Two numbers follow every appliance and every generator around, and mixing them up is the single most common reason a generator disappoints someone. One is the power a device uses while it's running. The other is the extra gulp it takes for a second or two when it first switches on. Get comfortable with the difference and generator sizing stops being guesswork.
Where the difference actually bites
If everything you plan to run is lights, a laptop, and a phone charger, you can almost ignore all of this, because those loads draw about the same whether they're starting or running. The distinction starts to matter the moment a motor enters the picture. A refrigerator, a sump pump, a well pump, an air conditioner, a power saw, each of these asks for a hard, brief pull of power to get moving, and that pull is what decides whether your setup holds or trips.
Why a motor gulps power at startup
A motor at rest has to overcome its own inertia to start spinning. For a split second it draws far more than it needs to keep turning, often two or three times as much, sometimes more, and then the draw drops back to a steady level once it's up to speed.
The comparison that tends to click is a car pulling away from a stop sign. Getting the car rolling from a standstill takes noticeably more effort than keeping it cruising once you're moving. A motor's starting watts are that first push off the line.
So the running figure is the steady draw once a device is going, and the starting figure is that short push to get it there. Purely electronic things, lights, chargers, a TV, barely have a starting figure at all. Anything with a motor has a big one.
A generator carries the matching pair. Its continuous rating is what it can supply all day and needs to cover your total running draw; its peak rating is a short ceiling for surges and needs to cover your single biggest startup on top of the rest. Worth remembering: the model number is usually that peak figure, so size from the continuous one. There's more on putting this into practice in How to Choose the Right Generator Size.
What the numbers look like in real life
These are rough ranges to show the pattern rather than exact specs, real values sit on your appliance's nameplate, so check yours:
| Device | Running watts | Starting watts |
|---|---|---|
| A few LED lights | 60–150 | same, no surge |
| Wi-Fi router and modem | 10–30 | same, no surge |
| Laptop or phone charging | 50–150 | same, no surge |
| Refrigerator or freezer | 100–250 | 600–1,200 (motor surge) |
| Sump pump | 800–1,050 | 1,300–3,000 (motor surge) |
| Microwave | 900–1,500 | same, draw is high but steady |
| Circular saw | 1,200–1,500 | 2,000–2,500 (motor surge) |
The pattern jumps out once you see it laid out. Electronics and heating-type loads barely move between the two columns. Motor-driven gear spikes hard for a moment, and the microwave is the odd one out, it pulls a lot but steadily, with almost no surge. That mix is exactly why you can't judge a load by a single number.
The traps worth sidestepping
Sizing to the running column alone is the classic slip. The math looks fine on paper, then the generator trips the instant the fridge compressor fires.
Stacking every starting figure together is the opposite mistake. In real life motors rarely start at the same instant, so you size for the single largest surge landing on top of the running load, not for all of them at once.
And it's easy to assume a big running draw means a big surge. The microwave says otherwise, high steady pull, no real spike, while a modest little pump can surge hard. They're different problems and they need different headroom.
The numbers don't change the safety rules
Understanding all this keeps you from overloading the generator, but it doesn't change how the machine has to be run. Outdoors only, well clear of windows, doors, and vents, never in a garage or any closed space, because the exhaust carries carbon monoxide you can't see or smell. And it's never wired into the house through a regular outlet; that job needs a transfer switch and a licensed electrician.
The complete rundown is here: Portable Generator Safety guide.
How this plays out across the lineup
Every model lists both numbers, so you can size honestly instead of hoping:
| Model | Continuous | Peak |
|---|---|---|
| S2500iS | 1.9 kW | 2.5 kW |
| S3200iS | 2.6 kW | 3.2 kW |
| S3600iS | 3.0 kW | 3.6 kW |
| S4000iS (Open-Frame) | 3.2 kW | 4.0 kW |
| S4500iS | 3.6 kW | 4.5 kW |
Match your running total to the continuous rating with headroom, then check the peak clears your biggest surge. The inverter models also deliver clean, steady power, which matters if you're running sensitive electronics. Learn more
When your load outgrows the range
If the running load by itself is already close to the top model's continuous rating, or several large motors have to start close together, this portable class may not be enough on its own. Trim what runs at the same time, stagger the startups so surges don't collide, or run your numbers past our team before buying.
Questions people ask
Which number do I size from?
From running watts for the steady load, then make sure the peak rating covers the single biggest startup surge.
Do all appliances have starting watts?
No. Motor-driven ones, fridges, pumps, AC, tools, surge hard. Electronics, lights, and chargers barely do.
Why did my generator trip when the fridge came on?
The fridge's startup surge likely pushed the total past the generator's peak for that instant. Size with more surge headroom, or run less when motors start.
Is 'surge watts' the same as 'starting watts'?
Yes. Surge, peak, and starting watts all name the same brief spike at startup.
Last updated: July 3, 2026 · Reviewed by: SIOKIUU Power Support