One of the most difficult tasks, when selecting a UPS, is to calculate the correct power consumption of your equipment. This measure is required to establish UPS’s rating or the amount of load, in volt-amperes (VA), that it’s designed to support. After all, the amount of power drawn will affect such important parameters as:
Most UPSs are rated at both Volt-Ampere (VA) and Watts (W), the difference between these two values will be explained below, but it's important to note that both are significant. At this stage it is timely to explain the units of electrical power and how they are measured. Remember, power is the rate at which energy is generated or consumed and the unit of measurement for power is the Watt (W).
A watt is the unit of electric power. One watt is a measure of the power consumed when a current of one "ampere" flows under a pressure of one "volt".
On all electrical appliances you will find the "rating" marked in watts (for example, iron - 600 watts, heater - 1 000 watts, lamps - 100 watts. The power taken by any appliance (such as a toaster, heater, kettle etc.) is found by multiplying the voltage by the current:
Watts (W) = Volts (V) x Amperes (A)
It is straightforward when dealing with Direct Current (DC) circuits, where both Volts and Amps are constant. However, this concept becomes more complicated when we apply it to Alternating Current (AC) circuits. And thus we need to introduce a new unit of measure – Volt-Amperes (VA)
Put it simply, in AC circuit, VA is a measure of how much current is going through the system at particular voltage, whereas Watts is a measure of how much real power is going through the same system. So in AC circuit:
Volt-Amperes (VA) = Volts (V) x Amperes (A)
In Australia, as of 2000, the mains supply voltage specified in AS 60038 is 230 V with a tolerance of +10% -6%. However 240 V (and less commonly 250 V) is within tolerance and is commonly supplied
Therefore, a UPS rated at 1000 VA, can support equipment that is drawing up to 4.3 Amps:
Amps (A) = 1000VA / 230 V = 4.3 Amps
In DC circuit, Volts and Amps are constant and therefore multiplying one by the other yields Watts which is also equals to VA, however, in AC circuit, Volts and Amps are represented by sinusoidal curves which can be misaligned (out of phase). Therefore, multiplying Volts and Amps in AC circuit will provide the correct apparent power (VA) but not necessarily correct real Power (W).
For many types of electrical equipment the difference between apparent power (VA) and actual power (Watts) is very slight and can be ignored, but for some computers the difference is very large and important. Many desktop personal computers present a nonlinear load to the AC supply. This is because they have a power supply design known as a "capacitor input switch mode power supply". In a study done by PC Magazine, it was found that typical personal computer systems exhibit a power factor of .65 which means that the apparent power (VA) was 50% larger than the actual power (Watts)
To find out more about the relationship between Watts and VA (power factor), please refer to this White Paper
Or if you'd like to know what 'Power Factor' is and how it relates to UPS, please refer to this White Paper
Nowadays, most of the electrical equipment comes with a rating plate which identifies the amount of electrical power it consumes. So the ‘name plate’ or a ‘rating plate’ should be your first point of reference when establishing power requirements. Note, rating plates typically overestimate the amount of electrical power required as manufacturers tend to allow for some tolerance. Relying solely on nameplate ratings may lead you to oversize the UPS system, so always use your equipment manufacturer’s sizing calculator tools as well, if available. Most major manufacturers have Web-based or downloadable sizing tools that can closely estimate your equipment’s power draw based on the configuration you are using.
The rating plate could specify Watts, Volt-Amperes or Amps. Be sure to apply the correct unit of measure to the UPS rating. For example if the rating plate lists power consumption in Watts – than use Watt-rating of the UPS, if it lists Volt-Ampere – than use VA-rating of the UPS, if it lists Amps – than multiply Amps by nominal Voltage (230V for Australian households) and use the VA-rating of the UPS.
You can also measure power consumption of your equipment using test and Measurement tools such as Ammeter or Multimeter. However, it is important to note that the power measured is an instantaneous value and can vary greatly (example the amount of power drawn by a desktop PC will be a lot smaller when on standby as opposed when playing a HD computer game).
When choosing the size of the UPS based on power consumption of your equipment it is important to consider the maximum current that such equipment may draw. If your UPS will be supporting motors, variable-speed drives or laser printers, add more VA capacity to your requirements to account for the high power inrush that occurs when those devices start-up.
More often than not, a UPS will be supporting a number of loads (computers, servers, monitors, etc.). So, once you know the power requirement for each component, just add the Watt, VA or Amp figure together to arrive at the total Power rating that is required to support your equipment.
Users that anticipate rapid near- or medium-term growth should multiply the total power rating by 1.2 or higher, to build in room for growth.