Sizing a generating set can be more critical than it might seem. Imagine you have spent weeks preparing for the launch of a new production line. Everything is in place: machinery, staff, deadlines agreed with the client. Then the power goes out. Your generating set kicks in — but it cannot handle the load. The protection trips, the line shuts down, and the cost of that failure, in time and reputation, far outweighs what it would have cost to choose the right equipment from the start.
This kind of situation is more common than you might think, and it almost always has the same root cause: the generating set was not sized correctly.
The reasons vary. In some cases, the unit was purchased years ago for a smaller installation, and machinery was added over time without ever checking whether the output was still sufficient. In others, the equipment was chosen based solely on the nominal consumption of the connected loads, with no consideration given to the starting peaks of electric motors, which can multiply that figure by four or six for a matter of seconds. It is also surprisingly common for nobody to have factored in that the site sits at 900 metres above sea level, or that summer temperatures inside the facility regularly exceed 40°C — two factors that can significantly reduce the generating set’s actual output.
At the other extreme are those who, out of caution or simply without a proper calculation, install a unit far larger than necessary. The generating set never operates within its optimal range, the engine suffers from internal carbonisation, fuel consumption is disproportionately high, and maintenance becomes more frequent. The result is a needlessly expensive investment that also causes problems.
In this article, we explain step by step how to size a generating set for commercial and industrial use: what factors to consider, how to apply the sizing formula, and how to select the most suitable range for your installation.
What does sizing a generating set mean?
Sizing a generating set is the process of calculating the power output needed to meet the actual electrical demand of your installation. This means looking beyond the nominal consumption of your equipment and accounting for factors such as starting peaks, load type, environmental conditions, and an appropriate safety margin.
Getting the sizing right guarantees three fundamental things:
- Continuity of supply: the generating set can power the full load without voltage drops or outages.
- Energy efficiency: a correctly sized unit operates within its optimal range, using less fuel.
- Longevity: avoiding overloads and low-load operation protects the engine and alternator, extending the service life of the unit.
A generating set running continuously above 80–90% of its capacity will deteriorate prematurely. Equally, one that runs well below its rated output (under 30%) is also harmful to the engine, leading to internal carbonisation and a loss of performance.
Key concepts before we start: kW, kVA and Power Factor
Before getting into the calculations, it is worth clarifying three essential concepts.
kW (kilowatt): active power This is the energy that electrical equipment actually consumes and converts into useful work. It is the figure that typically appears on the nameplate of industrial machinery.
kVA (kilovolt-ampere): apparent power This is the unit used to classify generating sets. It encompasses active power (kW) plus reactive power — which produces no useful work but is nonetheless necessary for the system to function. A 100 kVA generator can supply a maximum of 80 kW of real power.
Power factor (cos φ) This is the ratio between kW and kVA. In industrial installations, the standard value is 0.8. The conversion formula is straightforward:
kVA = kW ÷ power factor (0.8)
In installations with a large number of electric motors, the power factor may be slightly below 0.8; with modern electronic devices, it can approach 1. When in doubt, 0.8 is the safest figure to use as a reference.
It is worth noting that maintaining a power factor above 0.8 does not allow the generating set to deliver greater active power (kW), as this is limited by the mechanical output of the engine. For this reason, attempting to improve the power factor in this context serves no practical purpose. Furthermore, whenever an installation is running on a generating set, capacitor banks must be disconnected, as their simultaneous operation can cause disturbances or abnormal behaviour in the electrical installation.
How to size a generating set: step by step
Step 1: Load inventory
The first step is to draw up a detailed list of all the equipment you intend to connect to the generating set. For each item, you will need to know:
- Its rated power in kW (found on the nameplate or in the manual).
- The load type: resistive (heaters, incandescent lighting), inductive (motors, pumps, compressors, air conditioning) or capacitive (LED lighting).
- Whether it will run simultaneously with other equipment or at different times.
Step 2: Simultaneity factor
Not all equipment runs at the same time. The simultaneity factor allows you to align your calculation with the actual operating conditions of your installation. In an industrial facility with lighting, production machinery and climate control, it is unlikely that everything will be running at full capacity simultaneously. Applying this factor helps avoid unnecessary oversizing.
