Square footage does not size a generator

Ask the internet what size generator a 2,500 square foot house needs and you get a confident answer in kilowatts. Ask it why floor area decides that, and the answer falls apart.

Two houses of the same size can sit 10,000 watts apart. One has gas heat, a gas range and a gas water heater, so its outage load is lights, a fridge, a furnace blower and the television. The other does all three electrically and has a heat pump. Nothing about the square footage tells you which one you are standing in.

What actually sets the number

Three things, none of which is area.

The first is which appliances you intend to run. Not own, run. A generator sized for an outage is sized for the list you would actually switch on while the power is out, which for most households is a short list and a short one on purpose.

The second is which of those appliances contains an electric motor or a compressor, because those draw far more at the instant they start than they do while running. The third is whether more than one of them can start at the same moment, which is the thing that trips generators that looked correctly sized on paper.

The same house, two load lists

Take a fridge at 700 running watts, a sump pump at 800, a few hundred watts of lights, and a furnace blower at 600. That is roughly 2,300 watts running, and with surge headroom a 3,500 to 4,000 watt unit covers it.

Add a 3-ton central air conditioner at 3,800 running watts and the picture changes completely. Running load is now past 6,000, the surge when the compressor kicks in is thousands more, and you are shopping at 12,000 watts and up. Same house, same square footage, three times the generator, because of one appliance.

That is the whole argument against the area tables. Air conditioning, electric heat, well pumps and electric ranges are step changes, and a table built on floor area cannot see any of them.

Building the list that matters

Walk the house and write down what you would genuinely run. Most people find it is shorter than they expected once they separate the things they need from the things that are merely plugged in.

The home wattage calculator carries typical running and starting figures for common appliances, which saves reading nameplates in a dark basement. Treat those as starting points, since a nameplate on your own machine beats any table, and older motors draw more than their younger equivalents.

Then feed the list into the generator size calculator. It adds the running watts, adds the single largest surge on top, applies headroom so the unit is not pinned at its limit, and rounds up to a size that is actually sold.

Leave it room to breathe

Running a generator at its rated output continuously is not what the rating is for. The common guidance is to stay at or under about 90% of capacity, and there are two reasons. Sustained full load runs hot and shortens the life of the machine, and it leaves nothing in reserve for the moment a compressor restarts.

This is also why buying the next size up is often sensible and buying three sizes up is not. A generator loafing at 25% of capacity burns more fuel per useful kilowatt hour than one working at half to three quarters, so oversizing has a running cost that never shows up in the purchase price.

When the honest answer is a smaller list

If the number that comes out is larger than you want to spend, the productive move is usually to cut the list rather than to hunt for a cheaper unit of the same size.

Air conditioning is the usual candidate. So is the electric range, which almost nobody needs during an outage and which single-handedly adds thousands of watts. Dropping both often halves the generator, and a load manager that sheds one circuit when another starts can do the same job automatically for less than the price difference between two sizes of machine.