All power supplies apart from the key parameters i.e. output voltage or current capacity are also defined with a parameter specifying how much power it can feed to a connected device. The parameter is expressed in watts (for the British engineer and inventor James Watt). 1 W (watt) is a power input of a device fed with 1 V (volt) at 1 A (ampere). The power is calculated from the following equation:
P = U x I
Where: P – power [W] U – voltage [V] I – current [A]
Using the equation with a known supply voltage and current we can calculate a power input of the connected load and based on that choose a compatible power supply. Using the same method, we can determine the power output of the power supply by multiplying the maximum output current by the output voltage. Example switched mode power supply: 12 V/5 A/P (Fig. 1) available in Delta offer has an output voltage of 12V at current capacity of 5A. Using the equation, the power output of the power supply is 60W.
Fig. 1. Example switched mode power supply 12 V/5 A/P available in Delta offer.
While for the receivers and power supplies the power is given in watts (W), for the power supply with AC output voltage the power is specified in volt-amperes (VA). Since we discuss the alternating current, i.e. a current whose value changes in time, the electrical power is a complex value and it includes an active power, a reactive power and an apparent power. The latter is expressed in volt-amperes.
A power factor used to describe all devices fed with alternating current is also a key parameter. The power factor is a ratio of the active power (expressed in watts) to the apparent power expressed in (volt-amperes).
For a standard resistive device, e.g. incandescent light bulb or an electric heater, the power factor is close to one. We can assume that the active power equals the apparent power. However, for other capacitive or inductive devices, e.g. capacitors and motors, the power factor is significantly lower. The devices may consume much more power than suggested by the calculation using the equation given above. The power factor must be allowed for when designing any electrical system and selecting the AC power supply. An example device is a TR-60VA AC power supply (Fig. 2) with a 60VA output power.
Fig. 2. Example AC power supply available in Delta offer.
Specifying the active power in watts and the reactive power in volt-amperes is also used in relation to UPS devices. As mentioned before, the power expressed in watts and volt-amperes for certain electric loads is the same, however, the values differ for IT devices.
Some manufacturers of UPS power supplies give the apparent power only i.e. the power expressed in volt-amperes, since its value is significantly higher than the active power. The practice shows that UPS power in watts is 60% of the value in volt-amperes.
Remember that neither active or reactive power can be exceeded for any power supply, since it will cause a permanent damage. Also, correct operating conditions preventing device from overheating must be maintained to reduce the failure rate and extend the service life.