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 \u2014 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.<\/p>\n
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<\/strong>.<\/p>\n 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\u00b0C \u2014 two factors that can significantly reduce the generating set’s actual output.<\/p>\n 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.<\/p>\n 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.<\/p>\n 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.<\/p>\n Getting the sizing right guarantees three fundamental things:<\/p>\n A generating set running continuously above 80\u201390% 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.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n Before getting into the calculations, it is worth clarifying three essential concepts.<\/p>\n kW (kilowatt): active power<\/strong> 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.<\/p>\n kVA (kilovolt-ampere): apparent power<\/strong> This is the unit used to classify generating sets. It encompasses active power (kW) plus reactive power \u2014 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.<\/p>\n Power factor (cos \u03c6)<\/strong> This is the ratio between kW and kVA. In industrial installations, the standard value is 0.8. The conversion formula is straightforward:<\/p>\n kVA = kW \u00f7 power factor (0.8)<\/em><\/p>\n 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.<\/p>\n 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.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n Step 1: Load inventory<\/strong><\/p>\n 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:<\/p>\n Step 2: Simultaneity factor<\/strong><\/p>\n 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.<\/p>\n Step 3: Starting peaks<\/strong><\/p>\n 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.<\/p>\n 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.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\nWhat does sizing a generating set mean?<\/h2>\n
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Key concepts before we start: kW, kVA and Power Factor<\/h2>\n
How to size a generating set: step by step<\/h2>\n
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