Generator Sizing
People size a generator off the running total and then wonder why it stalls every time the air conditioner kicks in. Motors pull a surge on startup, sometimes double their running draw, and that surge sets the size you need. Tick what you run and the math below shows both numbers.
Live · Updates as you tick Generator Sizing
What do you run?
Recommended generator
6,500W
Running watts
3,700 W
Peak starting watts
5,300 W
| Step | Value |
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Assumes one motor starts at a time (the realistic worst case) and adds 20% headroom, because running a generator at its ceiling is how you shorten its life. Wattages are typical mid-range figures; your appliance's own label is the real number. A planning estimate, not a system design.
Carbon monoxide kills people every year. A generator's exhaust has no smell and no color, and it does not need to be indoors to reach you: running one near a window, a vent, or under an awning is enough. Keep it well away from where anyone is sleeping, never run it in an enclosed or partly enclosed space, and fit a working CO detector wherever you sleep. Read the safety guidance before you run one, not after.
Frequently Asked Questions
Anything with an electric motor or a compressor pulls a big spike of current for the first fraction of a second while the motor gets moving. An air conditioner that runs on 1,400W can demand 3,000W or more for that instant. Your generator has to survive that spike or it trips and everything stops. This is why the running total is the wrong number to shop with, and it is the single most common sizing mistake.
Because the surge lasts a fraction of a second, and the odds of two compressors kicking in at the exact same instant are low. The standard approach is to take everything running, then add the single largest startup surge on top. If you have two air conditioners you genuinely start together, size for both, but most people do not.
Often not on its own, and it is the most common disappointment in this category. A 13.5k BTU unit can surge past 3,000W, which is beyond what a 2,000W generator delivers even briefly. Two of them paired, or a soft-start kit fitted to the air conditioner, are the usual fixes. A soft start limits the spike and is cheaper than a second generator.
Because a generator held at its maximum output runs hot, burns more fuel, and wears out faster, and its rated peak is a short-duration figure rather than something it can sustain. Altitude makes it worse: engines lose roughly 3% of their output per 1,000 feet, so a generator that copes at sea level can struggle at 8,000 feet in a national forest. Headroom is what stops that being your problem.
For most loads, yes, and it is quieter, has no exhaust, and does not annoy everyone within a quarter mile. Solar plus a decent battery bank covers a fridge, lights, laptops, and charging comfortably. Where generators still win is heavy intermittent loads, air conditioning in particular, and long stretches of bad weather. Plenty of people carry a small one purely as insurance and run it twice a year.