Starting watts vs running watts
Running watts is the power a generator sustains continuously; starting watts is the extra surge it can supply for a few seconds while a motor spins up. Size a generator so running loads fit the running rating and the single largest motor start fits within the starting rating.
Motors — refrigerator compressors, well pumps, air conditioners, sump pumps — draw two to six times their running power for the moment it takes to reach speed. Resistive loads like heaters, lights, and electronics have no meaningful surge.
The practical sizing method: add up the running watts of everything that will run at once, then add the largest single starting surge (starting minus running for that one appliance) — motors rarely all start in the same instant. Manufacturers commonly name models by starting watts, so a “9500-watt” generator may sustain only 7,500 W.
| Typical surge multiple | 2–3× running for refrigerators and freezers; 3–5× for well pumps and air-conditioning compressors |
|---|---|
| Surge duration | Motor inrush lasts roughly 0.5–3 seconds; generator surge ratings assume seconds, not minutes |
| Sizing rule | Running total + the single largest starting surge above running + about 20% headroom |
| Altitude derating | Roughly 3–3.5% of engine output lost per 1,000 ft above sea level |
| Temperature derating | Roughly 1% per 10 °F above about 77 °F |
| Soft starter effect | Cuts compressor inrush by roughly 50–70%, often turning a 3-ton AC into a startable load |
Where the surge comes from
At the instant power hits an induction motor the rotor is stationary, so the motor behaves like a shorted transformer and draws locked-rotor current — the LRA figure stamped on the nameplate, typically four to eight times full-load amps. As the rotor accelerates it generates back-EMF and current collapses toward normal within a second or two. Nothing about that is a defect; it is how the machine works.
One nuance saves a lot of money. Locked-rotor current is largely reactive: power factor at the moment of starting is roughly 0.3–0.5, so the real watts drawn are well below LRA multiplied by voltage. A compressor with 60 LRA at 240 V is drawing about 14,400 volt-amperes at that instant, but only perhaps 5,000–7,000 watts of real power — which is why generators rated in watts can start motors whose apparent starting VA looks impossible. Generator surge ratings and motor LRA figures are not in the same units, and comparing them directly overstates the problem.
Resistive loads have no surge at all. Heaters, water-heater elements, incandescent lamps, toasters, and coffee makers draw their rated power from the first cycle. Electronics with switch-mode supplies draw a brief inrush of a few cycles, small enough to ignore. Modern ECM and variable-speed motors — condensing furnace blowers, inverter mini-splits, variable-speed pool pumps — ramp up under electronic control and have essentially no surge, though they care a great deal more about waveform quality than the old motors did.
The sizing arithmetic, with real numbers
Take a typical essentials list. Refrigerator: 150 W running, about 1,000 W starting. Furnace blower: 600 W running, about 1,800 W starting. Sump pump: 800 W running, about 1,800 W starting. Half-horsepower well pump: 1,000 W running, about 2,500 W starting. Lights, modem, and phone chargers: 300 W with no surge. Running total is 2,850 W. The largest single surge above running belongs to the well pump: 2,500 − 1,000 = 1,500 W.
That gives two separate requirements. The generator needs a running rating of at least 2,850 W, and adding 20% headroom for measurement error, derating, and the load you forgot puts the target near 3,400 W. Separately it needs at least 2,850 + 1,500 = 4,350 W of surge capability. A generator rated 4,000 W running and 5,000 W starting clears both comfortably; a 3,500 W running unit with 4,375 W starting clears the running requirement but is marginal on surge.
The one assumption to check is that only one motor starts at a time. That is true almost always and false during a storm, when the sump pump and the well pump can call within the same second. Where two motors genuinely can coincide, add both surge deltas. Where a motor is on a thermostat you control, you can also simply not run it during the start of another.
Why the model number lies, and what to read instead
There is no rule forcing a model number to reflect either figure, and brands are inconsistent about it. Some name models after the starting watts, so a “9500” is a 7,600 W machine. Others name after running watts and quote 12,500 W of surge on the same badge. Ignore the number on the box and find the spec table, which will list rated or running watts alongside surge, peak, or maximum watts. If it lists only one figure, assume it is the flattering one.
Then check three things that silently reduce it. Fuel: propane running and surge ratings are typically about 10% below the gasoline figures, natural gas 20–30% below, and dual-fuel spec tables publish all of them. Connection: a generator rated 9,500 W running with only an L14-30 outlet can deliver just 7,200 W through that cord, because 30 A at 240 V is the connector’s ceiling. And balance: each 120 V leg carries at most half the total, so a load list that is entirely 120 V and lands on one leg can trip at half the generator’s rating.
When a bigger generator is the wrong answer
A soft starter fitted to the compressor is usually cheaper than the next generator size up. These devices ramp the compressor’s voltage over a second or two instead of applying it instantly, cutting inrush by roughly 50–70%. That is routinely the difference between a 3-ton central air conditioner being startable on a 7,500 W generator and needing 12,000 W. Hard-start kits — a capacitor pack that boosts starting torque — are cheaper again but far less effective, and they raise starting current rather than reducing it.
Oversizing has real costs beyond price. An engine held at 3,600 RPM under 10–15% load burns nearly as much fuel as at 40%, runs cool enough to accumulate carbon and dilute its oil, and is louder for no benefit. The efficient operating band for a portable generator is roughly 25–75% of its running rating. Buy for the load you will actually carry plus headroom, not for the largest number you can afford.
Frequently asked questions
How many starting watts does a refrigerator need?
A typical household refrigerator draws 100–200 W while running and surges to roughly 800–1,200 W for a second or two when the compressor cycles on. A chest freezer is lower — around 100–150 W running and 500–800 W starting. Because they cycle rather than run continuously, their average draw over an hour is well below the running figure, but the generator still has to cover the surge whenever the compressor starts.
Do I add the starting watts of every appliance together?
No — that overbuilds the generator badly. Add the running watts of everything that will be on at once, then add only the largest single starting surge above its own running figure, because motors almost never start in the same instant. The exception is a case where two motors genuinely can coincide, such as a sump pump and a well pump during a storm; there, add both.
What happens if a generator does not have enough starting watts?
The voltage sags when the motor tries to start. Starting torque falls with the square of voltage, so a deep sag means the motor cannot break away and sits at locked-rotor current — several times normal — until its internal thermal overload opens. Repeated attempts overheat the windings, which is how compressors die during outages. On the generator side you will usually see the breaker trip, the engine bog, or an overload light. Occasionally nothing trips and the motor simply cooks.
Are starting watts and peak watts the same thing?
Yes. Starting watts, peak watts, surge watts, and maximum watts all describe the same short-duration output rating, held for a few seconds. Running watts, rated watts, and continuous watts describe the sustained figure. The only one a generator can hold for hours is the running rating.
Can a 5,000-watt generator run a well pump?
A half-horsepower pump, comfortably: about 1,000 W running and 2,500 W starting leaves room for a refrigerator and lights. A one-horsepower pump at roughly 2,000 W running and 4,000–6,000 W starting is marginal on a 5,000 W unit and will fail to start if much else is running. A one-and-a-half or two-horsepower submersible needs a substantially larger generator or a soft starter. Check the pump’s nameplate rather than guessing from the well depth.