1. Define what the generator must power
List every circuit or item that must operate during a mains failure or off-grid run. Record the voltage, phase, running watts or amps and whether each load contains a motor, compressor, pump, transformer, UPS or sensitive electronic equipment. Decide which loads are essential and which can be shed.
A generator should be sized from a measured or properly calculated load schedule. Simply matching the building’s incoming supply rating often produces an oversized, inefficient system. Use the Blackwell generator power calculator to convert between kVA, kW, amps and horsepower.
2. Allow for motor starting and step loads
Motors can demand several times their normal running current while starting. The generator must accept that step without excessive voltage or frequency dip. Record the largest motor, its starting method and the sequence in which loads will connect. Variable-speed drives, soft starters and staged loading can materially reduce the required generator size.
3. Choose the correct operating rating
Standby power is for emergency supply during a normal utility failure and is not intended for overload. Prime power applies where the set supplies a variable load for extended periods, subject to the engine manufacturer’s annual-hour and average-load rules. A rental, construction or off-grid application normally needs prime-duty assessment. Read our prime versus standby guide before comparing headline kVA figures.
4. Confirm voltage, phase and frequency
Most UK commercial systems use 400/230 V, 50 Hz, three phase. Smaller property and site applications may need 230 V single phase. Export projects can require different voltages, 60 Hz operation or alternative connection arrangements. Phase imbalance matters: large single-phase loads on a three-phase alternator need careful review.
5. Select open or acoustic construction
An open generator is intended for a suitable plant room or container with engineered ventilation, exhaust and access. An acoustic canopied set provides weather protection and noise attenuation for outdoor installation. Noise limits are site-specific, so assess the measurement distance, boundary, nearby receptors and time of operation rather than relying only on a nominal canopy label.
6. Plan the fuel system and autonomy
Define how long the generator must operate without refuelling. An integral base tank may be sufficient for short standby events; longer autonomy can require an extended tank or a separate bulk tank with transfer controls. Include fuel-quality management, bunding, vents, fill points, leak detection and safe delivery access. Stored diesel is a maintenance item, not an indefinite reserve.
7. Decide how power will transfer
An automatic transfer switch (ATS) monitors the utility supply, starts the generator and transfers the load after a confirmed failure. When the mains stabilises, it returns the load and cools the generator before stopping. Manual changeover, mains paralleling, multiple-generator synchronising, remote monitoring and load shedding are alternatives that depend on site criticality and electrical design.
8. Survey the installation
Check access, lifting, foundation, cable route, earthing, ventilation, exhaust termination, fuel delivery, fire strategy, drainage, security and service clearances. Confirm local planning and environmental requirements. A well-sized generator can still perform badly if hot air recirculates, exhaust back pressure is excessive or cables create unacceptable voltage drop.
9. Specify commissioning and future maintenance
Commissioning should test more than an unloaded start. Verify protections, phase rotation, voltage and frequency, control logic, ATS operation, alarms and performance under an appropriate load. Agree routine inspection, servicing, battery replacement, coolant and fuel care, and periodic load-bank testing before the generator enters service.
Information to include with a quotation request
- Site location and application
- Standby, prime or temporary duty
- Load schedule, maximum demand and largest motor
- Voltage, phase and frequency
- Indoor open set or outdoor acoustic canopy
- Required fuel autonomy
- Manual or automatic transfer, monitoring and synchronising needs
- Delivery access, installation and commissioning scope
- Noise, emissions or environmental constraints