How to Size Commercial Standby Generators: kVA, Load and Phase Explained

Choosing the right size for commercial standby generators comes down to three questions: how much power your site really draws, how hard your equipment hits the generator when it starts, and whether your supply is single phase or three phase. Get those right and the generator starts cleanly and carries the load. Get them wrong and you either pay for capacity you never use or watch the set struggle the first time the mains fails.

This guide walks through the sizing process step by step, with a worked example you can adapt.

Why Sizing Matters

An undersized generator can overload, trip, or fail to start large motors. An oversized one costs more to buy, takes up more space, and can run inefficiently at a very light load. Proper sizing is the cheapest way to protect the investment.

A standby generator is permanently installed and starts automatically when mains power is lost, usually through an automatic transfer switch. Because it becomes part of the building's electrical system, it has to be matched to that system from the start.

Step 1: Understand kVA and kW

Generators are rated in kVA (apparent power), but most equipment is rated in kW (real power). The link between them is the power factor:

kVA = kW ÷ power factor

Generator sets are commonly rated at a power factor of 0.8. That means a 100 kW load needs roughly 125 kVA of generator capacity (100 ÷ 0.8). Always confirm the exact ratings for the model you are considering.

Step 2: Build Your Essential Load List

List everything you need to keep running during an outage, with each item's power rating. Decide early whether the generator will back up the whole site or only essential circuits. Backing up only critical loads often allows a smaller, cheaper set.

Typical items to consider:

• Emergency and general lighting
• Servers, networking and communications equipment
• Refrigeration and cold storage
• Pumps, compressors and lifts
• Heating, ventilation and air conditioning
• Production machinery
• Fire, security and access control systems

Not everything runs at once, so aim for the realistic maximum demand rather than the sum of every nameplate figure.

Step 3: Allow for Starting Loads

Motors, compressors, pumps and refrigeration units can draw several times their running current for a moment when they start. The generator must ride through that surge without the voltage dropping too far. On sites with large motors, the starting load, rather than the running load, can decide the generator size. Soft starters, variable speed drives or staggered starting can reduce the surge.

Step 4: Check Your Phase and Voltage

The supply type decides which generators are suitable. Single phase sets suit smaller commercial premises and light-duty applications, while three phase sets suit larger sites with higher demand and specialised equipment.

For three phase supplies, you can estimate the current a generator delivers with this formula:

kVA = √3 × volts × amps ÷ 1000

For example, a 100 kVA set at 400V delivers about 144 amps per phase at full load (100,000 ÷ (1.732 × 400)). Comparing that with your distribution board and cabling is a quick sanity check. If your site mixes single phase and three phase loads, the way they are balanced across the phases matters, so get the load assessed before committing.

Step 5: Add Headroom for Growth

Businesses change. A sensible allowance above calculated demand covers future equipment, expansion and unexpected loads, but heavy oversizing is worth avoiding. How much margin suits your site depends on how likely growth is and how certain your load figures are.

A Worked Example

Suppose a small warehouse has an essential running load of 60 kW.

1. Convert to kVA at a 0.8 power factor: 60 ÷ 0.8 = 75 kVA.
2. Add a 20% allowance for growth and starting loads: 75 × 1.2 = 90 kVA.
3. Check the starting load of the largest motor, such as a compressor or lift, and increase the size if it demands more.
4. Choose the next suitable standby rating above that figure.

The 20% allowance is an assumption for the example, not a rule. A real project should be confirmed against measured or documented loads.

Standby Rating vs Prime Rating

Most generators carry two ratings. A standby rating applies to emergency use during mains failures, while a prime rating applies to longer or more regular running. A 110 kVA standby set, for instance, may carry a prime rating of around 100 kVA. Make sure the rating you size against matches how the generator will actually be used.

Common Sizing Mistakes

1. Adding up every nameplate rating instead of the realistic maximum demand.
2. Ignoring motor starting loads.
3. Confusing kW with kVA.
4. Forgetting future growth.
5. Choosing a phase configuration that does not match the site's supply.
6. Comparing a standby rating with a prime rating as if they were the same.

Questions to Ask Before You Buy

• Will the generator back up the whole site or only essential circuits?
• What is the largest motor on site, and how does it start?
• Is the supply single phase or three phase, and at what voltage?
• Is the rating quoted standby or prime?
• Does the generator need an automatic transfer switch?
• Is the set silent (canopied or containerised) or open for a plant room?
• How much space and ventilation is available, and how will fuel be stored?

Final Thoughts

A careful sizing calculation takes an afternoon and can save thousands of pounds, either by avoiding an oversized purchase or by preventing a failure at the worst possible moment. Use the steps above to get a solid first estimate, then have a specialist check it against your real loads and site conditions. If you want to see what sizes are available, the standby range at Global Generators lists sets from 10 to 3000 kVA.