Watts, Amperes or Amps, Volts, PF, KVA and kWh? Some of the power ratings used in the data center industry. This short blogpost will explain in simple terms the difference between the power rating and describe when each should be used in your data center architecture planning.
A KVA (Kilo Volt Ampere) is simply 1.000 volt amperes. The voltage, abbreviated as volts, indicates how much voltage there is on a connection. The full description can be found here: https://en.wikipedia.org/wiki/Volt. The number of amps indicates the size of the amount of current (Ampère is a physical unit). A term called apparent power (the absolute value of complex power) is equal to the product of the volts and amps without the losses due to mechanical / electrical losses (Power Factor = 1).
On the other hand, a watt (W) is a measurement of real power. Real power is the amount of actual power that can be drawn from a circuit. When the voltage and current of a circuit coincide, the real power is equal to the apparent power. However, as waves of current and voltage coincide less, less real power is transferred, even though the circuit is still carrying current. Differences between real and apparent power, and thus watts and volt amps, arise because of inefficiencies in electrical transmission.
The terms that you encounter most in data centers are watts and kilowatt hours. This unit indicates how many kilowatts are consumed in an hour. For example, if you have a cabinet that uses 1000 watts this state is on for one hour, then you have consumed one kWh. The number of kWh can be calculated by multiplying the number of amperes by the amount of volts, and dividing this by 1000.
The resulting inefficiency of electrical transmission can be measured and expressed as a ratio called the power factor. The power factor is a ratio (a number from 0 to 1) of real power and apparent power. In the case of a 1.0 power factor, the real power equals the apparent power. In the case of a 0.5 power factor, real power is approximately half that of the apparent power.
Deploying systems that have higher power factors result in less electrical loss and can help improve your Power Usage Effectiveness (PUE). Most Uninterruptible Power Supply (UPS) units will list the average power factor and real-time load capacity of the UPS, in addition to the KVA.
Example: You own a 1.000 KVA UPS unit (apparent power) with a 0.95 power factor. The resulting real power is 950 kilowatts.
Some useful conversion factors and formulas:
- VA = Voltage x Amps
- Watts = Voltage (root-mean-squared) x Amps (root-mean-squared) x Power Factor (PF) (a three-phase circuit would multiply the voltage by the square root of 3 or approximately 1.732)
- kWh = watts / 1000 per hour
- 1 BTU (British thermal unit) = Watts x 3.413
- 1 BTU = 1,055.053 joules (J)
- 1 watt = 3.413 BTU/hour
- 1 ton = 200 BTU/minute
- 1 ton = 12,000 BTU/hour
- 1 ton = 3.517 kilowatts