What Size Generator Do I Need to Hire?

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Getting your generator sizing wrong is one of the quickest ways to ruin a project. Under-spec the unit, and you’ll spend your day resetting tripped breakers or damaging sensitive electronics. Over-spec it by miles, and you’ll burn through fuel and money for capacity you never touch.

Whether you’re keeping a festival stage buzzing, powering a welfare cabin, or running heavy site tools, here’s how to calculate the exact generator size you need.

Step-by-Step: How to Calculate Your Power Load

You don’t need an engineering degree to get a solid estimate. Just follow these four simple steps to get a rough idea:

List simultaneous equipment: Write down everything that needs power at the same time. If a compressor and a table saw only run in turn, you don’t need to add both running totals together.

Find running power (kW or Watts): Check the nameplates or spec sheets on your kit. Sum up the wattage to find your baseline continuous load (1,000 Watts = 1 kW).

Factor in starting surge: Anything with an electric motor (pumps, compressors, air-con units, heavy saws) draws 2 to 3 times its continuous rating when kicking into gear. Always account for that initial burst.

Apply the 80% rule: Never run a generator continuously at 100% capacity. Add 20% to 25% safety headroom so the engine runs efficiently without straining.

Convert kW to kVA: Generators in the UK are rated in kVA (apparent power). Standard diesel generators have a power factor of 0.8. So to convert your required kW into kVA, simply divide your total kW by 0.8.

Real-World Sizing Scenarios

Small maintenance job: Running floodlights (500W), a hammer drill (1,200W), and battery chargers (300W) totals around 2 kW. Factoring starting surge and headroom, a 3 to 5 kVA portable unit does the trick.

Active construction site: A welfare cabin (kettle, heater, microwave ~5 kW) plus simultaneous site tools and compressors (~10 kW) needs roughly 15 kW. Applying the formula gives around 18.75 kVA, making a 20 to 30 kVA road-tow diesel generator ideal.

Outdoor wedding or festival stage: Catering vans, chillers, PA systems, and decorative lighting can easily demand 35 to 45 kW of mixed loads. A 60 kVA+ silent canopy generator ensures balanced three-phase distribution with zero sound bleed.

Typical Generator Sizing at a Glance

Small tools and domestic backup (3 to 6 kVA): Ideal for running hand tools, small task floodlights, and everyday domestic gear on single-phase power.

Site welfare and small commercial (10 to 20 kVA): The go-to size for powering standard site cabins, kettle runs, drying rooms, and light office setups.

Medium construction and marquee events (30 to 60 kVA): Delivers robust three-phase power for active building sites, festival catering vans, PA rigs, and event lighting.

Industrial sites and multi-stage festivals (80 to 100+ kVA): Heavy-duty continuous power designed for large commercial shutdowns, manufacturing plant backup, and major outdoor productions.

Common Sizing Mistakes to Avoid

Ignoring starting spikes: Forgetting motor inrush current is the number one cause of nuisance trips.

“Wet stacking” an oversized set: If you run a massive diesel generator at under 30% load for days, unburnt fuel clogs the exhaust and damages the engine.

Skipping earthing and safety: Every temporary installation must have proper RCD protection, earth spikes, and connections verified by a competent person.

Let Loadgen Calculate Your Load for You

Not sure if you’ve counted every watt? Not confident in your numbers? If that’s the case, send your equipment list over to Loadgen. Our temporary power specialists will run a free, precise sizing check and provide a fast, no-obligation quote tailored to your exact site requirements.