What size genset do you need? kVA, kW and a worked example

Most people buying a diesel generator ask for a size in kVA: 15 kVA for a shop, 62.5 kVA for a small factory, 125 kVA for a hospital wing. But the things you want to run are labelled in watts or kilowatts. Getting from one to the other is simple once you know three things.

kVA and kW are not the same

A genset's kVA rating is its apparent power. The kW figure is the real power it can deliver to your load. The two are linked by the power factor:

kW = kVA × power factor

Generator sets in India are rated at a power factor of 0.8. So:

Genset rating	Real power it can supply
15 kVA	12 kW
62.5 kVA	50 kW
125 kVA	100 kW
500 kVA	400 kW

If a dealer quotes only kVA, multiply by 0.8 to see what you actually get.

Step 1: add up what will run at the same time

List everything you want on backup power and write its rating in kW (1 kW = 1,000 W). Count only what will run together. A shop does not run its air conditioner and its water pump at full load at the same moment in every hour, but plan for the busiest hour you expect.

Step 2: allow for motors

Anything with a motor (pumps, compressors, air conditioners, lifts) draws several times its running current for a second or two while it starts. A motor started directly on line can draw five to seven times its running current. The genset has to carry that surge without the voltage collapsing.

Two practical ways to handle it:

• size the genset with room for the largest motor's start, or
• fit a soft starter or star-delta starter to big motors, which cuts the surge.

Step 3: don't run it flat out, or nearly empty

Makers generally advise that a diesel genset runs best at roughly 70 to 80 percent of its rating for long periods. Run it far below that for months and the engine does not get hot enough; unburnt fuel builds up (diesel engineers call it wet stacking). Run it at 100 percent all day and it wears faster.

A worked example: a small shop

Load	kW
Lights, fans, two computers, billing machine	3.0
Refrigerator and display cooler	1.5
One 1.5-ton split air conditioner	1.8
Water pump, 5 HP (3.7 kW)	3.7
Total running load	10.0

10 kW at a 0.8 power factor is 12.5 kVA. To keep the set near 75 percent load, divide by 0.75: about 16.7 kVA. Add the pump's starting surge and the sensible choice is a 20 kVA set, or a 15 kVA set if the pump has a soft starter and never starts while the air conditioner does.

These are planning numbers. Before you buy, give the dealer your actual load list and ask them to confirm the size in writing.

Prime or standby?

Spec sheets often carry two ratings for the same set, defined in the international standard ISO 8528-1:

• Standby: for power cuts only. The set is not meant to run for long hours at this rating.
• Prime: for running as your main power source for long hours at a load that goes up and down.

If the genset will run for many hours a day, size it on the prime rating, which is the lower of the two.

The rule every new genset now meets

Diesel gensets up to 800 kW sold in India since 1 July 2023 must meet the CPCB IV+ emission norms. These sets carry newer engine and exhaust technology and generally cost more than the older CPCB II models they replaced. Ask the dealer for the CPCB IV+ type approval certificate.

Who makes them

India's listed engine makers that sell gensets include Cummins India, Kirloskar Oil Engines and Greaves Cotton. Their quarterly results are on each company's page here.

Sources: ISO 8528-1 (rating definitions); Central Pollution Control Board, CPCB IV+ norms for diesel gensets up to 800 kW (in force from 1 July 2023). The worked example uses typical appliance ratings and is for planning only.