What size generator for a 4000 square foot house running everything except AC
| Generator size | 18kW |
|---|---|
| Running loadeverything on together, steady state | 11.7kW |
| Peak when the largest motor startsthis is the number that sizes the machine | 14.1kW |
| With load management4 kW smaller — a $200–500 module | 14kW |
| What it runsadds kitchen, laundry and the rest of the lighting | the house except air conditioning |
Notes
- The starting surge sizes the generator, not the running load. This house draws 11.7 kW steadily, but when the air conditioner or well pump starts, demand spikes to 14.1 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 14 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.
- 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.
18 kW, and the surge is what decided it
The house draws 11.7 kW steadily. When the largest motor starts, demand spikes to 14.1 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 18 kW and 14 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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- What size generator for a 4000 square foot house running an all-electric house
Read more
- Derating — A Generator Produces Less at Altitude and in HeatThe nameplate assumes conditions the machine may never see.
- Reading a Generator's Ratings — Standby, Surge and ContinuousTwo generators both called 22 kW can be rated on different bases, and only one of the numbers is the one to compare against a load list.
- Portable or Standby — The Difference Is Whether Anyone Has to Be HomeBoth produce electricity. Only one of them does it while the household is away or asleep.
- What Size Standby Generator — Start From What Has to Run at OnceThe house draws 200 amps of connected load and perhaps 30 amps at three in the morning. The generator only has to cover the second number.