Understanding the difference between kVA and kW is critical to proper operation of a data center. The equation is simple – kW equals kVA multiplied by the power factor, but the implications can be a little more complex.
Power factor is the ratio of resistive to reactive power. In layman’s terms resistive power is burned on the spot whereas reactive power does not burn all of the power and “bounces” some back into the system. A lightbulb is an example of resistive power while an electric motor produces reactive power. Most electric systems and utility grade equipment are designed around a power factor of 80%. Most data centers, on the other hand, are almost completely resistive load and have power factors approaching unity (100%).
Since the data center power factor is high and, as such, kW is very close to kVA, some people discount the difference and use the two interchangeably. This is very dangerous and can lead to the creation of “phantom” capacity.
The fundamental limitation crops up in how much capacity critical equipment such as the generator and UPS have. Most of these systems are designed for a power factor of 80%, which means that they can only produce kW equivalent to 80% of kVA.
What does this mean. Let’s walk through a sample calculation. A data center is filled full of 120 V circuits where the customers are allowed to draw up to 16 Amps, which is equivalent to a kVA of 1.92. If we assume that this equipment has a power factor of 95%, this is equal to 1.82 kW. If we look at the other side, the 600 kVA UPS has a power factor of .8, so it produces 480 kW. So the capacity of the UPS is 480/1.82=263 circuits. If we had done “simple” division on the kVA side, we would have had a capacity of 600/1.92=312 circuits. So we actually have 49 fewer circuits than the “simple” math would have indicated, hence the reason that it is critical to convert all loads and capacities to kW!
Recently, manufacturers have begun to address this by designing systems with higher power factor ratings. A few companies are also marketing retrofit kits for existing UPS systems to increase the power factor rating.
Check back in a few weeks for the next installment – “Implications of Data Center Power Factor for the Utility Grid”.