Generator power explained
How to use the Blackwell generator power calculator
Select the conversion you need, enter the known value and choose the electrical supply. For alternating-current calculations, use the voltage and power factor shown on the equipment nameplate or electrical design. The result updates immediately and can be printed for your project notes.
Most UK commercial and industrial generator installations use 400 V or 415 V, 50 Hz, three phase. Smaller property, agricultural and site applications may use 230 V single phase. Export projects can require different voltages or 60 Hz equipment, so always confirm the destination electrical standard.
Standard kVA, kW and amps formulas
| Calculate | Single phase | Three phase |
|---|---|---|
| kVA from amps | I × V ÷ 1,000 | I × V × √3 ÷ 1,000 |
| kW from amps | I × V × PF ÷ 1,000 | I × V × √3 × PF ÷ 1,000 |
| Amps from kW | kW × 1,000 ÷ (V × PF) | kW × 1,000 ÷ (V × √3 × PF) |
| Amps from kVA | kVA × 1,000 ÷ V | kVA × 1,000 ÷ (V × √3) |
In these formulas, I is current in amps, V is voltage and PF is power factor. Three-phase calculations use √3, approximately 1.732. One mechanical horsepower is equivalent to 746 watts.
kVA and kW are not interchangeable
A generator's kVA rating describes apparent power, while kW describes real power available to the load. The relationship is determined by power factor: kW = kVA × PF. At the 0.8 power factor commonly used for generator ratings, a 100 kVA generator corresponds to 80 kW.
That conversion does not mean every 80 kW load is suitable for a 100 kVA set. The nature and sequence of the connected equipment can be more important than the running total.
Motor starting current and step loads
Pumps, compressors, fans, refrigeration equipment and other motors can draw several times their normal current while starting. If a large motor starts across the line, the generator may need additional alternator and engine capacity to keep voltage and frequency within acceptable limits.
Record the largest motor, its starting method and whether loads start together or in stages. Soft starters, variable-frequency drives and managed load sequencing can change the required generator size. UPS systems, transformers, welders and non-linear electronic loads also need application-specific assessment.
How to calculate the generator capacity you need
- List every circuit and item that must run during a mains failure or off-grid operation.
- Record voltage, phase, running amps or kW, power factor and motor efficiency where available.
- Convert every load to consistent kW and kVA values, then establish the realistic simultaneous maximum.
- Identify the largest starting load and the order in which equipment will connect.
- Allow for site temperature, altitude, duty rating, future expansion and any required load reserve.
- Have the final selection checked against the generator engine, alternator and control-system performance.
Use our diesel generator buying guide for the wider specification process, or compare available models in the Blackwell generator catalogue.