxide kills people every storm season

  • Outdoors only, at least 20 feet from the house, with the exhaust pointed away from windows, doors and vents. That is the CPSC’s guidance, not a suggestion of ours. CPSC (opens in new tab)
  • Never in a garage, shed, basement, crawlspace or porch, even with the door open. Opening doors and windows does not provide enough ventilation to stop CO building to lethal levels. CPSC (opens in new tab)
  • Working CO alarms on every level, outside sleeping areas, with battery backup. Test them before hurricane season, not during an outage.
  • Never back-feed your panel. Plugging a generator into a dryer or range outlet to power the house is dangerous and illegal: it can electrocute a utility worker and start a fire. Have a licensed electrician install a transfer switch or an interlock kit.
  • Let it cool before refueling, and keep gasoline away from the hot exhaust.

If a CO alarm sounds, get everyone outside into fresh air and call 911. CO symptoms look like the flu — headache, nausea, dizziness, confusion — without a fever.

Next

Common questions

How do you calculate generator size?

Add up the running watts of everything you want on at once. Then add the single largest starting surge from that list, because only one motor starts at a time. That total is the starting watts the generator has to handle. It is the method the Homelite wattage worksheet uses.

Why add only the biggest starting surge instead of all of them?

Because motors do not all start at the same instant. The surge lasts a fraction of a second, and planning for every motor starting simultaneously would size the unit far larger than you need. Adding the single largest surge to the running total is the standard worksheet approach.

What size standby generator do I need for a whole house?

Most whole-house installs on this coast land in the 20 to 26 kW air-cooled range, because that is what carries central air plus the rest of the house. Your actual number depends on your air conditioning tonnage and your electrical service. Use this calculator for a planning figure, then have an installer do a proper load calculation.

Is this calculator accurate enough to buy from?

No, and no online calculator is, including this one. Real appliances differ from table values, your air conditioning draw depends on the actual equipment, and the load calculation that matters is done at the house against your panel. Treat this as a way to find out which category you are in.

Do you store what I enter?

No. The math runs in your browser and nothing is sent anywhere. The optional email field at the bottom does nothing in this preview either, and we never put the result behind an email.

Where these numbers come from

Anything marked estimate is our own rough planning figure, not a sourced number and not a quote.

We don’t install, sell or service generators. We’re a local resource that connects homeowners with installers in their area. How we work.