Part 2 – Sizing Generator

Sizing a generator to you specific needs and requirements is a essential part to installing a backup power solution. Whether you need a rescue generator for an immediate emergency backup power, or to use one in a remote location where access to the electrical grid can be difficult or non existent, understanding the how generator sizing works is necessary.

When is a generator required?

If electrical equipment has to be supplied with power which is longer than the run time of what the UPS is capable of doing, a generator will be required.


What you find is that when a generator starts up it needs to take its time to synchronise with the UPS units electrical current. If the generator is working close to full capacity, there is a chance that the generator will drop the load. The only way to stop this from happening is by installing a larger generator.

Quick Checklist

•Verify the effect on your company should certain electrical equipment fail

•Identify levels of power protection required for different pieces of electrical equipment

•Identify generator power loading

•Increase size to cater for surge current, UPS power efficiency, harmonics and synchronisation. operating temperature and future electrical loads

•A rule of thumb is that the generator size should exceed the UPS power rating by a factor of between 1.25 to 3.00, or more

Surge current

Once all electrical equipment has been identified, careful consideration should be given to electrical inrush or surge current. This occurs when electrical devices start up and where the initial start-up power consumption greatly exceeds the normal working load.

The generator needs to support both the UPS power rating and an additional 20%-30% to cover the ‘power conditioning’ process.

If you're unsure about what you need or would like further explanation, please contact us and we are happy to help


Harmonics are a distortion of the smooth waveform of the AC electrical supply. They can be explained by looking at the waveform of the basic or first harmonic, which cycles at 50Hz. You will have noticed on many European electrical appliances, 220V 50Hz. The 50Hz refers to the first harmonic, and these appliances are designed to work at this frequency.


Temperature also plays a significant role. With the generator running, a rise of 10°C is usual within the plant or engine room. If the ambient temperature is already 20°C the resultant 30°C temperature will be detrimental to the generator.

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