Generator Sizing & Fuel

Size a temporary generator from your load: kW to kVA, headroom, step-load allowance and altitude / temperature derating. Then estimate fuel burn, runtime per tank, refuel times across the show day, and CO2e for your sustainability report.

Guidance only. Results are estimates based on the figures you enter and published tables. All electrical work must be designed, checked and tested by a qualified electrician to the regulations that apply where you are (e.g. BS 7671, NEC).

Load

kW
Expected running load (after diversity), not the sum of nameplates.
PF
0.8 is the rating basis of three-phase gensets.

Sizing Allowances

% of prime
75–80 % leaves headroom for peaks and additions.
kVA
Biggest block switched on at once: motor start kVA, a rig power-up, a chiller.
% of rating
Placeholder — use the datasheet's ISO 8528-5 first-step figure.

Site derating

m
°C
Air at the set's intake, hottest hour.
%/100 m
above m
%/5 °C
above °C
Defaults are a typical rule of thumb only. Published figures vary widely by engine and alternator (we found 0.4–1.3 % per 100 m starting from 200–1000 m, and 0.4–3 % per 5 °C starting from 25–40 °C). The manufacturer's derating table for the actual set always wins.

Generator Size

Running load
—
Site derating factor
—
Minimum prime rating
—
Suggested hire size
—
Loading at running load
—
Usable capacity on site
—
Prime rating × derating factor

Fuel & Runtime

L/h
Interpolate the datasheet's 50/75/100 % figures.
%
HVO is less dense, so slightly more litres.
L
% left
h
Total hours across the event.
Fuel burn
—
Runtime per tank
—
Fuel for the run
—
Refuels needed
—
Refuel schedule

    Emissions

    Factor used
    —
    CO2e for the run
    —
    Per hour
    —

    Prime vs Standby Ratings

    • Prime (PRP, ISO 8528-1) is the rating for a set that is the only supply, running a variable load for unlimited hours. The average load over 24 h should not exceed 70 % of PRP. Size event generators on the prime rating.
    • Emergency standby (ESP) is a higher figure for back-up use only, typically limited to around 200 h a year with the same 70 % 24‑hour average. Hire listings sometimes quote it; don't size a show supply on it.
    • kVA vs kW. Three-phase sets are rated in kVA at 0.8 PF, so a 100 kVA set is an 80 kWe set. A load with PF below 0.8 is limited by kVA (alternator current) before kW (engine power).
    • Light loading. Diesels run below about 30 % load for long periods don't get hot enough: unburnt fuel and soot build up in the exhaust (“wet stacking”), which costs fuel and maintenance. Pick a smaller set, use a hybrid battery system, or add a load bank.

    Notes & References

    • Sizing. Minimum prime kVA = max(running kVA ÷ target loading, largest step kVA ÷ step acceptance) ÷ site derating. Running kVA = kW ÷ PF. Hire sizes are a typical UK fleet list (prime kVA); check what your supplier stocks.
    • Ratings and reference conditions. ISO 8528-1 (rating classes PRP, ESP, COP, LTP) and ISO 3046-1 / ISO 8528-1 standard reference conditions (100 kPa, 25 °C, 30 % relative humidity). Step loading performance is defined in ISO 8528-5 and depends on the engine's BMEP, so use the manufacturer's step figure.
    • Derating. Derating = (1 − altitude loss) × (1 − temperature loss). Example published rules: 1.0 % per 100 m above 400 m and 3.0 % per 5 °C above 40 °C (a 1 MW diesel datasheet); 1.5 % per 305 m above 1006 m (MTU); 1.3 % per 100 m above 200 m and 2 % per 10 °C above 25 °C (Kohler gas sets). Use the actual set's curve.
    • Fuel model. F (L/h) = a × rated kWe + b × load kW, the linear fuel curve used by HOMER and in microgrid studies. Typical: a = 0.028, b = 0.243 L/kWh, our least-squares fit to the widely published “approximate diesel fuel consumption chart” (e.g. Depco) for 60–1000 kW sets. Conservative: a = 0.08145, b = 0.246 L/kWh, the coefficients commonly cited in hybrid-microgrid literature; reads about 20–40 % higher than the chart. Rated kWe = prime kVA × 0.8. Real burn varies by engine, age and load profile; the datasheet figure is always better.
    • HVO (EN 15940) is less dense than EN 590 diesel and has slightly less energy per litre, so volumetric consumption is typically a few percent higher (energy-content estimates give about 4–5 %; one engine study measured up to about 6 %). Check the set is approved for HVO.
    • Emission factors (kg CO2e per litre, UK Government GHG Conversion Factors for Company Reporting 2026, DESNZ): diesel average biofuel blend 2.58354 (WTT 0.61101); diesel 100 % mineral 2.66155 (WTT 0.62409); gas oil 2.75541 (WTT 0.62665); HVO 0.03558 (WTT 0.56439), plus 2.43 biogenic CO2 reported “outside of scopes”. Red diesel can only be used legally in the UK for certain exempt purposes since April 2022.
    • Useful for Theatre Green Book and similar reporting; record actual litres delivered for the final figure.
    • Related tools: Three-Phase Load Balancer, Power Calculator, Cable Gauge Calculator.
    • This is planning guidance. It does not replace the generator manufacturer's data, BS 7671 / BS 7909, or the judgement of a competent person.