Choosing a Generator for a Home, Office or Telecom Site

Generators for a house, a small office and a telecom tower may all fall in the same 10 to 50 kVA range, yet they are chosen differently. A house needs backup for a few hours of outage, mostly at light load. An office needs automatic changeover and enough capacity for air conditioning. A remote telecom site may run on its generator most of the day with a refuelling visit only every few weeks. This guide covers the decisions that matter, from the load audit to fuel logistics, and ends with a worked example.

Start with a load audit

List every load that must stay on during an outage: lighting, IT, refrigeration, pumps, air conditioners, lifts, security. For each record the running kW, the power factor, the quantity, and whether it contains a motor that starts direct on line.

Be honest about what really needs backup. Water heaters, ovens and large air conditioners can double the set size while adding little to the essential service. Splitting the board into an essential section, fed through the changeover switch, and a non-essential section that stays dead on generator often affects cost more than any other decision. Where possible, measure: a logging power analyser on the existing supply for a week shows the real peak and typical load, whereas nameplate figures are maximums.

Running kVA and starting kVA

The running load is found by adding real powers (kW) and reactive powers (kVAr) separately and combining them as kVA = √(kW² + kVAr²). Because most sets are rated at PF 0.8, the set must also cover the running kW divided by 0.8; the larger figure governs. The guide on kW versus kVA explains why.

Starting is the second demand. Fixed-speed compressors, refrigerators, borehole pumps and lift motors draw several times their running current for a second or two. The set must absorb that step without its voltage dipping so far that other equipment drops out. Inverter air conditioners and pumps on variable-speed drives start gently. See sizing a generator for motor loads for detail.

Single phase or three phase

Most homes are single phase, and a single-phase set is the natural match. Offices and telecom sites are often three phase, and any three-phase motor needs a three-phase set. A three-phase set feeding mainly single-phase loads must have them spread across the phases: the alternator tolerates only limited unbalance, and its single-phase output is a fraction of its three-phase rating. The manufacturer states both limits.

Standby, prime and continuous ratings

The same engine and alternator can carry different ratings depending on use. ISO 8528-1 defines the rating types, and a datasheet quoting one number without naming the type is incomplete.

• Emergency standby power (ESP). The maximum power available during a variable load sequence when the normal supply has failed, for a limited number of hours per year. The manufacturer states the permitted annual hours and average load.
• Prime power (PRP). The maximum power available during a variable load sequence for unlimited hours per year, provided the 24-hour average load stays within the percentage of prime rating the manufacturer specifies.
• Continuous operating power (COP). The maximum power at constant load for unlimited hours per year, within the prescribed maintenance intervals.
• Limited-time running power (LTP). The maximum power at constant load for a limited number of hours per year; less common in small sets.

A home or office on a reliable grid usually needs a standby rating. Where the generator is the main source, such as an off-grid telecom tower or a building whose supply fails for many hours daily, choose by the prime rating.

Duty, runtime and service

Think in hours as well as kilowatts. Engines are serviced by running hours, so oil changes and overhauls come round far faster on a set running twelve hours a day than on one running a few dozen hours a year. Water-cooled, slower-speed engines generally suit long daily running better than small air-cooled units designed for occasional use. Multiply the service schedule by your expected annual hours before comparing prices.

Fuel logistics and tank size

The fuel rate depends on the average load, not the rating, and diesel engines burn more per kWh at light load. The fuel consumption calculator applies a part-load curve to give litres per hour. A city office may be refilled within a day; a telecom site needs autonomy for the whole refuelling interval plus a margin for delays. Stored diesel should be kept dry and checked for water and microbial growth, especially in standby sets that sit idle for months.

Changeover: manual or automatic

The generator must never feed back into the utility network. A mechanically interlocked manual changeover switch is often enough for a home. Offices and telecom sites normally use an automatic transfer switch (ATS) whose controller detects mains failure, starts the set, transfers once voltage and frequency are stable, and transfers back with a cool-down run when mains returns. Whether the neutral is switched (four-pole changeover) depends on the earthing arrangement and local regulations. Servers still need a UPS to bridge the seconds the set takes to start; the UPS sizing calculator helps there.

Noise, enclosure and location

Manufacturers usually quote sound pressure in dB(A) at a stated distance, commonly 7 m. Compare figures at the same distance and check local limits, often stricter at night. A sound-attenuated canopy, a position away from windows and an exhaust pointed away from occupied areas matter as much as the set. Hot radiator air must not recirculate to the intake.

Derating for altitude and temperature

Ratings apply at stated reference conditions. Thinner air at altitude and hot intake air both reduce engine output, and the alternator may also derate. The amount varies by engine, so use the derating table in the manufacturer's data for the specific model, not a generic rule. Rooftop and enclosed installations, where intake air is warmer than ambient, deserve particular attention.

Telecom sites: what is different

• The load is mostly DC. Radio equipment runs from a 48 V DC plant fed by rectifiers, so the generator sees the equipment load plus rectifier losses plus battery recharge current. After a long outage recharge can exceed the equipment load for hours; size for both.
• Cooling. Shelter air conditioners are often the largest load and the largest starting step.
• Battery hybrid or cyclic operation. Rather than running continuously at light load, the set runs at a healthy load to supply the site and recharge the batteries, then stops while the batteries carry the site. This cuts running hours and fuel, provided the battery is sized for the cycling depth and the rectifier capacity matches the set.
• Remote monitoring. Reporting fuel level, running hours, alarms and battery voltage lets refuelling and maintenance be planned and exposes fuel theft early.

A worked sizing example

Running load with a 20 % margin and no derating:

One air conditioner starting while everything else runs:

The pump start peaks lower (16.7 + 11.25 = 27.9 kVA). With no credit for short-term overload capability, the requirement is 33.2 kVA and the next common size is 40 kVA, although the running load needs only 25 kVA. With inverter air conditioners (about 1.5 × running kVA) their peak falls to about 20 kVA, the pump governs at 27.9 kVA, and a 30 kVA set suffices. The generator sizing calculator reproduces these figures.

Fuel follows average load. Suppose the office averages 10 kW over a 10-hour day. A 30 kVA set is 24 kW at PF 0.8, so it runs near 42 % load. With the default curve and 0.24 L/kWh at full load:

On the 40 kVA set (32 kW, about 31 % load) the multiplier is 1.33, giving 3.19 L/h, roughly 10 % more fuel for the same work. The curve is generic; use the manufacturer's fuel table once a model is chosen.

Summary

1. Audit loads, separate essential from non-essential, and measure where you can.
2. Check running kVA, running kW and the largest starting step.
3. Match the phase arrangement to the building.
4. Choose the ISO 8528-1 rating type that fits the duty.
5. Size the tank for the real average load and refuelling interval.
6. Specify changeover, enclosure and location, and apply the manufacturer's derating.

This guide is general education. Installation, earthing and changeover arrangements must follow local regulations and be checked by a qualified electrician or engineer.