Reading a UPS spec sheet without being fooled
Almost nothing on a UPS datasheet is untrue. Quite a lot of it is true under conditions the sheet does not print.
Comparing two UPS quotations is harder than it should be, because the two datasheets are rarely quoting the same thing under the same conditions. Here is what to look at, in the order that changes decisions.
kVA is not kW, and the load runs on kW
A UPS rated 10 kVA at 0.8 power factor delivers 8 kW. Your equipment consumes watts. If the sheet leads with kVA and prints the power factor somewhere in the small type — or not at all — that is the first thing to establish, because it sets the real capacity of the machine.
Output power factor has moved over the years. Older designs sit at 0.8, newer ones at 0.9 or unity. A 10 kVA machine can therefore mean 8 kW or 10 kW, a 25 per cent difference in what you are actually buying, from an identical headline number.
"Efficiency: 96%" — at what load, and in what mode?
Efficiency varies with load. A UPS is usually at its best somewhere between half and three-quarters load and less efficient at the extremes, so a single headline figure is the peak of a curve, not the value you will run at. Ask for the curve, or at least for the figure at the load you expect.
The bigger question is mode. A high figure quoted in eco or line-interactive mode is not comparable to one quoted in double-conversion, because in eco mode the machine is passing raw mains through and is not doing the job you bought it for. Both numbers can appear on the same sheet without either being wrong. Compare double-conversion against double-conversion.
Overload rating always has a time attached
"110% overload" on its own means nothing. The specification is a pair: a percentage and a duration. 110 per cent for ten minutes and 125 per cent for one minute are ordinary; 150 per cent for a few hundred milliseconds is a different claim about a different situation. If the duration is missing from the sheet, it is missing from the design conversation too.
Transfer time on a machine that should not have one
A true online double-conversion UPS has no transfer time to battery, because the load is already running from the inverter and only the DC source changes. If a datasheet for an "online" machine quotes a transfer time in milliseconds, it is describing a line-interactive or offline topology, and the two are not interchangeable for a load that objects to a break.
Input power factor and input current THD
These describe what the UPS does to your supply rather than what it does for your load, and they decide the size of the cable, the breaker and — if you have one — the generator. A machine with input power factor above 0.98 and input current THD below five per cent is well behaved. Older designs without power factor correction can pull heavily distorted current and force a generator to be oversized well beyond the UPS rating.
Backup time figures, and the missing conditions
A backup time is meaningless without three things stated alongside it: the load it was measured at, the battery bank it was measured with, and the temperature. Thirty minutes at half load on a new bank at 25 °C is a different promise from thirty minutes at full load on a bank in its fourth year in a plant room at 38 °C.
Where a supplier quotes backup time without naming the bank, they are quoting a bank they have not yet decided to sell you.
The table version
| Printed on the sheet | What to ask |
|---|---|
| Capacity in kVA | What is the output power factor — so what is the kW? |
| Efficiency percentage | At what load, and in which mode? |
| Overload rating | For how long, and what happens after that? |
| Transfer time | Why does an online machine have one at all? |
| Input current THD | At full load or at the load I will run? |
| Backup time | At what load, which bank, and at what temperature? |
| Operating temperature | Is the upper figure the machine, or the batteries? |
| Standards compliance | Self-declared, or third-party tested with a certificate? |
Two questions that settle most of it
First: can I see the unit running on a load bank at my load before it ships? A supplier who can say yes has the equipment and the confidence, and you get to see the numbers rather than read them.
Second: will you put the quoted figures in the purchase order? Almost every specification argument disappears at that question, because the figures that survive it are the ones the supplier actually stands behind.
On our own sheets
Our published capacity ranges come from our parameter sheets. Where a detailed figure has not been measured on our own machines we would rather leave the row empty than fill it with a plausible number — a spec sheet is a commitment, and one wrong line costs more trust than ten blank ones.
Questions we get asked
Anything not covered here, call us — most of it is faster to answer on the phone.
Why do two 10 kVA UPS quotations differ so much in price?
Usually topology and power factor. A 10 kVA online double-conversion machine at unity power factor delivers 10 kW and conditions the supply continuously. A 10 kVA line-interactive machine at 0.8 delivers 8 kW and only intervenes when the voltage strays. Both are honestly labelled 10 kVA.
What output power factor should I look for?
Higher is better value for the same kVA, since more of the rating is available as real power. What matters more is that you know the figure before comparing quotations — otherwise you are comparing two different capacities under one number.
Is a higher efficiency figure always better?
Only if the two figures were measured the same way. An eco-mode figure and a double-conversion figure are not comparable, and a peak-of-curve figure is not the value you will run at. Ask for the load and the mode before treating it as a comparison.