Step 3: Starting peaks
This is one of the most common mistakes when sizing a generating set: overlooking starting peaks. Electric motors, pumps, compressors and air conditioning systems all require significantly more power at start-up than during normal operation.
Starting current can be anywhere from two to eight times higher than the steady-state consumption. This is particularly critical when several pieces of equipment start up at the same time. A common solution is to stagger start-ups with delays of five to ten seconds between groups.
| Equipment type | Rated consumption (kW) | Approximate starting peak |
|---|---|---|
| Electric motor (direct-on-line start) | X kW | 4-8 × rated |
| Air compressor | 3,7 kW | ~11 kW |
| Water pump (2 HP) | 1,5 kW | ~3 kW |
| Air conditioning system | Variable | 2-3× rated |
| UPS | Variable | Requires specific calculation |
*Peak values are indicative only. The actual multiplier depends on the starting method: direct-on-line (4–8×), star-delta (2–3×) or variable frequency drive (1.2–1.5×). For critical equipment, always refer to the technical datasheet.
Step 4: Safety margin
It is always advisable to add a safety margin of 15–25% on top of the calculated power requirement. This allows you to:
- Prevent the generating set from running continuously close to its capacity limit.
- Leave room to add new equipment in the future.
- Compensate for potential performance variations due to environmental conditions.
If the generating set is to serve as the primary power source rather than a backup, the recommended margin rises to 25–35%, allowing the unit to operate within its continuous power range — with better fuel efficiency and a longer service life.
Step 5: The sizing formula
Once you have the total consumption in kW (including starting peaks for the most demanding group), apply the following formula:
Required kVA = (total kW × safety margin) ÷ power factor
Example: (40 kW × 1.20) ÷ 0.8 = 62.5 kVA
Environmental factors: altitude and temperature
One aspect that is frequently overlooked, yet can prove decisive, is the environmental conditions of the installation site. The loss of output resulting from high temperatures or altitude is known as derating.
Temperature
Manufacturers certify the performance of their engines and alternators at temperatures below 40°C. Each engine has its own characteristics, but as a general approximation, above that threshold the generating set’s actual output begins to decrease by around 3% for every 5°C of excess. In addition, the cooling system loses efficiency and fuel may reach the engine at too high a temperature, affecting combustion.
In the UK, where summer temperatures are generally more moderate, this factor is less commonly an issue — though it remains relevant for installations in poorly ventilated or heat-intensive environments such as foundries or large manufacturing facilities.
Altitude
The higher the altitude, the less dense the air, the lower the oxygen content, and the leaner the fuel-air mixture. In naturally aspirated engines, this effect can become noticeable even below 1,000 metres above sea level. In turbocharged engines, the loss begins to be significant above that threshold. As a general reference, a reduction of approximately 3% is applied for every additional 500 metres of altitude.
Practical tip: if your installation is situated above 800 metres or in an environment with high ambient temperatures, always share these details with your supplier so they can adjust the sizing to reflect the actual effective output, not just the rated figure.
Standby, prime or continuous power: which do I need?
Another fundamental consideration when sizing a generating set is defining its intended use:
- Standby (ESP): the generating set acts as a backup in the event of a mains failure. This is the most common scenario for grid-connected businesses. Sizing is based on maximum emergency power output.
- Prime (PRP): the generating set is the primary power source, operating with variable load. Typical for off-grid locations or industrial processes subject to frequent outages.
- Continuous (COP): the generating set runs 24 hours a day, seven days a week, under constant load. This requires more conservative sizing and a more demanding maintenance regime.
Which generating set range does your business need?
At Electra Molins, we offer three ranges of industrial generating sets designed to cover the most diverse requirements, from medium-sized businesses to large industrial installations and high-demand projects.
GlobalGen: 25 to 750 kVA
The GlobalGen range is designed for businesses that need a compact, versatile and easy-to-install generating set without compromising on reliability. Its lightweight design simplifies transport and commissioning in sites with restricted access or limited space.
