Generator Sizing Guide: How Many kVA Do You Actually Need?
The Short Answer
To size a generator, add up the running watts of everything it must power, add the largest single motor's starting demand (typically 3× its running load), convert to kVA (kW ÷ 0.8 power factor), then add a 20–25% safety margin. A generator should run at 50–80% of rated load for best fuel efficiency and engine life — never size it to run flat out or barely loaded.
Step 1: List your running loads
Write down every tool, motor, pump and appliance the generator must power at the same time, using the rating plate on each item. Resistive loads (heaters, lighting) draw exactly their rated power. Motor-driven loads are the trap — they draw far more at start-up than while running.
Step 2: Account for motor starting current
An electric motor draws roughly three times its running current for the few seconds it takes to spin up — a compressor, pump or crane motor rated at 10 kW can demand 30 kW at start. Size for your normal running total plus the starting surge of the single largest motor (you can stagger-start the rest).
Step 3: Convert kW to kVA and add margin
Generators are rated in kVA (apparent power). For typical industrial loads, divide your kW total by a power factor of 0.8: 40 kW of load needs 50 kVA of generator. Then add a 20–25% margin for future loads, hot Australian ambient temperatures, altitude derating, and to keep the engine in its efficient band.
Common sizing mistakes
- Undersizing for motor starting — the generator stalls or trips every time the compressor kicks in
- Oversizing dramatically — diesel engines that run under 30% load for long periods suffer wet stacking (unburnt fuel fouling the exhaust)
- Ignoring ambient derating — output falls in 45 °C summer heat and at altitude
- Forgetting single vs three phase — check what your equipment actually needs before you order
Which Bobcat generator class fits?
If you'd rather not do the maths, send us your equipment list — our team will size the generator for you. Call 03 7053 2937 or request a quote online.
- 20–60 kVA: site sheds, small crews, welding and power tools
- 70–340 kVA: crushing and screening support, events, medium construction sites
- 410–931 kVA: mining, prime power for remote facilities, large standby installations
Frequently Asked Questions
What is the difference between kVA and kW?
kW is real power doing work; kVA is apparent power, which includes the reactive component drawn by motors and transformers. For typical industrial loads, kW = kVA × 0.8. A 100 kVA generator delivers about 80 kW of usable power.
What size generator do I need to run a 10 kW motor?
Allow roughly three times the running load for start-up: a 10 kW motor can demand about 30 kW when starting. With a 0.8 power factor, that suggests a generator of around 37.5 kVA minimum — in practice a 40–50 kVA set is the right choice.
Is it bad to run a diesel generator on low load?
Yes. Sustained running below about 30% load causes wet stacking — unburnt fuel and carbon fouling the engine and exhaust. Size your generator so normal operation sits between 50% and 80% of its rating.
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