The honest way to size a generator is to stop thinking about the number on the box and start with what you actually plan to run. Make a list of the things you'd have going at the same time, add up the power they draw while running, and note the one that demands the biggest jolt when it starts. You want a model whose continuous rating covers the running total with a bit to spare, and whose peak rating covers that biggest startup jolt. Do that and you'll land on the right unit instead of guessing.
Who this is for
This is written for anyone picking a portable generator to keep the essentials on during an outage, to power an RV or a campsite, or to run tools on a small jobsite. It's built around units in the 2,500 to 4,500-watt range, which is enough to carry the things that matter, not to run an entire house at once. If your goal is to run central air, electric heat, and the whole panel together, that's a different class of machine and a different conversation.
The two numbers behind every sizing decision
Sizing goes wrong when people look at one number and ignore the other. Every device you plug in has two.
The first is its running draw, the power it uses while it's working normally. The second is its startup draw, a brief surge that anything with a motor, a fridge, a pump, an AC unit, a saw, pulls for a second or two when it first kicks on. That surge can be several times the running figure, and then it vanishes.
Your generator is described the same way. Its continuous rating is what it can supply steadily, and that has to cover your total running draw. Its peak rating is a short-term ceiling for surges, and that has to cover your single biggest startup on top of whatever is already running.
One thing worth knowing before you shop: a generator's model number is usually its peak figure, not the continuous power it can hold all day. Size from the continuous number first. There's a fuller explanation in Running Watts vs. Starting Watts.
Four steps to the right size
Work through these in order:
- List only what runs together. A fridge, a few lights, the Wi-Fi, and a phone on the charger is a realistic set; you rarely run everything you own at once.
- Add up the running watts of that set. This total has to sit under the generator's continuous rating, not right at it.
- Find the single largest startup surge in the group, usually the fridge or a pump. The peak rating needs to cover that surge on top of the running load already in use.
- Leave yourself twenty to twenty-five percent of headroom. A generator that never runs flat out stays cooler and quieter, lasts longer, and shrugs off surges without tripping.
A quick example. Say your outage essentials come to about 1,800 running watts together, and the refrigerator is the biggest surge in the group. You'd want a model rated comfortably above 1,800 watts continuous, with enough peak capacity to absorb the fridge's startup, which points you toward the middle of the lineup rather than the smallest unit.
Where people go wrong
The most common error is adding up every appliance in the house. You only need to cover what runs at the same time, not the whole inventory.
Close behind is ignoring the startup surge entirely. A generator that handles the running load with ease can still trip the moment a motor kicks in, if its peak rating is too low.
Two smaller ones round it out: sizing right to the limit, which leaves no headroom, and reading the model number as the real continuous wattage, when it's usually the peak. And bigger isn't automatically better, an oversized unit costs more, drinks more fuel, and is heavier to haul out for power you may never draw.
A word on safety while you plan
Sizing is about capacity, but capacity and safety go together. A generator runs outdoors only, never in a garage, basement, shed, tent, or any enclosed space, because its exhaust carries carbon monoxide. Keep it well away from windows, doors, and vents, with the exhaust aimed away from the house and your neighbors.
And never wire a generator into your home through a regular outlet. If you want it connected to your house circuits, a qualified electrician installs a transfer switch to do it safely. The full list is here: Portable Generator Safety guide.
Matching your load to the lineup
Once you know your running total and your biggest surge, here's the rough fit across the range. Continuous is the number to size from; peak is your surge headroom.
| Model | Continuous | Peak | Suits |
|---|---|---|---|
| S2500iS | 1.9 kW | 2.5 kW | Lightest essentials, charging, a CPAP |
| S3200iS | 2.6 kW | 3.2 kW | Small home backup, RV essentials |
| S3600iS | 3.0 kW | 3.6 kW | Mid-range backup, RV, light tools |
| S4000iS (Open-Frame) | 3.2 kW | 4.0 kW | Jobsite and heavier mixed loads |
| S4500iS | 3.6 kW | 4.5 kW | Largest supported loads, tough surges |
Size from the continuous rating first, then confirm the peak covers your biggest surge. Caught between two models, take the larger one for margin. If you need more than the top unit gives you, two identical models can be paralleled with a kit for roughly 80% of their combined continuous rating; for two different models, check with our technical support first. Learn more
When this range won't cut it
If what you really need is to run central air, electric heat, a well pump, or your whole panel all at once, the 2,500 to 4,500-watt portable class isn't the answer. The better move then is to cut down what runs at the same time, run the heavy items one at a time, or talk it through with our team before buying. We'd rather point you to something that does the job than sell you a unit that can't.
Questions people ask
Do I size from the continuous rating or the peak?
From the continuous rating for your running load, then use the peak as headroom for the biggest startup surge.
How much headroom should I leave?
Around twenty to twenty-five percent above your continuous load, so the generator isn't working at its limit.
Can I just add up the wattage labels on my appliances?
No. That ignores startup surges and assumes everything runs at once. Use the four-step method above.
Two models both seem to fit. Which do I pick?
The larger one. Headroom is cheap insurance against overload trips.
Can I combine two generators for more power?
Yes, two identical models parallel for about 80% of their combined continuous rating. For different models, check with support first.
Last updated: July 3, 2026 · Reviewed by: SIOKIUU Power Support