Sizing a Generator for a Construction Site

An undersized generator trips out or struggles every time a large motor starts. An oversized one costs more to hire and to fuel, and a diesel set that runs for long periods at a light load can suffer from wet stacking: unburnt fuel building up in the exhaust. Getting the size right starts with a list.

List every load

Write down everything the set will power, from cabins and lighting to pumps, compressors, tools and welders, with each item’s rated power and how many run at the same time. Cabins and lighting draw steadily; pumps and hoists come and go.

Allow for starting current

Electric motors draw several times their running current for a moment as they start. Submersible pumps, compressors and hoists are the usual culprits. Size the set to carry the steady load plus the largest motor starting, and start large motors one at a time rather than together.

kVA and kW

Generators are commonly rated in kVA and loads in kW. The difference is the power factor. Many industrial generators are rated at a power factor of 0.8, so a 100 kVA set delivers about 80 kW. Make sure the figures you compare are in the same unit.

Other things to settle

• Single-phase or three-phase supply, and the voltage your equipment needs.
• A sound-attenuated canopy where noise is limited.
• Fuel tank size, bunding and how often the set will need refuelling.
• Two smaller sets in parallel where demand swings between day and night.
• Distribution and earthing designed and checked by a qualified electrician.

Browse our generators and compressors, or send us your load list and we will help you size the set.