Choosing a UPS: how many VA is enough?

The VA rating is the number people look at when buying a UPS. On its own it does not tell you how long your systems will stay up when the power fails.

VA and watts are not the same

VA (apparent power) and watts (real power) differ by a figure called the power factor, typically between 0.6 and 0.9.

So a 1000 VA UPS typically carries 600–900 watts. If your equipment's consumption is given in watts, compare in watts.

Catalogues lead with VA because it is the bigger number. When comparing, ask for both.

Measure your real load

Do not estimate. The figures on equipment labels are maximum consumption; real usage is usually far lower.

A server labelled 750W does not draw 750W continuously. Under typical load it may draw 200–300W.

The right approach is to measure. Even a simple plug-in meter gives the real number. Size from labels and you will buy an oversized, expensive UPS.

You decide the runtime

The real question is: how long should the UPS last?

There are two different goals:

• For an orderly shutdown: 5–10 minutes is enough. The aim is for servers to close down without losing data. For that, the UPS must be able to signal the server — software and a data cable are essential.

• To keep working: if outages are short and frequent, aim for 30–60 minutes. This is markedly more expensive and may need an extra battery cabinet.

Until you decide which you want, the right UPS cannot be chosen.

Common mistakes

• Plugging a printer into the UPS. Laser printers draw a large surge when the fuser heats and will trip the UPS instantly. Never connect one.
• Not installing the shutdown software. The UPS exists but cannot tell the server; when the battery runs out the system still loses power abruptly. The whole point is lost.
• Forgetting battery life. Batteries degrade in three to five years. A five-year-old UPS may last minutes even though the display says OK.
• Skipping network equipment. If the server is up but the switch and firewall are down, nobody can reach it. Include them.
• Never testing. Pull the plug once a year and watch what happens.

The right order

1. List the equipment to be protected — server, storage, switch, firewall
2. Measure real consumption, not label values
3. Decide the runtime you want
4. Add 20–30% for growth
5. Install shutdown software and monitoring
6. Put the battery replacement date in the calendar

We do this calculation and give it in writing. Selling an oversized UPS serves nobody.