Size Matters: Optimizing Your Backup Power System

Sizing a generator requires some decision making. Generators are typically sized to back up the critical load in a facility, since the costs can be quite significant for backing up the entire facility's electrical load. IT and communications systems will be critical during an outage as well as lighting, heating, office equipment and perhaps cooling.

Calculating the Load

First, decide which loads need power during an outage. Next, determine how much electrical capacity is required to operate this critical load. You may need a generator supplier to assist in these estimations.

Don't overlook critical loads. As an example, the heating system may be fueled by natural gas, but the blower fan is electrical. If there's a 1 horsepower (hp) electrical motor on the blower, this will require just under 1 kilowatt (kW) of power as the following formula shows:

kW = 0.746 kW/hp x hp / motor efficiency

kW = 0.746 kW/hp x 1 hp / 0.85

kW = 0.88

This formula can be used to convert horsepower to kilowatts for other electrical equipment. Once you have estimated your total required electrical capacity for emergency power, you can select a generator. Once again, an electrician or generator supplier should be involved in this decision, but the general rule of thumb is to size the generator 30% to 40% larger than the calculated load.

Adding Capacity: Factors to Consider

This additional capacity helps account for the starting current requirements of motors. The inrush current during motor startup can be three to 10 times the normal operating current, so a generator will need extra available power to accommodate this and to add a safety margin. Consider electronically commutated motors for an equipment upgrade; they have no sudden inrush.

Mechanical compressors used in cooling equipment also have large inrush currents. Computer room air handling units, because they use chilled water and have no internal compressors, can eliminate part of this problem. However, dehumidification systems can draw a lot of power, so they must also be accounted for in load calculations.

Uninterruptible power supplies typically add 10% to 15% to the total load. The capacity of the automatic transfer switch should match the generator load. Include block heaters in generators, as well as fans, pumps and other equipment when calculating the total load. For computer room cooling equipment, having redundant units requires upsizing of all upstream electrical requirements. For multiple units, stagger startups to minimize inrush current, which will have less impact on generator sizing requirements.

Power factor correction capacitors do use some energy, but not much. The larger concern is producing a leading power factor situation or producing harmonic resonance conditions.

Consult with the Experts

Work with a generator specialist to determine the power draw of the equipment you need to operate and the generator size that will do the job.

For larger generator applications, and for those situations where there's some uncertainty, consult a generator expert. Frequently, the companies selling or leasing generators will make the calculations for you, though you may want to consult with more than one supplier to confirm your sizing requirements.

Remember that upgrading your equipment to reduce energy consumption and eliminate energy waste elsewhere can reduce your total load and the generator size required.