Generator Sizing Guide

Standby generators run intermittently when started-up in response to a mains power failure. Often standby power generators are undersized leading to a failure when the generator tries to ‘pick-up’ a load in excess of its rating. As part of a power protection strategy it is vital to ensure size the generator correctly to avoid overloads and match it to future capacity plans. This will also help to ensure the generator itself reaches its planned for life expectancy and avoids unnecessary maintenance or repair costs.

Correctly sizing a generator and selecting a generator means choosing between single-phase or three-phase, knowing and fully appreciating your power requirements, load characteristic (is it reactive or resistive?), the type of equipment it will be used to protect, its application (domestic, industrial, business?), and the synchronisation requirements of the uninterruptible power supply (UPS) it will be running alongside. It can be confusing, and for that reason, it is best to consult with a certified and experienced electrician in order to get it right.

Firstly, you need to calculate the power rating of all the equipment that needs to be powered by the standby generator. This information can be found in user manuals or on nameplate labels on the outside of the equipment. However, beware: some are rated in Watts and others in Amperes. Also, most electrical equipment has different starting and running Wattages (often more at start up and less when running) and this must be factored in. You need the final measurement to be in either kW or kVA. The following calculations will help:

Wattage = Amperes x volts (for resistive loads)
Wattage = (Amperes x volts) x load factor (for reactive loads)

A resistive load is one that has no initial switch on surge and the current steadily rises to a stable running state.

A reactive load (also known as an inductive or capacitive) is one that has a potentially high inrush current, which could cause system overload if not accounted for in sizing a generator.

When synchronising a generator with a UPS, you need also to take into consideration the harmonics that the UPS’s rectifier may generate as this will have a bearing on generator size. The generator must be able to accept the load of the UPS and the UPS rectifier and static bypass supplies must be able to operate with and synchronise to the output of the generator. For this to occur, the UPS must be able to synchronise to the voltage waveform supplied by the generator.

The correct rule of thumb is to oversize a generator rather than opt for one that is smaller than required. This is particularly true for standby generators, where the generator supplies power as an alternative to mains power but for a short duration (typically when mains power fails). Standby generators are normally sized 10% larger than Prime Power Generators (that supply power constantly as an alternative to mains). Typically, a generator should be 25-100% larger than the UPS that is it working alongside and this should be increased further when additional essential loads (air-conditioning) are to be powered.