How to size a UPS for an industrial load
Most undersized UPS installations we are called out to went wrong at the quotation stage, not on site. Almost always for the same three reasons.
Sizing a UPS is arithmetic, and the arithmetic is not hard. What makes it go wrong is that two of the numbers involved are routinely misread — the difference between kVA and kW, and the difference between running current and starting current. Get those two right and the rest follows.
Work in kilowatts, not kVA
A UPS is sold by its kVA rating, but your equipment consumes watts. The two are related by the output power factor, and that figure belongs to the UPS, not to your load. Our units are rated at 0.8 lag, so a 10 kVA machine delivers 8.0 kW and a 60 kVA machine delivers 48 kW. If you total 9 kW of equipment and order a 10 kVA UPS because the numbers look close, you have bought a machine that is already overloaded.
Total the continuous wattage of everything going on the circuit. Use the nameplate figure where you have it, and a clamp meter reading over a working day where you do not — nameplates are usually conservative and a measured figure will save you money.
Add headroom, but add it for a reason
Twenty to twenty-five per cent is the usual allowance. It covers two things: load that will be added over the machine’s life, and the transient current when equipment powers up. If you know the site is at its final configuration and everything on it is electronic, the lower end is honest. If there is a plan to add a rack next year, size for the rack now — a UPS is not a component you want to replace because the load grew ten per cent.
Motors change the calculation completely
This is where most specifications fail. A motor starting direct-on-line draws six to eight times its running current for a second or two. A UPS has to survive that surge, not just the steady state. A 5 kW compressor with a running load well inside a 10 kVA machine can still trip it on every start.
Which starting method is used matters as much as the motor rating. Direct-on-line is the worst case. Star-delta reduces the inrush to roughly a third. A soft starter or a VFD reduces it much further and changes the picture entirely. Tell whoever is quoting which one you have — it is the single most useful sentence in an enquiry.
The short version
Total watts ÷ UPS power factor = kVA needed. Add 20–25%. Round up to the next model. Then check separately that the machine survives your largest motor starting.
Capacity and runtime are two separate purchases
The UPS rating sets how much load you can connect. The battery bank sets how long it holds. The same 10 kVA machine can run eight minutes or two hours depending entirely on the bank behind it, and on most installations the bank is the larger part of the invoice.
Decide the runtime you need first, and be specific about why. Covering a DG start window is ten to fifteen minutes. Riding out the feeder blips typical of a Pune MIDC estate is two or three. Shutting down a server stack cleanly is however long your shutdown scripts actually take, which is usually longer than people assume.
A worked example
A small server room: two rack servers at 450 W each, a switch and router at 60 W, an NVR at 180 W, and two workstations at 200 W each. That totals 1,540 W. At 0.8 power factor that is 1.93 kVA. Add 25% and you need 2.4 kVA, so a 3 kVA machine covers it with room to spare.
Now the same room with a 1.5 kW precision air conditioner on the same supply. The AC alone is nearly as large as everything else combined, and its compressor starts direct-on-line. That is not a 3 kVA job any more. It is also worth asking whether the AC belongs on the UPS at all — in most server rooms the answer is no, and moving it to the DG side halves the machine you need to buy.
What to send when you ask for a quote
- The equipment list with wattage, or a clamp meter reading if you have one
- The backup time you need, in minutes, and what it is covering
- Whether the supply is single or three phase
- Any motor loads, with their rating and starting method
- The room: ambient temperature, ventilation, and where the battery bank can go
That is enough for a quotation that will not change materially after the site survey. Without the motor detail, any figure is a guess.
Questions we get asked
Anything not covered here, call us — most of it is faster to answer on the phone.
Can I just add up the nameplate ratings?
You can, and you will usually oversize. Nameplates state the maximum a device can draw, and most equipment runs well below it. A clamp meter reading over a normal working day gives a truer figure and frequently lets you buy a smaller machine. Where you have no way to measure, nameplate plus headroom is the safe default.
What power factor should I use if I do not know it?
Use the UPS output power factor, which for our range is 0.8 lag — that is the figure that converts the machine’s kVA rating into the kilowatts it can actually deliver. It is a property of the UPS, not of your equipment, so it does not change with what you plug in.
Is it worth oversizing so I have room to grow?
Within reason. A UPS running at very light load is less efficient and the battery bank is sized to the machine rather than the load, so a large machine with a small load is money spent twice. Twenty-five per cent headroom is sensible; a hundred per cent usually is not, and if you genuinely expect the load to double it is often cheaper to plan a second unit than to buy one enormous one now.