Generators

Types of generator and where each one is used

Generators are classified first by fuel and then by enclosure and mode of use. Fuel decides running cost and how long the set can run continuously; the enclosure decides portability, noise level and how the set is installed.

How many kVA do you need? Sizing a generator

The only sound way to choose a set is to add up the loads it has to carry. Resistive loads such as lighting and heaters draw their rated power. Inductive loads — electric motors, compressors, pumps, lifts — draw three to six times their rated power at start-up. So the calculation adds a starting-current allowance and roughly 20% for growth on top of the total.

The table below shows what a set's kVA rating means in continuous load terms, and what each class is typically specified for.

The difference between kVA and kW

kVA is apparent power and kW is active power. The ratio between them is the power factor (cos φ), taken as 0.8 as standard on generator sets. So kW = kVA × 0.8 and kVA = kW ÷ 0.8. A 100 kVA generator delivers 80 kW of active power.

The kVA figure on the nameplate describes what the alternator can deliver; the kW figure describes what the loads actually consume. Load schedules are usually drawn up in kW, so dividing the kW total by 0.8 gives the kVA rating to specify.

Standby, prime and continuous ratings

A generator datasheet normally carries two ratings. Standby is for backup duty during a mains failure: variable load, limited hours, no overload permitted. Prime is for continuous running under variable load, and typically allows 10% overload for one hour in twelve.

The prime rating is roughly 90% of the standby rating. On a site with no grid, an agricultural installation or a production line that runs continuously, size against the prime rating; for a set that only runs during an outage, standby is the right figure.

Single-phase or three-phase

A single-phase generator delivers 230 V and suits homes, small businesses, lighting and heating. A three-phase generator delivers 400/230 V, which is what three-phase motors, lifts, industrial machinery and larger facilities require.

A three-phase set can also supply single-phase loads, provided they are balanced across the phases. If the site has even one three-phase motor, specify a three-phase set; otherwise a single-phase set is both cheaper and simpler to install.

Canopy, open-set and automatic transfer configurations

A canopy set is housed in a sound-attenuated enclosure: it can be installed outdoors, is protected against rain and dust, and typically measures 65–85 dBA at 7 m. An open set is lighter and cheaper but needs a plant room with the ventilation, exhaust routing and acoustic treatment already in place.

An automatic transfer switch (ATS) starts the generator when the mains fails, transfers the load and hands it back when the mains returns. In hospitals, data centres, cold stores and continuous production it is a requirement rather than an option. On a manual set an operator does that work.

• Canopy set: outdoor installation, sound-attenuated, lockable doors
• Open set: needs a plant room, lighter and lower cost
• Automatic (ATS) set: starts and transfers without an operator
• Trailer-mounted set: for projects that move between sites

Fuel consumption and running cost

As a rule of thumb a diesel set burns about 0.25 litres per kW per hour at full load: a 100 kVA (80 kW) set uses roughly 20 l/h flat out. Consumption falls with load — the same set burns about 25% less at 75% load and about 45% less at 50%. Every product page publishes the manufacturer's figures at full, 75% and 50% load.

Run time is the tank capacity divided by full-load consumption. Where the set must run through long outages, an external tank with automatic refuelling extends it. Running a diesel set continuously below about 30% load is not recommended: light loading causes carbon build-up.

Installation, maintenance and spare parts

A set performs as well as it is installed: adequate ventilation area for intake and hot-air discharge, an exhaust routed outside, anti-vibration mounts, earthing, and a mechanically interlocked transfer arrangement between mains and generator.

Routine maintenance is usually an oil and filter change every 250–500 running hours, checks on the battery and block heater, and at least one on-load test run per year. Seza Global supplies load and site assessment, installation, scheduled maintenance and spare parts alongside the sets it delivers.