UPS Sizing Guide

What do I Need to Know to Size a UPS System?

There are several factors that influence the sizing of a UPS system. These are:

1. The Combined Load

Initially, you’ll need to list all of the equipment that you would like to be protected by the UPS in the event of a loss in electrical supply. This list can then be refined to establish those items of equipment that are critical loads (which require the backup of the UPS) and those that are non-critical (and can be allowed to fail if the mains supply does).

2. The Power Range

Once you have identified the critical pieces of equipment, add together all of the individual loads (the amount of power each device will consume) to calculate the combined load figure. It is essential this figure is based on the equipment being used during peak working hours and not at quieter times of the day (please refer to equipment labels and technical data sheets for this information).

Where power consumption is stated in Watts (W) this will need converting to Volt-Amperes (VA) to match UPS system ratings, that are measured in VA or kVA. This can be done by dividing the power consumption (W) by the power factor.

It is important to always allow for scope for future expansion of any UPS system. This can be done by multiplying the total VA figure by 1.2 or 1.25 to accommodate for this.

Please note: A UPS should never be sized to run at 100% load capacity as this can greatly affect both the overall reliability and stability of the system. 

3. Leading/Lagging Loads

When calculating the combined load figure, it is also essential to accommodate for any loads that could be problematic; such as motors, compressors, air conditioning units, laser printers etc that may draw higher currents during start-up or as part of their normal operating process. If not taken into consideration, these may cause the UPS to overload or be sent into bypass mode at the point when the higher current is called into action.

4. Battery Autonomy

When sizing a UPS you will need to consider how long the UPS will need to run for to bridge the gap between any power failure and generator start up (if there is one), or how much time will be needed to keep critical equipment operating until work-in-progress can be saved and the equipment shutdown safely.

5. What Environment Will the UPS be Stored In?

Manufacturers typically recommend that UPS batteries operate in an ambient temperature ranging between 20⁰C and 25⁰C as this provides the optimal balance for battery chemistry and performance. 

6. Is the UPS to be Stored in a Rack?

Rack-mounted UPS save on floor space due to their design, the UPS being mounted in server racks or cabinets rather than side by side. They also offer increased scalability so if you anticipate future growth, this style of UPS could be a good option for you.

7. Topology

There are three primary UPS technologies available, all offering different performance characteristics and varying degrees of protection - these are online, line-interactive and offline (also known as standby).