Generator Size Calculator: What Size Generator Do I Need?

A generator has to do two things: carry everything you leave on at the same time, and survive the jolt when a motor starts. Refrigerators, freezers, pumps, furnace blowers and AC compressors all pull a short surge on startup, often about three times their running power. This calculator adds up the loads you choose, adds the single largest starting surge, and picks a generator size with some headroom.

How it works

running    = Σ (running watts × how many)
peak       = running + largest single (starting − running)
needed     = the larger of peak × 1.10 and running ÷ 0.80
generator  = smallest common size ≥ needed

Planning values. Each appliance has a typical running and starting wattage, listed in the table below. Motor-driven loads assume a start of about three times running. Heaters, lights and electronics have no surge. Your own equipment can be quite different, so check the nameplate or manual and use the “other loads” fields to add anything that isn’t on the list.

One start at a time. The peak assumes only one motor starts at once, which is how you’d manage a generator in an outage. If two motors could start together, like a well pump and a furnace on a cold morning, add the second one’s surge as an “other” load.

Headroom. We add 10% to the peak, and we keep the running load at or below 80% of the rating. Generators run hotter and less efficiently near their limit, and their output falls at high altitude and in hot weather.

What it doesn’t do: fuel use, inverter versus conventional output quality, or the 120/240-volt split. Large 240-volt loads like well pumps need a generator with a 240-volt outlet or a transfer switch.

Worked example

A family wants to ride out storms with the refrigerator, a chest freezer, the sump pump, the gas furnace’s blower, 20 LED bulbs, the TV and internet, the microwave and a window air conditioner in one bedroom.

• Running: 700 + 500 + 1,050 + 800 + 200 + 250 + 1,000 + 1,000 = 5,500 W
• Largest start: the sump pump, 3,150 − 1,050 = 2,100 W extra. The window AC adds 2,000 W, just under it
• Peak: 5,500 + 2,100 = 7,600 W, and with 10% headroom, 8,360 W
• Running at 80%: 5,500 ÷ 0.8 = 6,875 W, which is lower, so the peak sets the size
• Result: a 9,500-watt generator

If they skip the window AC and add a 1-hp well pump instead, the well pump’s 4,000-watt start sets the peak at 9,000 W, and they’d need a 12,000-watt generator.

Generator wattage reference table

The running and starting watts the calculator uses for each appliance.

Tips and common mistakes

Measure the loads you care about. A clamp meter or the nameplate amps × volts gives running watts. Starting surges are harder to measure. Soft-start kits on AC compressors can cut them a lot.

Don’t forget 240-volt loads. Well pumps, some sump pumps, electric dryers and central AC run on 240 volts. A generator’s 120-volt outlets can’t power them.

Stage your loads. Start the largest motor first, then add the rest. Turn off loads you don’t need, like a second fridge, during long outages to save fuel.

Install a transfer switch. It lets you pick which circuits the generator feeds and makes back-feeding the grid impossible. It’s also the cleanest way to run furnaces and well pumps that are hard-wired.

Run it far from the house. CPSC and CDC guidance is clear: generators belong outdoors, well away from windows, doors and vents, with the exhaust pointed away from the house. Carbon monoxide from generators kills people every year.