Simple Guide on How to Size a Generator Correctly
December 17, 2025
Choosing the right generator size is one of the most important decisions you’ll make for backup power, prime power, or site power.
- Too small, and your generator won’t start key loads (or it’ll trip, stall, or brown out).
- Too large, and you’ll overpay on capital cost, fuel, servicing, and potentially run the set inefficiently.
This guide is written by YorPower to help you size a diesel generator correctly — using a practical, repeatable method that works for everything from small commercial standby installations to large multi-load industrial sites.
What does “generator size” actually mean?
Generator “size” is usually discussed in one (or more) of these ways:
- kW (kilowatts) = real usable power (what your equipment actually consumes to do work)
- kVA (kilovolt-amps) = apparent power (what the generator delivers as a system total)
- Amps (A) = current draw at a given voltage (common for electrical panels and motor data)
- Single-phase vs three-phase = affects distribution, current, and compatibility
In the UK and most commercial/industrial applications, generators are commonly rated in kVA, while many loads are specified in kW. That’s why proper conversion matters.
Step 1: Work out your load requirements
Start by identifying exactly what you need the generator to run.
Make a list of each item and total up the wattage. That combined figure shows how much electrical power your equipment needs, and helps you determine the minimum generator output required.
You can usually find the wattage on the unit’s rating/nameplate, or in the manufacturer’s manual/specification sheet.
Step 2: Convert to kVA from kW
Here’s the sizing mistake we see constantly:
“My site needs 100kW, so I’ll buy a 100kVA generator.”
That’s rarely correct.
Generators are often referenced at a typical power factor (commonly 0.8 in many generator rating conventions).
A simple rule of thumb used in practice:
- kVA = kW ÷ PF
Example:
- If your load is 100kW
- And PF is 0.8
- Then kVA = 100 ÷ 0.8 = 125kVA
Step 3: Consider future growth
If you’re planning to add more equipment, expand your premises, or increase operating hours, your load demand will rise—and a generator sized only for current usage can quickly become restrictive, leading to overload risk, reduced efficiency, and more wear. Building in sensible headroom from the start gives you flexibility, helps the generator handle starting currents and peak demand more comfortably, and can reduce the need for costly upgrades later.
Need help sizing a generator? YorPower can validate it properly
If you want a generator that starts your largest loads reliably and runs efficiently in the real world, the final selection should be validated by a generator specialist.
At YorPower, we size generator systems based on:
- your load schedule and motor starts,
- whether you need standby, prime, or hybrid operation,
- phase balancing and distribution,
- and real site constraints.
If you already have a load list (even a rough one), that’s enough for us to begin narrowing down the correct kVA range and configuration. If you’re still unsure, please get in touch to arrange an on-site assessment with our experienced generator projects team.
