Understanding Generator Sizing & kVA Calculations

Specifying temporary power for construction sites, events, and emergency backup systems requires strict attention to electrical loads and engine mechanics. Sizing a generator purely based on the total kW load without factoring in Power Factor (cos φ) or continuous running limits will result in catastrophic failure, voltage drops, and severe engine damage.

1. The Difference Between kW and kVA

Generators are rated in kVA (Apparent Power), which is the total power the alternator can produce. However, most equipment is rated in kW (Real Power), which is the actual power consumed to perform work.

• Real Power (kW): The amount of power converted into useful work (heat, light, mechanical force).
• Reactive Power (kVAR): The power drawn by inductive loads (like motors and transformers) to sustain their magnetic fields, which does no useful work.
• Apparent Power (kVA): The vector sum of kW and kVAR. It is the total power the generator must physically supply.

2. Power Factor (cos φ) Explained

The Power Factor is the ratio of Real Power (kW) to Apparent Power (kVA). Unless you are running purely resistive loads (like incandescent heaters where the PF is 1.0), you must account for reactive power.

For standard commercial and industrial site setups involving motors, power tools, and LED drivers, the industry standard Power Factor is 0.8. This means that a 100 kVA generator will only supply 80 kW of usable real power.

3. The 80% Continuous Running Rule

Diesel generators should never be run at 100% of their Prime Rating continuously. Doing so causes engine overheating and alternator degradation.

Conversely, running a diesel generator on a very light load (under 30%) causes "bore glazing" (unburnt fuel and oil baking onto the cylinder walls), destroying engine efficiency. The engineering best practice is to size your generator so that your maximum sustained, continuous load operates at exactly 80% of the generator's total capacity.