Get the size right the first time
Pick a generator too small and it trips, stalls your tools and wears out fast. Pick one too big and you pay more upfront and burn extra petrol every day. The right size sits comfortably above your real load with a little room to spare. Here is how to work it out for a construction site, step by step.
This article covers petrol generators. See the full GenEx generator range for portable and silent models.
Step 1: list every tool that runs at once
Walk the site in your mind. What actually runs together? Lights and fans run all day. A grinder and a drill might run together. A mixer or a pump runs when needed. You are not sizing for every tool you own, only for the ones switched on at the same time.
Write each one down with its wattage. The watt rating is on the tool label or in the manual. If a tool is marked in amps, multiply amps by 230 to get roughly the watts.
Step 2: add up the running watts
Add the watts of everything that runs together. As a rough guide for common site tools:
- LED flood lights: 50 to 200 watts each
- Hand drill: 600 to 900 watts
- Angle grinder: 1,000 to 2,500 watts
- Small concrete mixer: 1,500 to 2,200 watts
- Water pump: 750 to 2,200 watts
- Vibrator: 1,000 to 2,000 watts
These are general figures, so always check your own tool labels. Say your total running load comes to 4,000 watts.
Step 3: add startup surge for motors
This is the step people skip, and it is why their generator trips. Any tool with a motor, like a pump, mixer, grinder or compressor, pulls two to three times its running watts for a second when it starts. A 2,000 watt mixer can demand 5,000 watts at the moment of startup.
So take your biggest motor and add its extra surge on top of the running total. If your steady load is 4,000 watts and your largest motor adds 3,000 watts of surge, plan for around 7,000 watts at the peak.
Step 4: convert watts to kVA and add headroom
Generators are rated in kVA. The simple conversion: 1kVA gives about 800 usable watts. So divide your peak watts by 800 to get the kVA you need.
For a 7,000 watt peak, that is about 8.75kVA. Round up, not down. And never plan to run a set at full rating. Keep steady use to about 70 to 80 percent so the engine stays cool and lasts. That points to a 9.5kVA set for this example.
A worked example
| Load | Running watts |
|---|---|
| 4 LED flood lights | 600 |
| Angle grinder | 1,500 |
| Hand drill | 800 |
| Concrete vibrator | 1,500 |
| Running total | 4,400 |
| Surge from vibrator at startup | +3,000 |
| Plan for peak | 7,400 |
That peak points to the GenEx 9.5kVA Portable Petrol Generator for comfortable headroom.
Matching common sites to GenEx models
For a small site running lights and a couple of hand tools, the GenEx 3.5kVA Portable Petrol Generator is enough. For a typical site with drills, a grinder and a small mixer, the GenEx 5.5kVA Portable Petrol Generator handles the load. When you add a pump or compressor to the mix, step up to the GenEx 7.5kVA Portable Petrol Generator. Large sites running several tools together suit the 9.5kVA.
One last tip
If your loads will grow as the project moves through stages, buy one size up now. It costs a little more, but it saves you renting a second set or replacing the generator mid-project. If you are unsure, send HMS your tool list and we will size it for you. Contact us for the current price and stock.
