Selecting the right generator size for a UK worksite or commercial facility is a cost-effective choice that has tangible repercussions. Oversizing the unit means you lose money and fuel. Undersize it and you risk shutdown and equipment destruction. If you’re looking for new generators for sale in the UK, this guide will assist you in determining needs, doing calculations and making a sound purchase.

Why sizing matters

The generator needs to handle peak demand, absorb motor starts and operate dependably in the desired duty cycle. The correct size keeps things operating, saves sensitive devices and minimises maintenance headaches. Getting it incorrect causes downtime, safety risks and increased operating expenses.

Steps to get the right size generator for worksites and commercial premises

Step 1 — audit your loads

List all the electrical devices you require the generator to power:

• Critical processes (assembly lines, pumps, compressors)
• Safety and lighting (alarm systems, emergency lighting)
• Office and site welfare (heaters, kettles, IT)
• Ancillary systems (security, refrigeration, control systems)

Record operating power in kilowatts (kW) where practicable and comment on equipment with large start-up current (motors, compressors). For three-phase equipment, use kW or kVA ratings from nameplates or manufacturer information.

Step 2 — convert kW to kVA and take account of the power factor

Most generators are rated in kilovolt-amps (kVA). To convert from kW, divide by the power factor. A common assumption for mixed commercial loads is a power factor of 0.8. Example calculation:

• Total running load = 160 kW
• Power factor = 0.8
• Required kVA = 160 ÷ 0.8 = 200 kVA

Always check the actual power factor where possible; using a generic figure risks oversizing or undersizing.

Step 3 — accommodate motor starting and surge loads

Most motors take several times their operating current to start. Normal inrush factors range from 2.5 to 4.0 based on motor type and mode of start. You will need to either:

• Synchronise the generator to accommodate the most severe simultaneous starts, or
• Stage motor to start using soft starters or employ load management to stagger the load.

A common practice is to compute running kVA, include a conservative surge margin, and then select a generator that can handle continuous and transient loads.

Step 4 — select the appropriate duty rating

Learn about generator duty types:

• Standby rating: limited usage for emergencies with short runtime and no overload tolerance.
• Prime power: variable load continuous usage for worksites and continuous applications.
• Continuous rating: continuous steady output over extended periods.

Prime power duty is preferable for most commercial and worksite installations; refer to the manufacturer’s datasheet to ensure the selected model is capable of that duty.

Step 5 — provide for future expansion and redundancy

Include a headroom of 10–25 per cent for provision against future load growth and preventing the operation of the generator at full capacity continuously. For critical locations, consider redundant configurations: generators in parallel provide resilience and maintenance ease.

Step 6 — think about fuel, enclosure and site restrictions

Select diesel, gas or hybrid based on fuel availability, operating hours and emissions restrictions. For external installation, use an IP-rated enclosure appropriate for the UK climate and incorporate sound attenuation for town centre sites. Consider fuel storage, refuelling access and planned maintenance.

Step 7 — include transfer and control systems

A generator will need to be accompanied by suitable transfer equipment. An automatic transfer switch (ATS) offers quick, unattended changeover between mains and generator and is a must on installations where downtime is not permissible. Size the ATS to match or be larger than the generator kVA and check that protective devices and control panels are compatible.

Most common sizing errors and how to prevent them

Preventable and predictable errors occur commonly. First, do not size on nameplate ratings alone; meter actual demand with a load logger to measure running and peak currents. Second, do not assume all motors start at once; plot likely simultaneous starts and, where practicable, accommodate soft starters or staggered start controls. Third, do not overlook auxiliary loads like heaters, control panels and battery chargers when calculating requirements. Lastly, make sure switchgear and protection equipment are properly rated for the fault and short-circuit levels of the generator.

Maintenance, warranties and supplier support

Look at whole-life costs in choosing a unit. Discuss with suppliers the recommended service interval, spare parts availability and local emergency call-out support. A supplier who offers commissioning, load bank testing and planned servicing will minimise unscheduled downtime. Examine warranty conditions closely; warranties may be void if maintenance is not carried out at predetermined intervals.

When to call an expert?

If your installation involves parallel sets, control system integration, synchronisation or emergency critical infrastructure, hire a trained electrical engineer. They can simulate load dynamics, recommend protective settings and ensure compliance with UK electrical safety standards. Budget for fuel quality monitoring and staff training so workers use the equipment safely and effectively.

Decision checklist

• Did you measure running and start-up loads?
• Did you calculate kW to kVA from the actual power factor?
• Have you provided headroom and surge capacity?
• Is the selected duty rating correct (prime vs standby)?
• Is fuel type, enclosure and access being planned?
• Are the ATS and control gear sized correctly to be matched with the generator?
• Have you made commissioning, testing and maintenance arrangements?

Final thoughts

Generator sizing blends precise measurement, safe allowances and pragmatic planning. Use measured load data where available, engage a professional electrical engineer for calculations and commissioning, and size equipment to the intended duty. An accurately sized generator keeps a UK worksite productive, safe and compliant while minimising unnecessary cost.