Starting Watts vs Running Watts
Updated 2026-08-15 · 6 min read
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The single most common generator sizing mistake is adding up running watts and buying accordingly. The generator then trips or stalls the first time the well pump starts.
Why do motors draw more watts at startup?
An electric motor at rest has very low resistance and no back-EMF opposing the applied voltage. For the instant before it spins up, it draws far more current than it will once running — locked rotor current.
The surge lasts a fraction of a second. But the generator has to supply it, or voltage sags, the motor stalls, and either the generator's overload protection trips or the motor overheats.
Resistive loads have no surge. A heater, an incandescent bulb, or a toaster draws the same power at the instant of switch-on as it does a minute later. It's motors and compressors that cause the problem.
How much more power does a motor need to start?
| Load type | Starting watts vs running |
|---|---|
| Resistive — heaters, lights, kettles | 1× (no surge) |
| Electronics, LED lighting | ~1× |
| Small motors — fans, small pumps | 2–3× |
| Refrigerator, freezer | 3–6× |
| Well pump, sump pump | 3–6× |
| Air conditioner, heat pump | 3–5× |
| Air compressor, table saw | 3–6× |
Treat these as a planning range. The nameplate or manual is the authority for any specific appliance, and figures vary substantially between models — a modern inverter-compressor refrigerator surges far less than an older one.
How do starting watts affect generator sizing?
Add the running watts of everything that will run at once. Then add only the single largest starting surge.
Not the sum of all surges. Motors don't coordinate their startups, and the probability of two large compressors starting in the same fraction of a second is low enough that the industry sizes on the largest single one.
A worked example
A basic outage load:
| Load | Running W | Starting W |
|---|---|---|
| Refrigerator | 200 | 1,000 |
| Sump pump | 800 | 2,400 |
| Furnace blower | 600 | 1,800 |
| Lights (LED) | 150 | 150 |
| Phone/laptop charging | 100 | 100 |
| Running total | 1,850 |
Largest single surge is the sump pump at 2,400 W, which is 1,600 W above its own running figure.
So the peak requirement is roughly 1,850 + 1,600 = 3,450 W.
A generator rated around 3,500 running watts with a higher surge rating covers this. Sizing on the 1,850 W running total alone would have produced a generator that fails the first time the sump pump cycles.
Work through your own list with the generator sizing calculator.
What do generator watt ratings mean?
Generators are advertised with two numbers, and the larger one is usually the headline:
- Running / rated watts — what it can sustain continuously
- Starting / surge / peak watts — what it can supply for a few seconds
Size on running watts. The surge rating is a short-duration allowance, not capacity you can use.
A generator advertised as "5,500 watts" with 4,500 running watts is a 4,500-watt generator. Marketing leads with the bigger number.
What makes surges worse
Motors starting under load. A compressor restarting against system pressure, or a pump starting against a full head, draws more than one starting unloaded. This is why a refrigerator that just cycled off surges harder if it restarts immediately — and why generators have a harder time with short-cycling equipment.
Long or undersized extension cords. Voltage drop over an undersized cord means less voltage at the motor, which means more current for the same power — making a marginal start fail. See generator extension cords and wire gauge.
Cold. Lubricants are thicker and motors work harder.
Several starts at once. Rare, but it happens — a fridge and a furnace cycling together. Some headroom protects against it.
How do you reduce peak generator demand?
You can shrink the generator you need by managing what runs:
Stagger startups. Don't switch everything on at once when the generator starts. Bring loads up one at a time, largest first while the generator is unloaded.
Soft starters. A soft-start device on an air conditioner or heat pump compressor reduces inrush substantially — often enough to let a much smaller generator run a central AC that otherwise couldn't be started at all. Worth pricing against a bigger generator.
Load management. Automatic transfer switches with load-shedding disconnect lower-priority circuits when a large load starts. See transfer switch explained.
Choose what you actually need. A well pump and a furnace during an outage is a different requirement from air conditioning the whole house.
Where to go next
More in our generator and backup power guides.
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