Every portable generator label shows two wattage numbers: starting watts (sometimes labeled "peak" or "surge") and running watts (sometimes labeled "continuous" or "rated"). The two numbers can differ by 25% or more, and confusing them is the most common sizing mistake. A generator advertised as "5,000 watts" might mean either 5,000 peak (with 4,000 continuous) or 5,000 continuous (with 6,250+ peak) — and the difference determines whether it actually runs your house.
The two numbers, defined
Running watts (continuous, rated): The wattage the generator can sustain indefinitely. If your continuous load is below this number, the generator runs without strain until it runs out of fuel.
Starting watts (peak, surge): The wattage the generator can deliver for a brief moment — typically two to three seconds at most — to absorb the inrush current of starting motors. This is not a sustained capability. After the surge moment, the generator returns to its continuous rating.
The ratio matters. A typical inverter generator might be rated 1,800 running / 2,200 starting watts — a 22% surge margin. A typical conventional generator might be 5,000 running / 6,250 starting watts — a 25% surge margin. A few generators with aggressive marketing inflate the peak number further with a "Peak Power" rating that lasts under a second; treat those numbers with suspicion and rely on the continuous rating for sizing.
Why motors need surge watts
Electric motors — refrigerator compressors, well pumps, sump pumps, AC compressors, furnace blowers, power tools — draw significantly more current at the moment of starting than they do once they're running. The reason is physics: a stationary rotor has no back-EMF (the voltage the spinning rotor generates that opposes the supply voltage). With no back-EMF, the only thing limiting current draw is the motor's winding resistance, which is low. Once the rotor accelerates to running speed, back-EMF rises and current drops to the running level.
The result is an inrush spike of 2 to 4 times the running current, lasting one to three seconds depending on motor type, load, and starting method. Standard induction motors (refrigerators, AC compressors) spike about 3x. Capacitor-start motors (well pumps, large compressors) spike about 4x. Soft-start motors and inverter-driven appliances may spike only 1.5x.
The math, with a concrete example
Suppose you want to run a refrigerator and a furnace blower together off a generator. Specs:
- Refrigerator: 200W running, 1,000W starting (5x ratio)
- Furnace blower (1/2 HP): 800W running, 2,200W starting (2.75x ratio)
Once both are running steadily: 200 + 800 = 1,000W continuous. Easy.
The hard case is when the furnace blower kicks on while the fridge is already running. At that moment, the load is 200W (fridge) + 2,200W (blower starting surge) = 2,400W for about two seconds. The generator needs at least 2,400 starting watts and 1,000 running watts.
Critical point: you don't add both motors' starting surges. Real-world appliances don't start at the same instant; they cycle randomly. The realistic worst case is the largest single motor surge on top of everything else's steady-state running consumption.
The sizing formula
Step 1: Sum running watts of every load you'll have on at once. Call this R.
Step 2: Identify the largest single motor surge in your load list. Call this S.
Step 3: Identify the running watts of that same motor. Call this Rm.
Step 4: Peak demand = R - Rm + S. (You subtract the motor's running watts before adding its surge watts, because at the instant of surge, the motor isn't yet running.)
Step 5: Generator's starting-watt rating must exceed peak demand. Generator's running-watt rating must exceed R.
For the fridge + furnace example above: R = 1,000, Rm (blower) = 800, S (blower surge) = 2,200. Peak = 1,000 - 800 + 2,200 = 2,400W. Need a generator rated at least 2,400 starting / 1,000 running. A 2,200/3,000W inverter is enough.
Inverter generators handle surges differently
Inverter generators have a brief surge capability above the rated starting watts, sometimes called "peak boost" or just absorbed in the rated surge number. The Honda EU2200i, for instance, is rated 1,800 running / 2,200 starting and can hold the 2,200W surge for several seconds — long enough for any normal motor start. Conventional generators have less headroom; their starting-watt rating is closer to the actual one-second peak.
This matters in practice: an inverter generator at its rated starting wattage usually handles motor surges more reliably than a conventional generator at the same starting-watt rating, because the inverter's electronics can hold the surge longer.
Resistive loads have no surge
Resistive loads — incandescent bulbs, electric heaters, toasters, coffee makers, kettles, hair dryers, electric ranges — draw their running wattage immediately at startup. There's no surge to plan for. If a 1,500W space heater is on your load list, plan for 1,500W flat — no surge math needed.
Electronic loads — LED bulbs, laptops, TVs, phone chargers — also draw at or near their running wattage immediately. Computer power supplies have a small inrush as their capacitors charge, but it's milliseconds and well within any generator's capability.
Common appliances: surge multipliers
| Appliance | Surge multiplier | Notes |
|---|---|---|
| Refrigerator | 3–5x | Higher for older units with worn compressors |
| Freezer | 3–5x | Similar to refrigerator |
| Furnace blower (½ HP) | 2.5–3x | Less surge than capacitor-start motors |
| Furnace blower (¾ HP) | 2.5–3x | |
| Sump pump (½ HP) | 3x | Capacitor start |
| Well pump (1 HP, 240V) | 2.5–4x | Highly variable — check nameplate |
| Window AC | 1.5–2x | Modern units often have soft start |
| Central AC (2-ton) | 2.5–3x | Often the limiting surge in home backup |
| Power tools (circular saw) | 2x | Universal motors have lower surge |
| Microwave | 1x | Resistive after magnetron warm-up |
| Coffee maker | 1x | Pure resistive |
| LED lights | 1x | Negligible |
| Electric heater | 1x | Pure resistive |
The mistake to avoid
The biggest sizing mistake we see: someone adds the starting watts of every motor in their inventory and sizes the generator to that imaginary maximum. The result is a generator that's 30–40% oversized for actual use, which costs $500+ more than needed and runs at inefficient low load most of the time.
The correct approach: size running-watt total accurately, then add only the largest single motor's surge to the running total. This gives you a generator that handles every real worst case without paying for theoretical scenarios that physics won't produce.
For the full sizing process applied to a real home, see how to size a generator for your home.
Frequently Asked Questions
Why are starting watts higher than running watts?
Electric motors require a current spike at the instant of starting because the stationary rotor has no back-EMF to limit current draw. The spike is 2–4 times the motor's running current, lasting 1–3 seconds. Generators rate starting watts to indicate the brief surge capacity they can deliver to absorb that inrush.
Do I add the starting watts of every motor in my load list?
No. Multiple motors do not start at exactly the same moment in real-world use — appliances cycle randomly. The realistic peak demand is the running watts of everything plus the surge of the single largest motor. Adding every surge together produces a number that overstates actual demand by 50–100% and leads to oversizing.
How long can a generator sustain its starting wattage?
Typically 1–3 seconds, depending on the model. Inverter generators with electronic control can often hold the surge longer than conventional generators with direct mechanical output. The exact duration is usually specified in the generator's manual.
Do all appliances have starting watts higher than running watts?
No. Only motor-driven appliances have a surge. Resistive loads — heaters, toasters, coffee makers, incandescent bulbs, electric kettles, ranges — draw their running wattage immediately with no surge. Electronics also draw at running wattage from the moment of startup. Surge calculations only matter for compressors, pumps, blowers, and similar motor loads.
Is 'peak watts' the same as 'starting watts'?
Usually yes. The two terms are used interchangeably by most manufacturers. Some marketing departments add a third 'peak' number that lasts under one second — shorter than typical motor surges. Always size against the rated starting wattage, not against a marketing peak number with vague duration.