Generator Size Chart: What Size Generator Do I Need?

The generator size chart

A generator is sized in watts, and every appliance has two watt numbers: the running watts it draws while it runs, and the starting watts it needs for the first second or two while its motor spins up. A fridge runs at 700 watts but needs 2,200 watts to start the compressor; a chest freezer runs at 500 watts and needs 1,500 to start; a window air conditioner runs at 1,200 watts and needs 3,600 to start; and a 3-ton central air conditioner runs at 3,500 watts and needs 9,000 to start. Resistive loads — a microwave, a coffee maker, a TV, a laptop — need no extra starting watts at all: their starting watts are the same as their running watts. Add up the running watts of everything you want on at once, add the largest starting watts of any one appliance, and round up to the next common generator size.

The number that matters most is the starting watts: it is the surge that trips an undersized generator. A 2,000-watt generator will run a fridge, a freezer and a few lights (about 1,500 running watts), but it will not start a fridge that needs 2,200 watts to start — the fridge alone needs more than the generator can give in that first second. That is why the rule is running watts plus the largest starting watts, not running watts alone. Common generator sizes are 2,000, 3,500, 5,000, 7,500, 10,000 and 12,500 watts; anything over 12,500 watts is a 20,000-watt standby generator, wired into the house, not a portable one you wheel out of the garage.

The worked example, line by line

Setup: 2,500 running watts and a 2,200-watt fridge start; find the generator size.

Line 1 — the watts needed. 2,500 + 2,200 = 4,700 W.

Line 2 — the next common size. The common sizes are 2,000, 3,500, 5,000, 7,500, 10,000 and 12,500 W; the first one at or above 4,700 W is 5,000 W.

Line 3 — the answer. A 5,000 W generator.

Line 4 — the reverse. A 5,000 W generator with a 2,200 W fridge start leaves 5,000 − 2,200 = 2,800 W of running budget.

Every branch, checked

1,500 running + 1,200 starting watts: the need is 1,500 + 1,200 = 2,700 W, and the next common size is 3,500 W. That is a fridge, a freezer and a few lights, with a furnace fan starting — a small portable generator covers it.

4,000 running + 3,600 starting watts: the need is 4,000 + 3,600 = 7,600 W, and the next common size is 10,000 W. That is a house with a window air conditioner starting — the 3,600-watt surge is what pushes it past the 7,500-watt generator.

800 running + 0 starting watts: the need is 800 + 0 = 800 W, and the next common size is 2,000 W. That is lights, a laptop and a TV — no motor to start, so the starting watts are zero and the smallest common generator covers it with room to spare.

6,500 running + 9,000 starting watts: the need is 6,500 + 9,000 = 15,500 W, which is above the largest portable size of 12,500 W, so the answer is a 20,000 W standby generator. That is a house with a 3-ton central air conditioner starting — the 9,000-watt surge is what pushes it out of portable territory entirely.

Reverse — the 5,000 W generator: a 5,000 W generator with a 2,200 W fridge start leaves 5,000 − 2,200 = 2,800 W of running budget. That is the fridge (700 W), the lights and the phone chargers, with room to spare — but add a well pump at 1,000 W and a furnace fan at 800 W and you are at 2,500 W, which still fits, just barely.

What the defaults assume

This chart sizes by watts, not a runtime or fuel chart: it tells you how big a generator you need, not how long it runs on a tank of gas or how much fuel it burns. It assumes the watt numbers in the chart, which are the standard values generator manufacturers publish — your exact fridge or pump may draw a little more or a little less, so check the nameplate on the appliance if you want the exact number. It assumes only one motor starts at a time: if the fridge and the well pump start together, the surge is the two starting watts added, not the largest one. It assumes you add up the running watts of everything on at once — if you stagger the loads, turning the microwave off while the well pump runs, you can get by with a smaller generator. It says nothing about the generator’s fuel, its noise, or whether it is an inverter generator, which matters for electronics. Rounding convention: watts are whole numbers and the recommended size is always the next common size at or above the need.

Output · formula · meaning

Who uses this

• The homeowner buying a first generator: they add up the fridge, the lights and the phone chargers at 2,500 running watts with a 2,200 W fridge start, and learn they need a 5,000 W generator — so they buy the 5,000-watt one, not the 3,500-watt one that would trip on the first fridge start.
• The family with a well pump: their running watts are 4,000 W with a 3,600 W window AC start, so they need a 10,000 W generator — and they learn the air conditioner is what decides the size, not the pump.
• The renter with a small apartment: their running watts are only 800 W with no motor to start, so a 2,000 W generator covers them — and they stop shopping for a 5,000-watt machine they do not need.
• The homeowner with central air: their need is 15,500 W, which is above the largest portable size, so they learn they need a 20,000 W standby generator — and they price a wired-in unit instead of a portable one.

Accuracy & limitations

The math is exact for the numbers you enter: the need is the running watts plus the largest starting watts, and the recommended size is the next common size at or above the need. What the chart cannot do is know your appliances: the watt numbers are the standard values manufacturers publish, and your exact fridge or pump may draw a little more or a little less — check the nameplate on the appliance for the exact number. It assumes only one motor starts at a time; if the fridge and the well pump start together, add both starting watts. It assumes you add up everything on at once; if you stagger the loads, you can get by with a smaller generator. It says nothing about runtime or fuel: a 5,000-watt generator at half load burns about half a gallon of gas an hour, and a 20-gallon tank lasts about 40 hours, but that is a fuel question, not a size question. For the power that feeds the house, see solar panels vs grid power; for the heat that runs on it, see heat pump vs furnace.