How to size a UPS for your office

A UPS that is too small shuts down in minutes; one that is too big wastes money. A simple method for getting it right.

In an office in Ghana a UPS is not optional, but the wrong size causes as many problems as none at all. Here is the method we use.

1. List what must stay on

Decide what the UPS actually needs to protect. For most offices that is the server, the network equipment (router, switches, Wi-Fi) and a few key workstations. Printers, especially laser printers, should not go on a UPS: their power draw is large and spiky.

2. Add up the watts

Check the label or specification of each device for its power in watts, and add them. If a label only shows amps, multiply by the supply voltage (around 230 V) for a rough figure.

3. Understand VA versus watts

UPS units are sold by VA (volt-amperes) and watts. The watt rating is the real limit, and it is lower than the VA figure, often around 60 to 90 percent of it depending on the model. Always compare your total against the watt rating.

4. Leave headroom

Aim for your load to be no more than about 70 to 80 percent of the UPS watt rating. That leaves room for growth and gives the batteries an easier life.

5. Decide on runtime

Runtime depends on the load and the batteries. Manufacturers publish runtime charts for each model: find your load in watts and read off the minutes. If you need longer than the internal batteries give, choose a model that accepts external battery packs, or pair it with a generator or solar system.

6. Pick the right type

Line-interactive units suit desktop PCs and small network cabinets.
Online (double conversion) units give continuously clean power and suit servers and sites with poor power quality.

7. Look after the batteries

Heat shortens battery life. Keep the UPS in a ventilated or cooled space, run a self-test regularly, and plan to replace batteries every few years. A UPS with dead batteries gives no warning until the day the power goes.