Product guide

Diesel Generator Sets

A diesel generator set is an engine, an alternator and a control panel mounted on a common base frame and rated to a defined duty. This guide covers how the ratings on a datasheet are defined, which power range fits which job, and what has to be in an enquiry before a set can be selected.

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Ratings

How a generator set is rated

Two sets with identical hardware carry different numbers depending on the duty they are sold for. ISO 8528-1 defines the four ratings that appear on commercial datasheets.

Rating Duty Overload Annual hours
ESP — emergency standby Mains failure only Not permitted Up to about 500 h
PRP — prime power Continuous variable load 10% for 1 h in every 12 h Unlimited
COP — continuous power Constant load, no variation Not permitted Unlimited
DCP — data centre power Variable load, defined step limits Per manufacturer declaration Unlimited

ESP is always quoted higher than PRP for the same machine. If a tender asks for one figure without naming the rating, ask which one it is — the difference is typically 10% of the rating.

Power ranges

Which power range fits which job

Typical duty by rating. Final sizing always depends on your load list.
Rating Typical duty Usually configured as
20–100 kVA Retail units, telecom cabinets, small offices, villas Single-phase or three-phase, open frame or small canopy
100–300 kVA Offices, hotels, clinics, small factories Three-phase canopy, optional automatic transfer switch
300–800 kVA Hospitals, production lines, data halls Three-phase canopy or containerised, parallel-ready control
800–2000 kVA Data centres, large plants, marine and rental fleets Containerised or open skid, multi-set parallel configuration

Pages by power range

Each page carries the rating, the typical specification envelope for the class, the applications it suits and the questions we are asked most often at that size.

Form factors

Four ways to package the same set

  • Silent canopy

    Acoustic enclosure, typically 65–75 dB(A) at 7 m. Standard for hotels, hospitals and residential areas.

  • Containerised

    Set, tank and controls inside a 20 ft or 40 ft container with service access doors. Chosen when weather protection, security and long autonomy all matter.

  • Trailer-mounted

    Road-towable chassis, used where the set has to move between sites.

  • Open frame

    Bare skid-mounted set for plant rooms that already have acoustic treatment and ventilation.

Electrical system

Voltage and frequency combinations

Voltage Frequency Common in
400/230 V 50 Hz Middle East, Africa, Europe, South Asia
380/220 V 50 Hz Older installations and projects written to legacy specifications
415/240 V 50 Hz Specifications written to legacy UK standards
440/254 V 60 Hz West and Central Africa, legacy Gulf networks
480/277 V 60 Hz North American specifications and data centre tenders

Frequency is set by engine speed — 1500 rpm for 50 Hz and 1800 rpm for 60 Hz — so the frequency must be fixed before the set is built, not adjusted afterwards.

Construction

What is inside the set

Component What to check on the datasheet
Diesel engine Make and model, governor type (mechanical or electronic), rated speed, fuel consumption at 50/75/100% load
Alternator Make and model, insulation class, temperature rise, voltage regulation, IEC 60034 rating
Control panel Auto mains failure (AMF) capability, protections, remote monitoring options
Base frame and tank Anti-vibration mounts, base tank capacity, autonomy at full load
Cooling and exhaust Radiator sized for site ambient, silencer type and noise level at 7 m
Options Automatic transfer switch, parallel control, bundled tank, canopy or container, remote monitoring

Engine makes commonly specified in these power classes include Cummins, Perkins, Volvo Penta, Weichai, Yuchai, Deutz, Kubota, Yanmar and Lovol, paired with brushless alternators such as Stamford, Leroy-Somer or Marathon.

Selection

Five steps from load list to order

  • 1. List the loads

    Running kW and power factor for each load, plus the starting method of the largest motor.

  • 2. Fix the duty rating

    ESP, PRP, COP or DCP. This single choice changes every number that follows.

  • 3. Fix voltage and frequency

    Build the electrical system first — it cannot be changed on site later.

  • 4. Confirm site conditions

    Ambient temperature, altitude, noise limit, fuel storage and access route.

  • 5. Choose form and controls

    Canopy, container, trailer or open frame; single set or parallel; ATS or manual.

Questions

Common questions

What is the difference between kVA and kW?

kVA is apparent power and kW is real power; the ratio between them is the power factor of the load. Generator sets are rated at a 0.8 power factor as a convention, so a 500 kVA set is quoted as 400 kW. A load with a lower power factor uses more kVA for the same kW.

How much fuel does a diesel generator use?

Budget roughly 0.24 to 0.27 litres per kWh delivered. A 400 kW set running at 75% load, so about 300 kW, therefore burns in the region of 72 to 81 litres per hour. Part-load running is less efficient per kWh than running near rated load.

Should I choose 1500 rpm or 3000 rpm?

Most 50 Hz sets are 1500 rpm four-pole machines, and most 60 Hz sets are 1800 rpm; those speeds are the standard for continuous duty. Higher-speed sets are smaller and lighter but are generally used for light or portable duty.

What does an automatic transfer switch do?

It watches the mains supply, starts the generator when the mains fails, and transfers the load once the set is at voltage and frequency. Transfer typically happens within 8 to 15 seconds of a mains failure, and it transfers back when the mains returns.

What is derating and when does it apply?

Engine and alternator output both fall when ambient temperature is above 40 °C or the site is above 1000 m altitude. The exact derating curve is declared by the engine and alternator manufacturers, so the site conditions have to be known before the rating is finalised.

How much noise does a silent set make?

A canopy set is normally quoted between 65 and 75 dB(A) at 7 m, depending on the enclosure and the rating. If a tender sets a limit below 65 dB(A), the enclosure has to be specified for it rather than chosen from a catalogue.

Form factors

Pages by form factor

The same rating can be built four ways. Which one you need follows from where the set will stand, who will hear it, and how it will get there.

Applications

Pages by application

A data centre, a hospital, a hotel, a construction site and an oil and gas facility each impose a different governing requirement on the same machine.

Not sure which rating you need?

Send the load list, the site conditions and the frequency. We will reply with the rating and the build options that fit.

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