It is the ideal choice for offices, retail premises, hotels, clinics, industrial SMEs and any installation requiring agile and efficient backup power in the 25 to 750 kVA range.
Industrial: 35 to 550 kVA
The Industrial range units are fitted with the AUT-MP12 control panel and come fully equipped as standard, with a wide range of optional configurations available. They are engineered for industrial environments where reliability and adaptability to different operating scenarios are the priority.
With outputs of up to 550 kVA, this range covers everything from mid-sized installations to industries with high electrical demands and specific technical requirements.
TopGen: 550 to 4.000 kVA
The TopGen range is built for the most demanding projects. With outputs of up to 4,000 kVA and fitted with the high-performance MP15 control panel, this range is the answer for large industrial facilities, data centres, hospitals, critical infrastructure and any project where an uninterrupted power supply is non-negotiable.
The MP15 panel offers advanced monitoring, load management and remote connectivity capabilities, making integration with BMS systems straightforward and enabling real-time status monitoring of the generating set.
| Range | Power output | Typical applications | Key feature |
|---|---|---|---|
| GlobalGen | 25 – 750 kVA | SMEs, hotels, retail premises, clinics | Compact, versatile and lightweight |
| Industrial | 35 – 550 kVA | Mid-to-heavy industry, logistics, manufacturing | AUT-MP12 control panel, outstanding versatility and performance |
| TopGen | 550 – 4.000 kVA | Large industry, data centres, critical infrastructure | MP15 control panel, maximum power, control and performance |
Common mistakes when sizing a generating set
These are the most frequent errors to avoid:
- Overlooking starting peaks. This is the most common mistake, and it can leave the generating set without sufficient power at the moment the machinery starts up.
- Undersizing to save on the initial investment. A generating set running continuously at the limit of its capacity will deteriorate more quickly and may cause voltage instability.
- Excessive oversizing. A unit that regularly operates below 30–40% of its capacity also suffers: internal engine carbonisation, high oil consumption, disproportionate fuel consumption and more frequent maintenance.
- Ignoring environmental conditions. Failing to apply altitude and temperature correction factors can lead to selecting a unit that does not deliver the actual power output you need.
- Not planning for future growth. If your business intends to expand its production capacity over the coming years, it is far more cost-effective to factor this into the initial sizing than to replace the generating set further down the line.
Correct sizing is an investment
Sizing a generating set correctly is not merely a technical matter — it is an economic and operational decision. A well-chosen unit runs efficiently, lasts longer, consumes less and guarantees the continuity of your operations when you need it most.
At Electra Molins, we have spent decades advising businesses on the selection and installation of industrial generating sets. If you would like our technical team to assess your requirements and recommend the most suitable solution, do not hesitate to get in touch.
Frequently asked questions about generating set sizing
What is the formula for calculating the kVA of a generating set? The basic formula is: kVA = (total kW × safety margin) ÷ power factor (0.8). On top of this, you need to account for the impact of starting peaks and apply altitude and temperature correction factors if the site requires it.
Is it better to err on the side of too much power when choosing a generating set? Not necessarily. Excessive oversizing brings its own problems: a unit running continuously under low load is prone to engine carbonisation, high oil consumption and disproportionately high fuel consumption. The ideal approach is to size with a reasonable safety margin — 15–25% above the anticipated actual demand.
Does altitude affect the performance of a generating set? Yes. Above 1,000 metres above sea level (or even lower for naturally aspirated engines), generating sets progressively lose output — approximately 3% for every additional 500 metres. If your installation is in a mountainous area or on an elevated plateau, this factor must be included in the calculation.
What is the difference between standby power and prime power? Standby power (ESP) is the maximum output the generating set can deliver for short periods in emergency situations. Prime power (PRP) is what it can supply continuously under variable load. If the generating set is to serve as your primary power source, always work from the prime power rating, which is the more conservative figure.
When is it advisable to stagger start-ups? Whenever there are several pieces of equipment with electric motors that could start simultaneously. Staggering start-ups with delays of five to ten seconds between groups reduces demand peaks and makes it possible to use a lower-rated generating set — with the corresponding saving on investment.
