Undersizing a generator is the single most expensive mistake crews make with temporary power. A unit that is too small won't just run out of headroom — it will brown out, overheat, and eventually cook the motors on the tools you plugged into it. Oversizing has its own cost: you burn extra fuel, carry extra weight, and pay for capacity you never touch.
This guide walks through how to size a generator properly, in the order the decisions actually matter.
1. Starting watts vs running watts
Every motor-driven tool pulls a surge when it starts, then settles into a lower steady draw. That surge is what sizes your generator, not the number on the nameplate.
- Running watts — the continuous draw once the tool is up to speed.
- Starting watts — the inrush at startup, typically 2x to 3x running watts for a universal motor and as much as 5x to 7x for a capacitor-start induction motor like the one on a compressor or a big drill press.
The rule: add up the running watts of everything that will run at once, then add the single largest starting surge on top of that. You do not add every surge together, because you will never start every motor at the same instant — but you must have headroom for the worst one.
A worked example
Say a concrete crew is running a 1,800W vibrator, 900W of string lights, and a 1,500W (running) walk-behind saw whose motor surges to 4,500W.
- Running total: 1,800 + 900 + 1,500 = 4,200W
- Largest surge above running: 4,500 - 1,500 = 3,000W
- Minimum generator: 7,200W
An 8,000-watt ChoreMaster covers this with a margin. A 6,500W unit does not, and you would find that out the hard way the first time the saw kicked on under load.
2. Add 20 to 25 percent headroom — always
Generators are rated at sea level, at moderate temperature, on fresh fuel, with a clean air filter. Real jobsites are none of those things. Output drops roughly 3 to 4 percent per 1,000 feet of elevation and again in high heat.
Sizing to exactly your calculated load means running the unit at 100 percent continuously, which is where fuel consumption spikes and engine life collapses. Target 70 to 80 percent of rated capacity as your normal working load.
3. Decide on your voltage and phase before you shop
This is where most buyers get stuck, and it is worth settling early because it eliminates most of the catalog immediately.
- 120V only — hand tools, lights, chargers. Fine for a punch-list crew, useless for machinery.
- 120/240V single-phase — the default for most contractor work. Covers welders, compressors, most saws and pumps.
- 480V three-phase — required for larger industrial machinery, big pumps, tower cranes, and most permanently installed shop equipment. A 23kW 480V unit is a different class of machine from a portable, and you cannot adapt your way there from a 240V portable.
If the machine you are powering has a three-phase motor, you need three-phase power. Phase converters exist but they cost capacity and introduce their own failure points.
4. Match the fuel and runtime to the job
Gasoline units are cheaper up front, start easily in cold weather, and are the right answer for intermittent use. Diesel costs more but delivers better fuel economy under sustained load, longer engine life, and far better performance when the generator is going to run eight or ten hours a day, every day.
Check the runtime at 50 percent load, not at 25 percent — manufacturers quote the flattering number. If a unit runs 11 hours at quarter load, expect roughly half that in real work.
5. Consider a hydraulic power pack instead
If your actual need is to drive a single piece of hydraulic equipment — a wall saw, a wire saw, a core drill — a generator is the wrong tool. A dedicated hydraulic power pack delivers the flow and pressure that equipment expects, more efficiently and more quietly than a generator plus a separate pump.
Quick reference by crew size
- 2,000 - 3,500W — hand tools, lighting, a small pump. One or two workers.
- 5,000 - 8,000W — the workhorse range. A small crew with a saw, a mixer, lights, and chargers.
- 9,000 - 16,000W — multiple simultaneous machines, larger welders, sustained daily use.
- 20kW and up, 480V three-phase — industrial machinery, permanent installations, powering a shop rather than a task.
Before you buy, confirm three things
- The surge rating, not just the running rating.
- The receptacle configuration — a generator with the right wattage and the wrong outlets is still the wrong generator.
- Whether you need a transfer switch, if this will ever back-feed a building.
Still unsure? Tell us the machines you need to run and we will size it for you. Browse the full generator and power range, or request a quote and we will come back within 24 hours.