KwTallyRunning watts, starting watts, and the number that actually sizes the unit.

What size generator for a 3500 square foot house running the house with AC

3,500 ft², the house including one AC unit
28kW standby generator
18 kW with a load management module — worth pricing before buying the bigger unit
3,500 ft², the house including one AC unit28 kWGenerator size15.3 kWRunning load26.3 kWPeak when the lar…18 kWWith load managem…
Generator size28kW
Running loadeverything on together, steady state15.3kW
Peak when the largest motor startsthis is the number that sizes the machine26.3kW
With load management10 kW smaller — a $200–500 module18kW
What it runsa 3-ton condenser is the largest starting load in most homesthe house including one AC unit

Notes

  • The starting surge sizes the generator, not the running load. This house draws 15.3 kW steadily, but when the air conditioner or well pump starts, demand spikes to 26.3 kW for a second or two. A generator that cannot swallow that sags, and the sag is what damages motors and drops the whole panel.
  • Surges do not add up, and this is where people overbuy. Two motors almost never start in the same instant, so you size for the largest single surge, not the sum. Adding every appliance's starting watts together produces a number roughly twice what the house needs — and a generator twice the size costs more to buy, more to install, and burns more fuel at part load.
  • A load management module would let a 18 kW unit do this job. It sheds the air conditioner or the water heater for the few seconds another large load starts, so the generator never sees the combined peak. It costs a few hundred dollars against several thousand for the next size up, and it is almost never offered unprompted. Ask about it explicitly.
  • 28 kW is past ordinary residential standby equipment, which mostly stops around 26 kW. A number this size is a signal that the load list is too ambitious rather than that you need a light-commercial machine: shed the largest loads and the answer comes back into the normal range.
  • A soft starter on the air conditioner is the other lever. It cuts compressor inrush by roughly two thirds for a couple of hundred dollars, and on a house whose peak is set by the AC that frequently drops the generator a whole size.
  • Fuel matters more than the nameplate for a long outage. Natural gas runs indefinitely but delivers roughly 10% less output than propane. Propane means a tank, and a 500-gallon tank at half load runs a 20 kW unit somewhere around two to three days — not the week people assume.
  • The transfer switch is a separate decision and a separate price. A whole-house automatic switch costs more than an essential-circuits one, and it is what determines whether you choose loads in advance or the generator carries everything. Budget it alongside the machine rather than discovering it in the quote.

28 kW, and the surge is what decided it

The house draws 15.3 kW steadily. When the largest motor starts, demand spikes to 26.3 kW for a second or two, and a generator that cannot swallow that sags — which is what damages motors and drops the whole panel.

Surges do not add up

Two motors essentially never start in the same instant, so you size for the largest single surge rather than the sum of all of them. Adding every appliance's starting watts together produces a number roughly twice what the house needs, and a generator twice the size costs more to buy, more to install, and burns more fuel sitting at part load.

Load management is the cheapest kilowatt you will ever buy

A shed module drops the air conditioner or the water heater for the few seconds another large load starts, so the generator never sees the combined peak. Here that is the difference between 28 kW and 18 kW — a few hundred dollars against several thousand.

It is almost never offered unprompted. Ask about it by name.

A soft starter is the other lever

On a house whose peak is set by the air conditioner, a soft starter cuts compressor inrush by roughly two thirds for a couple of hundred dollars, and that frequently drops the generator a whole size on its own.

Fuel changes what "runs" means

Natural gas runs indefinitely and delivers about 10% less output than propane. Propane needs a tank, and a 500-gallon tank at half load runs a 20 kW unit somewhere around two to three days — not the week most people assume when they buy it.

Nearby sizes

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