Part 3 – Sizing Generators

As discussed in part 2 of the IT Professional’s Guide to Standby Power, (Power Quality Problems), if electrical equipment has to be supplied with power beyond the run time of the UPS, a generator will be required.

The correct sizing of a generator is not as simple as one might at first suppose.

Not all electrical equipment will need to be protected by a UPS, but some will still require a power source in the event of a power failure. Other electrical equipment not considered important, can be allowed to become non-operational until mains power is restored.

Sizing strategy

When deciding on the level of protection required for electrical equipment it helps to adopt a methodical approach.

A useful strategy is to list all electrical equipment and then ask the question, what would be the effect on the reputation, profits, employee safety and the smooth running of the company, should this electrical equipment fail.

Would it be a mild inconvenience or could it expose the company to extreme embarrassment, financial loss or even ruin? How much of an inconvenience would the loss of the organisation’s email, Internet access or primary business applications be?

Equipment to be protected

The UPS will need to be top of the list, as it will be protecting all critical systems.

Some equipment such as cooling plant will need to be included if associated with the server room. This is essential to prevent the servers, which will be running on Standby Power, shutting down because they become too hot.

Health and safety issues will need to be addressed, with lifts and emergency lighting being included.

Depending on the way your particular company is structured, there may well be across over with the Facilities Director, and it makes sense to work together when specifying a common resource such as a generator.

Surge current

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

All the above factors point to specifying a larger generator than may have at first seemed apparent, but there are other important reasons.

Generator size must exceed UPS power rating

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

Best practice would indicate that an on-line UPS with frequency regulation features should be specified in conjunction with a generator. As the load changes, due to equipment being switched on or off, so the generator frequency can change.

If the UPS detects that the frequency changes are out of tolerance, it will immediately revert to battery power. This can happen with both off-line UPS and line interactive UPS. Specifying an on-line UPS with frequency regulation will solve this problem.

An on-line double conversion UPS will typically operate at 90% efficiency on a full load. This is due to power losses incurred during the process of ‘cleaning’ incoming power.


From the perspective of sizing generators, another disadvantage of the UPS double conversion process is that harmonics are introduced.

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.

Inverters within UPS and other electrical equipment produce harmonics. The most problematic are the triplin harmonics. These are harmonics which are multiples of three. The third harmonic cycles at three times the speed of the first harmonic, which is 150Hz, the sixth at 300Hz and so on.

These much faster harmonics are superimposed on top of the basic 50Hz wave-form. The result of this, bearing in mind that equipment designed to operate at 50Hz may now be experiencing, for example, 300Hz, is that the electrical components heat up, reducing their working life.

A generator receiving triplin harmonics superimposed onto the basic 50Hz will experience higher running temperatures and can be expected to have both a shorter working life and a reduced power output. Although this undesirable side effect can be reduced with filters, the greater the size of the generator, the less it will be affected.


When a generator first starts up it needs to synchronise with the electrical current supplied by the UPS. If the generator is working close to full capacity, there is a chance that the generator will drop the load. Since this is clearly undesirable, this is yet one more reason to increase the size of the generator.


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. Larger generators with adequate capacity run cooler so heat becomes much less of a problem.

Extra capacity

If there is the potential that once the generator is installed, other needs may come to light or, during the next three to five years, expanded systems will require more power, extra capacity should be provisioned.

A good rule of thumb is therefore to oversize a generator between 1.25 and 2 times the output of the UPS where the generator is only supporting the UPS, and to increase this to the required rating when it has to support additional electrical equipment such as emergency lighting or air conditioning.

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, 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

The IT Professionals Guide to Standby Power  – contents

A Glossary of Standby Power Terminology

Part 1 – Planning your Standby Power Requirements

Part 2 – Potential Power Quality Problems

Part 3 – Sizing Generators

Part 4 – Specifying UPS

Part 5 – Matching a generator to your UPS

Part 6 – Making the right generator choices

Part 7 – How to ensure your batteries don’t fail

Should you wish to discuss any of the issues raised in these articles, please contact us.