How Do I Calculate the Size of UPS Required?

UPS systems are sized according to the amount of power the connected loads will draw. This is measured in VA (Voltage x Amps) or Watts. Voltage represents the local mains power supply voltage which in the UK is 230Vac, 50Hz (single phase) or 400Vac, 50Hz (3phase).

VA is referred to as the ‘Apparent Power’ drawn by an electrical load.

Apparent Power (VA) = Volts (V) x Amps (A)

For a computer type load drawing 4 Amps the calculation would be:

VA = 230 × 4 = 920VA

The nearest UPS size to this would be 1kVA. It is always recommended to size your UPS to run at 80% or less. For more applications we add a further 20% for headroom.

VA = 230 × 4 = 920VA x 1.2 = 1104VA or 1.1kVA

The nearest UPS size to this would now be 1.5kVA.

The Amps used in the formula can be obtained from rating plates, brochures, datasheets and manuals, or with the correct electronic/electrical devices, measured. It is important to note that when taken from rating plates and published data, the figures will be topside and actual running may be at 50% less.

For a three phase application the VA or kVA calculation is made per phase i.e.

Three phase Apparent Power = kVA Phase 1 + kVA Phase 2 + kVA Phase 3

It can be harder to calculate the Watts (W) required. Watts is referred to as the ‘Real Power’ required by the load. 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).

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)

or

Apparent Power (VA) = Real Power (W) / Power Factor (pF)

See our VA to Watts and Power Factor conversion table for more information.