How to Size a Diesel Generator: Getting the kVA Right
The most expensive generator mistake is buying the wrong size. How to calculate your real load, account for motor starting surge, and avoid both wet stacking and stalling.
Anil Menon·Power & Genset Specialist · 19 years in diesel generators (India market)·April 14, 2026 · Updated July 8, 2026 · 7 min read
The single most expensive mistake in buying a generator is buying the wrong size — and it happens constantly because people shop on price before they calculate load. Get the kVA right and a used genset serves reliably for years. Get it wrong and you either stall under load or slowly destroy the engine through under-loading. Sizing comes before shopping, always.
Undersizing vs oversizing: both are costly
Oversizing is not "playing it safe"
A generator that is far too big for its load runs lightly loaded and suffers "wet stacking" — unburnt fuel glazes the cylinders over time, fouling the engine and shortening its life. Undersizing is equally bad: the genset stalls, overheats or cannot start your motors. The goal is a machine that sits comfortably in its efficient loading band, not one that idles near-empty or strains at its limit.
Step 1: Add up your running load
List every load the generator must power and add up the running watts/kVA. Include lighting, HVAC, machinery, computers, refrigeration — everything that will run at once. Be realistic about simultaneous use: you rarely run absolutely everything at peak, but size for your genuine worst-case combination.
Step 2: Add the starting surge
This is the step people forget. Motors and compressors draw several times their running current at the instant they start — an air conditioner, pump or large motor can spike well above its steady draw. Your generator must handle the running load plus the largest starting surge occurring at once. Skip this and the genset that looked correctly sized stalls every time a big motor kicks in.
Step 3: Add headroom for growth
Add a margin (commonly 10–25%) above your calculated peak for future loads and safety.
But do not massively oversize "to be safe" — that reintroduces wet stacking.
Aim for the genset to run at a healthy fraction of its rating under normal load, not near-empty.
When in doubt, get a load calculation
Generator sizing spans from small 5 kVA units to multi-megawatt industrial sets, so the same "good deal" can be completely wrong for your site. If your load mix is complex — big motors, sensitive electronics, mixed phases — have an electrical engineer do a formal load calculation. It costs little and prevents an expensive mistake.
Then — and only then — shop
With your kVA number in hand, shop for a genset from a supported brand. Cummins and Caterpillar lead high-capacity industrial sets globally; Kirloskar, Mahindra Powerol, Ashok Leyland, Greaves Cotton and Jakson are strong choices in India, with Kirloskar and Mahindra especially popular in commercial bands. Buying to a number you calculated beats buying to a number a seller quoted.
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Add up the running load of everything that will run at once, add the largest motor’s starting surge (motors draw several times their running current at startup), then add 10–25% headroom for growth. Size the genset to run comfortably in its efficient band — not idling near-empty and not straining at its limit.
What happens if a generator is too big for the load?
An oversized generator runs lightly loaded and suffers "wet stacking" — unburnt fuel glazes the cylinders over time, fouling the engine and shortening its life. Oversizing also wastes capital. Bigger is not safer; aim for the genset to sit in its efficient loading band.
Why does motor starting surge matter for generator sizing?
Motors and compressors draw several times their running current at the instant they start. If you size only for running load, the generator stalls whenever a large motor kicks in. Always size for the running load plus the largest starting surge occurring at once.
What happens if a generator is undersized?
An undersized generator cannot carry the load — it stalls, overheats, trips, or fails to start your motors. This stresses the engine and leaves you without reliable power exactly when you need it. Undersizing is as damaging as oversizing.
Should I get a professional load calculation?
For complex sites — large motors, sensitive electronics, mixed phases — yes. Sizing ranges from small 5 kVA units to multi-megawatt sets, so a formal load calculation by an electrical engineer costs little and prevents an expensive sizing mistake.
Disclaimer: The buying and selling guidance in this article is based on EasiMachines' own experience and research. It is general information, not professional, legal or financial advice. Use your own judgement and carry out your own due diligence before you buy or sell; EasiMachines is not responsible for any loss or damage arising from decisions made without that due diligence. EasiMachines is independent and is not sponsored by, affiliated with or endorsed by any manufacturer or brand named in this article. Brand names and trademarks belong to their respective owners and are used only to identify products. Check current specifications with the manufacturer.
About the author
Anil Menon
Power & Genset Specialist · 19 years in diesel generators (India market)
Anil has specified and commissioned diesel gensets for factories, hospitals and commercial buildings across India, where reliable backup power is non-negotiable. He writes with a hard focus on sizing, emissions compliance and load testing.
