Sizing a UPS for a small server room

Most undersized UPS installations are not the result of saving money. They are the result of reading the wrong number off the label, and the wrong number is printed larger.

VA is not watts

A UPS is sold by VA — volt-amps — and the load on it is drawn in watts. The two are related by the power factor, and on a cheaper unit that can be as low as 0.6. So a "1500 VA" UPS delivers around 900 W, and a rack drawing 1100 W will overload it the first time the mains drops. It will not have failed slowly beforehand: on mains, the UPS was passing the load straight through and reporting nothing.

Read the watt rating. If the label does not give one, assume 0.6 and buy accordingly, or buy a unit that states its power factor.

Measure the load, do not add up the labels

The rating plate on a server says what its power supply can deliver, not what the machine draws. A server with two 750 W supplies typically draws 200–300 W. Adding plates produces a figure three to four times the real load and a UPS that is either far too large or, more often, dismissed as impossible and replaced by guesswork.

Plug a clamp meter or a metered PDU on the feed for a working day. Take the peak, add 25% for growth, and that is the watt figure to size against.

Decide what the runtime is for

There are two honest answers and they lead to different purchases.

• Ride through the flicker and shut down cleanly. Five to ten minutes. The UPS covers the short interruptions that are most outages, and for a real cut it signals the servers to shut down properly. This is the right answer for almost every small server room, and it is the cheaper one.
• Stay up until the generator takes over. Only meaningful if there is a generator, and then the runtime need only cover the generator's start time — usually under a minute, sized as five for margin.

"Stay up as long as possible" is not an answer. It is the phrase that buys a UPS with two hours of runtime under a load that nobody shuts down, so the batteries drain flat and the servers crash anyway — after two hours instead of at once.

The shutdown signal is the whole point

A UPS that cannot tell the servers the power has gone is a large battery. The USB or network card that lets it do so is the part most often left in the box. Install the agent on every server, test it by pulling the plug once, and put that test in the AMC schedule.

Batteries have a date

Sealed lead-acid batteries hold rated capacity for three to five years and then lose it quickly. A five-year-old UPS that has never been tested will report "battery good" right up to the outage, when it will give thirty seconds. Replace on schedule, not on failure; the batteries cost a fraction of the unit and the outage costs more than both.

Do not plug these in

Laser printers, space heaters, the kettle in the corner. A laser printer's fuser draws over a kilowatt for a second when it warms up, which is enough to trip a UPS sized for the rack — and it will do so on a Monday morning, on mains, with nothing else wrong.