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.