The two most common mistakes in choosing a generator are opposite in nature and equally expensive. The first is taking double the size «to be safe»: the set runs for years at 20% load, wet-stacks, burns fuel unburnt and fails earlier than its rated life. The second is adding up the nameplate figures and buying exactly that: the generator stalls every time a compressor starts.
kW and kVA are not the same figure
Generators carry two numbers. kVA is apparent power — what the alternator delivers. kW is active power — what actually becomes work: heat, rotation, light. The ratio between them is the power factor, and on most diesel sets it is 0.8.
So a 100 kVA set delivers 80 kW of active power. If a supplier says «a hundred», it is worth asking a hundred of what — the gap between 100 kVA and 100 kW is 20 kW, and that is exactly the difference that leaves a facility unable to start.
Your load has a power factor too. Incandescent lamps and heating elements are purely resistive. Motors, pumps, compressors and welding sets are inductive, with a factor between 0.6 and 0.85. The more motors on site, the wider the gap between what you «consume» and what the alternator must deliver.
Starting currents decide the rating more often than total load
An electric motor draws several times its rated current at start-up. A compressor, a lift, a pump or a chiller can briefly demand several times what it consumes in steady operation. A generator sized to the sum of steady-state figures will simply drop voltage and shut down at that moment.
This is why the specification must list not only total load but the largest single motor and whether starts are staggered or simultaneous.
Diversity: not everything runs at once
Adding up every nameplate on site gives a figure that never actually occurs. In practice a share of equipment is idle at any moment, and that share differs by facility type: a clinic, a warehouse and an office behave very differently.
The realistic approach is to define what must keep working during an outage — this is almost always a smaller list than «everything». Emergency lighting, the server room, medical equipment, heating circulation and lifts are typically essential; the rest can wait.
Headroom — but the right amount
Reasonable headroom is roughly 10–20% above calculated load: it covers future growth and keeps the set out of continuous full-load operation. Doubling the rating is not caution but harm — a diesel engine running lightly loaded for long periods deteriorates faster than one working near its design point.
What to include in the request
- A list of loads that must remain powered, with ratings.
- The largest motor and whether it starts simultaneously with others.
- How long the facility must run autonomously — this determines the fuel tank, not the rating.
- Installation: outdoors or in a plant room, canopy, exhaust routing, noise limits.
- Whether automatic start on mains failure is required (see our article on ATS).
We supply generating sets through direct imports, size them to the actual load and service what we deliver. From practice: 26 generators of 120 kW for humanitarian operations and 35 sets securing railway stations. Send the list of loads — or simply describe the facility — and our engineers will calculate the configuration.