What Size Generator Do I Need?

To find the generator size, add up the running power of your equipment, then add the extra power the largest motor draws at start-up; apply a safety margin of approximately 20% and divide by the 0.8 power factor to get the kVA rating. In short: kVA ≈ (running load + largest starting difference) × 1.2 ÷ 0.8. The motor starting method (direct-on-line, star-delta, soft starter) is the factor that changes the result most.

Step 1: List the loads

List all the equipment the generator will supply and write down the active power (kW) of each. This information is usually on the equipment rating plate. When preparing the list, separate:

• Resistive loads: Lighting, heaters, ovens, socket loads. They draw no extra power at start-up.
• Motor loads: Pumps, booster pumps, lifts, compressors, fans, on-off air conditioners. They draw several times their running power at start-up.
• Electronic loads: Computers, UPS units, drives (VFDs), LED drivers. They can generate harmonics; for large UPS and drive loads, the manufacturer’s recommendation should be obtained.

Including only the loads that really need to run during an outage prevents choosing an unnecessarily large generator.

Step 2: Add up the running load

Add up the power of the equipment that can run at the same time. This is the running load the generator must supply continuously. If there are several identical units, remember to multiply by the quantity.

Step 3: Add the largest starting load

Induction motors draw a high current for a short time at start-up. The generator must handle this moment without an excessive drop in voltage and frequency. The typical multipliers used in our power calculator are:

Starting method	Power drawn at start-up (approximate)
Direct-on-line (DOL) starting	~3 times running power
Star-delta	~2 times running power
Soft starter	~1.5 times running power

Motors do not usually start at the same time. So the calculation adds not the starting difference of every motor, but the largest single starting difference: extra power = motor power × (multiplier − 1). Where motors may start simultaneously (e.g. all pumps coming online together when the mains returns), either staggered start-up should be arranged or the calculation increased accordingly.

Step 4: Safety margin and kVA conversion

A margin of approximately 20% is added on top of the peak load. This margin covers future loads, calculation uncertainties and keeps the generator from running at its continuous limit. Then convert to kVA using the 0.8 power factor, the standard assumption for diesel generators:

kVA = kW ÷ 0.8

Worked example

A small business: lighting and socket loads 6 kW, office and inverter air conditioners 4 kW, one 11 kW pump. Running load: 6 + 4 + 11 = 21 kW.

Pump starting	Starting difference	Peak load	×1.2	÷0.8
Direct-on-line	11 × 2 = 22 kW	43 kW	51.6 kW	≈ 65 kVA
Star-delta	11 × 1 = 11 kW	32 kW	38.4 kW	≈ 48 kVA
Soft starter	11 × 0.5 = 5.5 kW	26.5 kW	31.8 kW	≈ 40 kVA

Simply by changing the pump’s starting method, the same site can run on a 40 kVA generator instead of approximately 65 kVA. This is why using a soft starter or variable frequency drive on large motors directly affects the generator investment.

Common mistakes

• Forgetting the starting load: On a site where the running load looks small, a lift or pump start can overload the generator.
• Confusing kW and kVA: A 100 kVA generator is not enough for a 100 kW requirement; approximately 125 kVA is needed.
• Oversizing: A diesel engine running continuously at very low load can suffer from unburnt fuel build-up (wet stacking). An oversized generator increases both investment and operating costs.
• Ignoring the standby/prime distinction: A generator that will run continuously should be selected on prime power, not standby; see our article on standby vs prime vs continuous power for details.
• Not accounting for high altitude and temperature: Engine power can fall at high altitude and in hot environments (derating); values should be checked against the manufacturer’s data sheet.

When is a professional analysis needed?

This method is sufficient for a preliminary selection. For lift groups, large pumping stations, loads dominated by UPS units and drives, medical equipment, data centres and industrial plants, however, voltage dip, harmonics and load steps should be analysed in detail. This analysis is based on the motor and alternator data sheets.

Run the calculation now

Use the power calculator to select your equipment, set the starting method and see the result instantly; then list matching models with the generator finder. For project-specific support on Terex Diesel sets in the 10–4,125 kVA range, contact us.