Generator Sizing Guide

Choosing the correct generator starts with the load.

Correct generator sizing is more than adding the wattage printed on equipment labels. Running load, motor starting current, phase configuration, operating duty, site conditions and future expansion can all affect the generator capacity required.

1. Start with what must operate during an outage

List the essential equipment that must run from the generator. This may include motors, pumps, compressors, refrigeration, air conditioning, lighting, office equipment, production machinery and other critical electrical loads. Separating essential loads from non-essential loads can prevent unnecessary oversizing.

2. Understand kVA, kW and power factor

Generator output is commonly expressed in kVA while many electrical loads are stated in kW. The relationship depends on power factor: kW = kVA × power factor. For example, at a 0.8 power factor, a 100 kVA generator represents 80 kW. Final sizing still needs to consider the actual load type and starting requirements.

3. Separate running load from starting load

Electric motors, pumps, compressors and refrigeration equipment can draw substantially more current while starting than during normal operation. A generator that carries the normal running load may still be too small to start the largest motor reliably. Starting method and whether large loads start together or in sequence also matter.

4. Confirm single phase or three phase

The site electrical system determines the generator phase arrangement. Confirm whether the installation is single phase, three phase or a mixed-load application. Three-phase load balance should also be considered when planning the final distribution.

5. Decide how the generator will operate

Tell us whether the generator is for automatic mains-failure standby power, prime power, solar backup, manual backup or another operating requirement. The duty and expected operating hours influence the generator and control configuration.

6. Allow for the site and future load

Altitude, ambient temperature, ventilation, enclosure requirements, exhaust routing, available installation space and future electrical expansion can affect the final selection. A modest allowance for known future load can be sensible, but excessive oversizing should be avoided.

7. Generator loading and fuel consumption

Diesel-generator fuel consumption changes with electrical load. GenTech specification pages show consumption at 25%, 50%, 75% and 100% load where confirmed data is available. These figures are useful for comparing expected operating costs, but actual consumption can vary with engine model, site conditions, load profile and generator condition.

8. For complex loads, use real electrical information

For factories, large motors, compressors, pumps, laboratories, commercial buildings and other critical applications, provide an equipment list, motor data, electrical drawings or measured load information where available. This gives the GenTech team a much better basis for generator selection than a rough estimate.