Diesel generator sizing guide: how to calculate the kVA your site really needs

A diesel generator that is too small trips the moment a big motor starts. One that is too big costs more to buy, burns more fuel per unit of power and suffers from running lightly loaded. Getting the size right is the single most important decision when you buy a DG set.

This guide walks through a practical sizing method you can check with your electrical consultant or supplier.

kVA vs kW

Generators are rated in kVA (apparent power), but your equipment consumes kW (real power). The two are linked by the power factor:

kW = kVA × power factor

Generator ratings normally assume a power factor of 0.8. So a 100 kVA generator delivers about 80 kW of real power. If your loads have a poorer power factor than 0.8 (common with many motors and older equipment), you need more kVA for the same kW.

Step 1: list your loads

Make a list of every load the generator must supply at the same time. For each, note:

• rated power (kW or HP; 1 HP is about 0.746 kW)
• whether it is a motor, and how it starts (direct-on-line, star-delta, soft starter, VFD)
• whether it runs continuously or intermittently
• single-phase or three-phase

Include lighting, air conditioning, compressors, pumps, lifts, IT equipment and machinery. Don't forget loads that switch on automatically, like pumps and cold-room compressors.

Step 2: add the running load

Add up the running kW of everything that can operate at the same time. This is your continuous running load. Be realistic: if two machines never run together, don't count both, but don't underestimate either.

Step 3: allow for motor starting

Electric motors draw several times their running current for a few seconds when they start. A direct-on-line motor can draw roughly 5 to 7 times its running current; star-delta starters reduce this, soft starters reduce it further, and variable frequency drives (VFDs) reduce it most.

The generator must handle the largest motor starting while everything else is already running. This is often what decides the generator size, not the running load. Starting large motors one after another, rather than together, can significantly reduce the size you need.

Step 4: convert to kVA and add margin

Convert your peak kW to kVA using the power factor, then add a margin for future growth. A common rule of thumb is to size so the generator runs at roughly 70 to 80% of its rating under normal load. This leaves headroom for motor starting and expansion while keeping the engine properly loaded.

Why oversizing hurts

Diesel engines don't like running lightly loaded for long periods. Below roughly 30 to 40% load, combustion temperatures stay low, unburnt fuel and carbon build up in the exhaust system (often called "wet stacking"), and maintenance needs rise. If your load varies a lot, consider two smaller generators running in synchronisation instead of one large one.

Prime, standby and continuous ratings

Generators have different ratings depending on use:

• Standby (emergency) rating: for backup during grid failures, limited hours per year.
• Prime rating: for use as the main power source with a variable load, unlimited hours.
• Continuous rating: for a constant load running all the time.

Make sure you compare quotes on the same rating, because the same set can carry different numbers under each.

Emissions and noise rules in India

In India, diesel generators up to 800 kW must meet CPCB IV+ emission norms, which took effect in 2023. These norms require cleaner engines with after-treatment, and generators come fitted with acoustic enclosures to meet noise limits. Ask the supplier for the type approval certificate for the exact model, and confirm it applies to the rating you are buying.

Fuel consumption

Fuel is the largest running cost over a generator's life. Ask each supplier for:

• fuel consumption in litres per hour at 25%, 50%, 75% and 100% load
• fuel tank capacity and running hours at 75% load

Compare models at the load you actually expect to run, not only at full load.

Controls and changeover

• AMF (automatic mains failure) panel: starts the generator automatically when grid power fails and stops it when power returns.
• ATS (automatic transfer switch) or changeover panel: switches loads between grid and generator safely.
• Synchronisation panel: lets two or more generators share load.
• Remote monitoring: alerts on fuel level, faults and running hours.

Installation and service

• Plan ventilation, exhaust routing and a level foundation.
• Check access for fuel delivery and maintenance.
• Ask about the service network near your site, response time, availability of spares, and the cost of an annual maintenance contract.
• Clarify warranty terms: years or running hours, whichever comes first.

Worked example (simplified)

A small factory has 40 kW of lighting, office and small machinery load, plus one 30 kW compressor motor started star-delta, and power factor around 0.8.

• Running load: 40 + 30 = 70 kW, about 87.5 kVA.
• The compressor's starting surge must be absorbed while the other 40 kW is running.
• Sizing for around 75% normal loading and allowing for the motor start, a set in the 125 kVA range would be a reasonable starting point, to be confirmed by the supplier's sizing calculation.

This is only an illustration. Always have the supplier or a consultant confirm the size using the motor data and starting method.

Frequently asked questions

How do I convert kVA to kW?

Multiply kVA by the power factor. Generators are usually rated at 0.8 power factor, so 100 kVA gives about 80 kW.

Can I run a generator at 20% load all the time?

It is not recommended. Long periods of light loading cause carbon build-up and wet stacking in diesel engines. If your load is low most of the time, consider a smaller set or two sets in synchronisation.

Why does motor starting matter so much?

A motor started direct-on-line draws several times its running current for a few seconds. The generator must handle that surge while carrying other loads, so the largest motor often decides the generator size.

What does CPCB IV+ mean for buyers?

It is India's current emission standard for diesel generators up to 800 kW. Compliant sets use cleaner engines with after-treatment and come with type approval certificates. Make sure the model and rating you buy are covered.

Source generators on GlobalMart

Compare manufacturers and dealers in diesel generators and the wider power generation category. When you post a requirement, include your load list, the largest motor and its starting method, standby or prime use, site location and the date you need it running.