How to size a generator for your site
Work out the kVA you need: list your loads, allow for motor starting current, convert kW to kVA, and leave headroom for UAE heat. With a worked example.
By the WME team · Updated 1 November 2026
A generator that is too small trips or stalls when a motor starts. One that is far too big wastes fuel and, if it is diesel, runs badly at light load. Sizing it properly takes four steps and a list of what you want to run.
1. List every load and its running power
Write down each tool or machine that will run at the same time and its rated power in watts (W) or kilowatts (kW). You will find it on the rating plate or in the manual. Typical figures:
| Equipment | Typical running power |
|---|---|
| Angle grinder (230 mm) | 2,000 to 2,600 W |
| Rotary hammer | 800 to 1,500 W |
| Site lighting tower (LED) | 300 to 1,000 W |
| Concrete mixer (small) | 650 to 1,200 W |
| Submersible water pump (1 HP) | 750 W |
| Split AC unit (1.5 ton) | 1,500 to 2,000 W |
| Inverter welding machine (200 A) | 5,000 to 7,500 W at full output |
2. Allow for starting power
Anything with an electric motor draws much more power for a second or two as it starts. Compressors, pumps, mixers and air conditioners usually need 2 to 3 times their running power to start, and some induction motors need more. Add the starting surge of the largest motor to the running total of everything else: in practice not every motor starts at the same moment.
3. Convert kW to kVA
Generators are rated in kVA. To convert, divide kW by the power factor, which is 0.8 for most site generators:
kVA = kW ÷ 0.8. So 8 kW of load needs at least 10 kVA.
4. Add headroom, especially for UAE heat
Add 20 to 25 percent on top for future loads and so the engine is not running flat out. Generators are rated at standard test conditions; in 45 to 50°C summer heat the usable output drops. Check the manufacturer's derating figures for high ambient temperature, and size up if the set will work outdoors through the summer.
Worked example
- Running loads: two angle grinders (2 × 2.2 kW), lighting (1 kW), a 1 HP pump (0.75 kW) = 6.15 kW
- Largest starting surge: the pump at 3 × 0.75 = 2.25 kW, so add 1.5 kW over its running power
- Total: 6.15 + 1.5 = 7.65 kW
- In kVA: 7.65 ÷ 0.8 = 9.6 kVA
- With 25 percent headroom: about 12 kVA. A 12 to 15 kVA set suits this site.
Single phase or three phase?
Hand tools, lighting and small pumps run on single phase (220 to 240 V). Larger compressors, welding machines and motors often need three phase (380 to 415 V). If even one machine needs three phase, choose a three phase generator and spread the single phase loads evenly across the three phases.
Petrol or diesel?
| Petrol | Diesel | |
|---|---|---|
| Typical size | Up to about 10 kVA, portable | From about 10 kVA to several hundred kVA |
| Best for | Short jobs, light tools, backup | Long daily running, site power, standby |
| Running cost | Higher fuel use per kWh | Lower fuel use, longer engine life |
| Noise | Louder for its size unless silent type | Canopied (silent) sets available |
Diesel sets prefer a steady load. Running one at very light load for long periods causes unburnt fuel build-up (often called wet stacking), so do not oversize a diesel generator by a large margin.
Quick checklist before you buy
- Total running load in kW, plus the largest starting surge
- Converted to kVA and with 20 to 25 percent headroom
- Single or three phase
- Hours of running per day (decides petrol or diesel, and fuel tank size)
- Noise limits on site (canopied set needed?)
- Automatic transfer switch if it is standby power for a building
Not sure about your numbers? Send your load list to our sales team and we will suggest a size from our range.