How to Size a Generator for Your Project: kVA, kW & Starting Loads

Calculate What Will Actually Run

Start with the equipment that will operate when the generator is on.

This could include:

• Air conditioners
• Lighting
• Refrigerators and freezers
• Computers and servers
• Pumps and motors
• UPS systems
• Office equipment
• Machinery

Use the actual equipment wattage or nameplate rating whenever possible.

You also do not necessarily need to add every appliance in the building at 100%. Generator sizing should consider which loads are likely to operate at the same time.

Understand kW and kVA

Most diesel generators are sold in kVA, while electrical equipment is commonly rated in watts or kW.

They are related through power factor:

kW = kVA × Power Factor

A generator rated at 100kVA at a 0.8 power factor therefore provides approximately 80kW of real power.

This is why generator sizing should check both kW and kVA, not just one number.

Understand Prime and Standby Generator kVA Ratings

Generators are commonly rated for either prime power or standby power. These ratings describe how the generator is expected to operate.

Standby rating is intended for emergency backup during a power outage. The generator may operate for limited periods and is generally not designed to carry a continuous load for extended operation.

Prime rating is intended for applications where the generator is the main or regular source of power. It can operate for longer periods, subject to the manufacturer’s load and maintenance requirements.

The standby rating is usually higher than the prime rating for the same generator. You should therefore not select a generator based only on its largest advertised kVA figure.

For regular or extended operation, size the generator using its prime rating. For occasional emergency backup, the standby rating may be appropriate.

Always confirm the applicable rating, operating hours, load profile and maintenance requirements before making a final selection.

Allow for Motor and Air-Conditioner Starting Loads

Motors, pumps, compressors and conventional air conditioners can require several times their normal current for a few seconds when starting.

This temporary demand is one of the most important considerations when choosing a generator.

But it does not necessarily mean the generator must have a continuous rating equal to the full starting demand.

Generators and alternators can normally tolerate a higher current for short periods. The important question is whether the generator can start the equipment while maintaining acceptable voltage and frequency.

Our Generator Sizing Calculator therefore considers normal running load and short-duration starting demand separately.

Check Single-Phase and Three-Phase Loads

If you are sizing a three-phase generator, single-phase equipment should be distributed reasonably across: L1, L2 and L3.

You should not multiply every single-phase load by three, and you should not automatically assume all single-phase equipment is connected to one phase.

Good phase balancing allows better use of the generator’s available capacity.

Consider UPS and Battery-Charging Loads

A generator supplying a UPS may need to carry more than the equipment connected to the UPS.

After an outage, the UPS may simultaneously: support its load + recharge its batteries.

Large inverters, rectifiers and battery chargers can behave similarly.

These additional loads should be considered when sizing a standby generator.

How Much Generator Reserve Do You Need?

Some spare capacity is useful for load changes, future expansion and operating conditions.

But adding excessive safety margins at every stage can result in a generator that is unnecessarily large.

The objective is not to buy the biggest generator possible. It is to select one that can carry the running load, handle starting conditions and operate reliably with the correct prime or standby rating and sensible reserve capacity.