The maths behind sizing an event generator
Every generator supplier asks the same first question, and it is never "how many guests?" It is "what's running?" Here is the actual arithmetic behind the number they come back with.
Size an event generator from the load list, not the guest count: add up every kW draw (catering leads by far), divide by 0.8 to convert real power to the kVA a generator is actually rated in, then add 25 to 30 percent headroom for motor start-up spikes. Round up to the nearest standard silenced set, typically 40kVA for a 120-guest catered event, 60kVA for 300, 100kVA for 800. Guest count alone gets this wrong in both directions.
It sounds backwards. Every other quantity on an event kit list scales with guests: toilets, tables, chairs, even trackway roughly tracks how many cars are coming. Power does not, because power is not consumed by guests, it is consumed by equipment, and equipment is decided by the caterer, the band and the marquee crew, not by the headcount. An 80-guest wedding with a full electric field kitchen can draw more than a 200-guest event running gas catering and an acoustic set. Get this the wrong way round and you either overpay for a set that idles all weekend, or you book too small and the ovens surge just as the band hits the chorus.
Step one: the load list, in kW
Everything with a plug gets added up in kilowatts. The big number is almost always catering: a hot cupboard pulls 2 to 3kW, a single oven 6 to 10kW, and a full electric kitchen serving over a hundred covers can add up to 20 to 30kW on its own, which is why the trade's first question to a caterer is for their kit list in amps or kW, not a guess.
| Load | Typical draw |
|---|---|
| LED lighting rig (full marquee) | 0.5–1kW |
| PA / band system | 2–5kW |
| Hot cupboard (each) | 2–3kW |
| Catering oven (each) | 6–10kW |
| Fridge trailer | 2–4kW |
| Luxury toilet trailer | 1–2kW |
Step two: why divide by 0.8, not just add it up
A generator's nameplate rating is in kVA, apparent power, not kW, real power. The gap between them is the power factor, and event equipment (motors, compressors, induction hobs, transformers in lighting and sound gear) is mostly inductive, which drags the power factor down to around 0.8. So a 100kVA set does not give you 100kW to spend: it gives you roughly 80kW of usable load, with the other 20 kVA absorbed by that electrical overhead. Work forwards from a kW total and you divide by 0.8 to find the kVA a set must be rated at to deliver it.
Step three: the headroom is not padding
The 25 to 30 percent on top exists mainly for inrush current: a motor, compressor or induction hob can draw several times its running current for a fraction of a second at switch-on. A set sized to the exact running total will brown out or trip the moment the fridge compressor and the oven both kick in together. Headroom absorbs that spike without anyone noticing, and it keeps the engine off its absolute ceiling, which is better for fuel economy and engine life across a weekend of running.
The other direction: why oversizing has its own cost
It is tempting to solve all of this by booking a much bigger set than the maths says. Diesel engines are designed to run under load, close to their rated output, so the fuel burns cleanly and the engine reaches full operating temperature. A set loaded well below 30 to 40 percent for hours at a stretch cannot get there: unburned fuel accumulates in the exhaust, a condition the trade calls wet stacking, and it can foul the engine over time. The working target suppliers aim for is 50 to 80 percent load, which is exactly why the sizing maths rounds up to the nearest standard set rather than the safest oversized one.
Reading the answer off the standard bands
Silenced event sets come in a handful of standard ratings, not a continuous scale, so the final step is always rounding up: 6kVA, 20kVA, 40kVA, 60kVA, 100kVA, and twin 100kVA sets beyond that. As rough working bands for a lit, amplified, catered event: 40kVA covers up to about 120 guests, 60kVA to around 300, 100kVA to around 800, with twin sets beyond that. Those bands are a sanity check, not a substitute for the load list; a small, gas-catered, acoustic event can sit well inside them, and a small, all-electric one can blow straight through.
What this means for your event
Get the caterer's real kit list before anyone books a set. Add the PA, lighting, fridge trailer and anything else with a plug. Divide by 0.8, add the headroom, round up to the next standard band, and hand the whole load list to the generator supplier so they can design the distribution, cable runs and RCD protection around it rather than guessing at your site plan. The generator calculator runs this exact arithmetic against your own load list, and the generator hire guide covers silenced sets, fuel and overnight running in full.
Generator sizing questions, answered properly
Why does an 80-guest wedding sometimes need a bigger generator than a 200-guest one?
Because power is sized from what runs, not from how many people are watching it run. An 80-guest wedding with a full electric field kitchen can pull more kW than a 200-guest event with gas catering and a covers band. Guest count is a proxy at best; the caterer's kit list is the real number.
What does the 25 to 30 percent headroom on a generator actually protect against?
Mostly inrush: motors, compressors and induction hobs draw several times their running current for a fraction of a second when they switch on. Headroom stops that spike tripping the set or browning out everything else on the circuit, and it keeps the engine away from its absolute ceiling where fuel efficiency and engine life both suffer.
Is it bad to hire a generator that is too big for the job?
Yes, in a specific way: diesel engines are designed to run hot, and a set loaded much below 30 to 40 percent for hours at a time cannot reach that temperature. The unburned fuel that results, wet stacking, coats the exhaust and can foul the engine. The trade's answer is the same headroom rule from the other direction: aim to run at 50 to 80 percent load, not 15 percent.
Why do suppliers sometimes recommend two generators instead of one bigger one?
Twinning isolates risk and improves loading. A dedicated set for catering absorbs the oven and hot-cupboard spikes without dimming the band's lighting rig, and each set sits closer to its efficient loading band than one oversized generator trying to cover a wildly variable combined load.
Why does the cabling sometimes cost as much as the generator itself?
Because a generator on its own does nothing: it needs a distribution board, armoured cable runs to every position, and RCD protection throughout, all designed from the actual site plan. A 60kVA set with two 40-metre cable runs and a distro box is a materially different quote from the same set on a 10-metre run, even though the generator line reads identically.
Run your own load list through the maths
Tick what your event runs and get a kVA answer with proper headroom, the same arithmetic this post walks through.