How to size a UPS system

Critical Power Supplies has pleasure in providing you with the following sizing guide for sizing your UPS

When calculating the size of a UPS it is important to take into account the following:

Apparent power (VA or kVA)

This is defined as S=VxI for single-phase loads, S=(VLI x ILI)(VL2xIL2)(VL3xIL3) for three-phase loads where V is the voltage, and I is the current absorbed by the load under normal operating conditions (EN50091-1-1). This information can usually be found on rating hardware labels, and in the documents and information supplied with the system(s) to be protected. It is generally over estimated.

Active power (W or kW)

Is defined as P=Sxpf where pf is the power factor. If the value of P and pf of the load(s) are not specified, the power absorbed must be precisely measured in order to correctly size the right UPS. The typical load of a computer is associated with a pf of between 0.65 and 0.8. Active power is particularly relevant when sizing batteries.


Are voltage and current demands on the UPS in excess of its specification. They may be temporary during initial energising of a system or constant where too much steady state load is connected to the UPS output.

System Expansion

When sizing a UPS two factors are important. The reliability of any electronic device is improved when run at less than 100% of capacity. For UPS the load should be around 90% of the system size to guarantee long-term reliability. A factor should also be added for future expansion of the protected load(s). This is typically taken as 25%.

High switch on current demands

At power-on, some loads have a high initial switch on current demand lasting for a short time period (50 to 100ms). For example laser printers, some types of lights, isolation transformers and pumps. For these types of load it is good practice to oversize the UPS by a factor of at least 3 or remove them from the protected system, especially when they can be allowed to power down on mains failure.

UPS runtime

Depending on your critical application and business environment you may only require the UPS to last 10minutes or 24hours. However, its important to know how long you require the UPS to last at this stage, as UPS recharge times can be within 4hours to 90% for small single phase solutions to typically ten times the discharge to beyond 12hours for long autonomy solutions.

Critical Power Supplies provides sizing assistance and if you have any queries or require additional information please do not hesitate to contact us or request a site survey

Things which you will require in order to make this calculation.

- Access to all equipment to be protected by the UPS.
- Idea of future expansion or growth of the load.
- How much runtime you would like the UPS solution to last.
- Calculator and pencil or paper.