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.

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 organization's email, Internet access or primary business applications be?

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 cross over with the Facilities Director, and it makes sense to work together when specifying a common resource such as a generator.

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 for specifying a more powerful generator.

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.

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.

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.