What Size Generator Do I Need?

Almost everyone shopping for a backup generator starts at the wrong end. They look at machines, compare peak watts, and pick something that feels about right. Then the power goes out, they start the well pump, the generator bogs down and trips, and they discover that the number on the box was never the number that mattered.

The question that decides which generator you need is not how big — it is what, exactly, do I need running at the same time. Answer that and the generator picks itself. That is what this hub is for, and the calculator below is the fastest route to the answer.

The two numbers, and why only one of them is on the box

Every motor-driven appliance draws far more power in the half-second it starts than it does while it runs. A refrigerator runs on around 700 watts and needs roughly 2,200 extra watts to get its compressor turning. A 1/3 HP well pump runs on 1,000 watts and wants another 2,000 to start.

So you need two figures: running watts, the steady total of everything on at once, and starting watts, that total plus the biggest single start-up surge. You add only the largest surge, not all of them, because motors do not all start in the same instant. That is the method published by both of the wattage charts this site cites, and it is the method the calculator uses.

That house needs a generator with about 3,200 running watts and 4,700 starting watts — not the 9,500-watt machine the search results push at it. A quiet inverter generator covers it, costs less, burns less fuel and annoys the neighbors less.

What actually pushes people into a big generator

Three loads, and only three, are responsible for most oversized generator purchases:

• Central air conditioning. A compressor's start-up draw is brutal and its running load is sustained. This is the single biggest reason people end up at 11,000 watts and above.
• A deep well pump. Not the running load — the surge. A soft starter can halve it, which is often cheaper than the next generator up.
• Electric heat of any kind. Resistive heating has no surge at all, but it draws its full rating continuously. An electric water heater is 4,500 watts, flat, forever.

If one of those is on your list, read generator size for air conditioner or generator size for well pump before you shop — there is usually a cheaper way through than buying more generator.

The connection limits the generator, not just the generator

A 9,500-watt generator feeding a 30 amp transfer switch can only deliver about 7,200 watts into your panel, because 30 A at 240 V is the ceiling regardless of what the engine can make. If your sizing math lands above roughly 7,500 running watts, you need a 50 amp transfer switch or an interlock kit, and that changes the installation budget. Size the connection and the generator together, in that order.

Where to go from here

Run the calculator. Then follow the wattage class it gives you into the matching roundup — the result card links straight there with a live price. If you want to understand a specific appliance before you commit, the sizing spokes below each take one load and work it properly: a whole house, a refrigerator and deep freezer, a furnace, a sump pump, or an RV.

The appliance wattage chart is the reference underneath all of it, with every figure attributed to the published chart it came from — including the places where the two charts disagree, which is the most useful thing on the page.