How to size a generator for a construction site
Undersize a generator and it trips when a motor starts; oversize it and you pay for fuel and hire you do not need. Here is a simple way to get the kVA right.
kW and kVA
Loads are usually listed in kilowatts (kW), the real power that does the work. Generators are rated in kilovolt-amperes (kVA), the apparent power they can supply. Most diesel generators are rated at a power factor of 0.8, so a 100 kVA generator supplies about 80 kW. To convert, divide your total kW by 0.8.
Step 1: list the running loads
The figures below are an example. Use the ratings from your own equipment.
| Load | Quantity | Running power each | Total |
|---|---|---|---|
| Site office cabins with air conditioning | 3 | 6 kW | 18 kW |
| Lighting and small power | 1 | 4 kW | 4 kW |
| Welding machines | 2 | 8 kW | 16 kW |
| Submersible dewatering pump | 1 | 11 kW | 11 kW |
| Total running load | 49 kW |
Step 2: allow for motor starting
Electric motors draw several times their running current as they start. The generator has to carry the starting surge of the largest motor while everything else is already running.
| Starting method | Typical starting current |
|---|---|
| Direct-on-line | 6 to 8 times full-load current |
| Star-delta | 2 to 3 times full-load current |
| Soft starter | 2 to 4 times full-load current |
| Variable-speed drive | 1 to 1.5 times full-load current |
In the example, the 11 kW pump started direct-on-line is the largest motor. Its start is the moment the generator is most likely to struggle, so check that case with your supplier instead of relying on the running load alone.
Step 3: convert and add a margin
The running load of 49 kW divided by 0.8 is about 61 kVA. Adding 20 to 25 percent for motor starting, unknown loads and future additions gives roughly 75 to 80 kVA, so choose the nearest standard size above that.
Step 4: check the conditions
- Heat: generators lose output at high ambient temperatures, and Gulf summers often exceed the conditions a generator is rated at. Ask for the derated output at your site temperature.
- Frequency and voltage: the UAE, Oman, Qatar, Kuwait and Bahrain use 50 Hz, while Saudi Arabia uses 60 Hz. Confirm the voltage and phase your equipment needs.
- Light loads: running a diesel generator for long periods at very low load can cause wet stacking, so avoid gross oversizing.
- Run time: tank size and refuelling arrangements decide how long the generator runs between fills.
- Noise: canopied, sound-attenuated generators help where a site is close to homes or offices.
Fuel and run time
As a rough guide, a diesel generator uses about 0.25 to 0.3 litres of fuel per kWh it produces, and is most efficient between about 50 and 80 percent of its rated load. In the example above, a 49 kW load would burn roughly 12 to 15 litres an hour. Use the manufacturer’s fuel table for the exact figure, and plan refuelling around your site hours.
For sites in the UAE, send us your equipment list and the largest motor, and we will propose a generator from our 20 to 500 kVA hire range.
Frequently asked questions
How do I convert kW to kVA?
Divide kW by the power factor. With the usual 0.8 rating, 80 kW divided by 0.8 is 100 kVA.
Is a bigger generator always safer?
No. Gross oversizing wastes fuel and hire, and long periods at very light load are bad for diesel engines. Size for the real load plus a sensible margin.
Can one generator run 50 Hz and 60 Hz equipment?
Not at the same time. A generator is set up for one frequency, so match it to the country and the equipment it will power.
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