Uninterruptible power supplies are typically sized in VA or kVA (1000VA) and will also have a Watts (or KW) rating. The relationship between VA and Watts is referred to as Power Factor. Where the VA=Watts, Power Factor=1 and is referred to as Unity Power Factor. For more information on UPS sizing calculations and formulae please refer to the section below or contact us for more information or a battery runtime sizing.
Most UPS systems are sized according to VA or kVA (where 1000VA = 1kVA). VA is referred to as the ‘Apparent Power’ drawn by an electrical load. VA is calculated by multiplying the RMS supply Voltage (V) by the load current draw Amps (A).
Apparent Power (VA) = Volts (V) x Amps (A)
In the UK and Europe the standard mains power supply voltage (single phase) is 230Vac. If the current drawn by a load is 2A, the VA value is:
Apparent Power (VA) = 230 (V) x 2 (A) = 460VA
For this load the nearest UPS size would be a 500VA, 600VA, 750VA or 800VA UPS system. It is recommended good practice to allow for future load expansion and to size a UPS with at least a 20% head room between the load VA and capacity of the UPS.
To calculate the three-phase mains power supply load size, the UPS sizing calculation consists of measuring the Amps drawn per phase and calculating the Apparent Power per phase and then adding these together.
Three phase Apparent Power = kVA Phase 1 + kVA Phase 2 + kVA Phase 3
In addition to leaving a 20% headroom it is also considered good practice to balance the loads across the three phases. Unbalanced loads can lead to lower UPS operating efficiencies and phase conductor overheating.
If the calculated load per phase = 10kVA Phase 1, 8kva Phase 2 and 7kVA Phase 3, the UPS system loads would be fairly well balanced. The overall three phase UPS size required would be 25kVA with the nearest model typically being a 30kVA UPS. For a 20% headroom and additional capacity a 40kVA UPS system would be recommended.
Note that three phase UPS are typically sized in kVA or for larger systems (mega data centre sized) the term MVA is used. UPS systems can also be calculated in Watts, kW or MW.
Calculating a UPS System Real Power Rating – Watts (W)
The Watt rating of a load is measured in Watts and this number is used to size batteries for a load size and runtime. Watts is a typical measurement for linear resistive loads and can be calculated as:
Real Power (W) = Amps (A) x Volts (V).
Electronic equipment such as computers and servers use Switch Mode Power Supplies (SMPS) to convert the incoming AC (alternating current) from the mains power supply to the levels of DC (direct current) required to power internal circuits. SMPS are capacitive and when sizing a larger UPS system it is important to take into considering the Real Power Watts.
The difference between Apparent Power and Real Power is known as the Power Factor. This is the degree to which the current and voltage waveforms are out of phase with one another.
Real Power (W) = Apparent Power (VA) x Power Factor (pF)
Apparent Power (VA) = Real Power (W) / Power Factor (pF)
When calculating the load to be powered by a UPS system and selecting the right sized mode for the application it is important to consider the following:
Please contact us for further information or to arrange a UPS site survey.