A UPS is bought by the VA on the box and disappoints by the minutes on the battery. Sizing one properly takes four numbers: the load in watts, its power factor, how long the load must be carried, and how much of the battery can really be used at that rate. This guide goes through them, explains the three topologies of IEC 62040-3 and which loads care about the difference, and ends with the small server room from the calculator, where the 2 kVA UPS is the easy part and the 12 Ah battery is the part that needs thought.
What a UPS is for and what it is not
An uninterruptible power supply does three things. It rides through the cuts and dips that last a few cycles to a few minutes, so that equipment never sees them. It gives the servers time to shut down cleanly when the cut is longer, by signalling them over USB or the network, which is the real reason a server has a UPS at all. And it bridges the 30–60 seconds a standby generator takes to start and take the load. Most of them also condition the supply on the way through, to a degree that depends on the topology. What it is not for is hours of backup: a UPS carries a load for minutes on a small bank of sealed batteries, with a charger sized to refill them over the rest of the day.
In Pakistan the word has been stretched. The "UPS" sold in every electrical market, a 12 V or 24 V inverter-charger with a 150–200 Ah tubular battery, is an inverter for load-shedding: it carries fans, lights, a fridge and a router for four to eight hours, transfers in 10–20 ms, and is sized on hours and depth of discharge, which the battery backup runtime calculator does. A true UPS is sized on minutes and on how fast the battery can give up its energy. They are different tools, and a server room in Lahore usually needs both: the UPS to bridge the seconds and shut things down, and an inverter or a generator for the hours.
VA, watts and the power factor trap
The load draws real power in watts, which is what heats the inverter, and apparent power in volt-amperes, which is the current it draws times the voltage and is what the inverter's transistors must carry. The ratio is the power factor. A PC or a server with an active power-factor-corrected supply, which is every supply made to IEC 61000-3-2 for more than about 75 W, runs at 0.95–0.99; older supplies, monitors and small adaptors sit at 0.6–0.7. The UPS must cover both: its VA rating against the load's VA, which is W ÷ power factor, and its W rating against the load's W.
The trap is that the UPS has a power factor of its own. A UPS rated 1 000 VA / 600 W is built for a 0.6 load; put 800 W of servers at 0.98 on it and it is overloaded at 800 VA of its 1 000. Modern UPSs are rated at 0.9 or 1.0 output power factor precisely because the loads have moved that way, so the W rating is now the one that usually governs. Read both numbers off the datasheet and check both; the UPS sizing calculator picks the rating that satisfies whichever is tighter. Then add a margin, 20–25 % for growth, for the loads that were not on the list, and for the battery losing capacity with age.
Topologies: standby, line-interactive, online
IEC 62040-3 classifies a UPS by how much of the input it lets through. A standby (offline) UPS, class VFD, voltage and frequency dependent, passes the mains straight to the load through a filter and a transfer switch and only starts its inverter when the mains fails, with a break of 2–10 ms. It is cheap and efficient and protects against outages and little else. A line-interactive UPS, class VI, voltage independent, puts a tap-changing autotransformer in the mains path that corrects sags and swells without touching the battery, and its inverter-charger takes over in 2–4 ms when the mains goes outside its window. It is the sensible default for desktops, small offices and network racks. An online double-conversion UPS, class VFI, voltage and frequency independent, rectifies the mains to a DC bus, holds the battery on that bus and makes the output with its inverter all the time: there is no transfer, the output voltage and frequency are the inverter's and not the supply's, and every disturbance on the input stops at the rectifier. It costs more, runs 3–5 points less efficient because both conversions are always working, and is the right answer for servers, medical and laboratory instruments, industrial control, and anything that will ever run on a generator. Most online units offer an "eco" mode that runs as a line-interactive UPS most of the time and returns to double conversion when the input misbehaves; the efficiency comes back, the transfer time returns with it.
Whether the transfer time matters depends on the load. A switch-mode power supply holds its output up for 10–20 ms after the input disappears, so a 4 ms break is invisible to a PC or a server. Some instruments, PLCs with no hold-up, and equipment with a contactor in its supply are not so forgiving, and a generator, whose frequency wanders as the load changes, pushes a line-interactive UPS in and out of battery all day. The classification code on the datasheet, VFI-SS-111 for a full online unit, is the quickest way to know which you are looking at.
Runtime: the battery does the work
The energy the battery must deliver is the load in watts times the runtime in hours, divided by the inverter's efficiency on battery: about 0.92 for an online unit, 0.95 for line-interactive, 0.96 for standby, at a reasonable load. Fifteen minutes of 1 200 W through 0.92 is 326 Wh. The battery must hold more than that, for two reasons. The first is Peukert's law: a lead-acid battery's capacity is printed at the 20-hour rate, and the faster it is discharged the less it gives before its voltage reaches the inverter's cut-off. At a 15-minute discharge a general-purpose VRLA block yields perhaps 50–65 % of its 20-hour rating, at 30 minutes about 70 %, at an hour about 80 %, and at four hours or more about 90 %. The second is ageing: the bank is replaced when it reaches 80 % of its rated capacity, so a bank sized exactly for today falls short in its third year.
The discharge rate is worth checking in C. The current is the watts through the efficiency divided by the bus voltage, and the C-rate is that current divided by the 20-hour Ah. Above about 1 C a general-purpose VRLA block is out of its design range, which is why UPS makers specify their blocks by watts per cell at a 15-minute rate to a cut-off of 1.67 V per cell rather than by Ah: a "high-rate" 12 V 9 Ah UPS block can deliver 30–40 A for fifteen minutes and an ordinary 9 Ah alarm battery cannot. Tubular flooded batteries are the opposite case, built for hours of slow discharge and hopeless at high rates, which is why they belong on inverters and not on UPSs. Lithium iron phosphate UPS batteries lose very little at high rates and last several times as many cycles; they need a UPS built or configured for them, since the charging profile and the battery management interface are different.
Measuring the real load
Nameplates lie, or rather they state the maximum a power supply can deliver, which is two to three times what it usually does. A server with two 750 W redundant supplies draws 250–400 W; a desktop with a 500 W supply draws 80–150 W; a 24-port PoE switch draws anything from 50 W to its full PoE budget depending on how many phones and cameras hang off it. The only good numbers are measured. A clamp meter on the UPS input on a normal day gives the current, and current × 230 V gives VA (watts need a meter that reads power, or a power factor to divide by); a plug-in energy meter does the same for a desk; a server's management interface reports its input power directly; and a UPS already in service shows its load as a percentage of its rating, which is the easiest reading of all. Take the reading at the busiest time of day, then add the growth margin on top of it, not instead of it.
Two things never go on a UPS. Laser printers, whose fuser draws a kilowatt in pulses; and anything with a compressor or a large motor, which draws several times its running current at start. Both trip a small UPS or shorten its life, and neither needs it.
Generators and UPSs
A UPS and a generator are natural partners and awkward ones. The UPS rectifier draws a distorted current, especially the older six-pulse type, and on a generator, whose source impedance is many times the grid's, that current distorts the voltage. The alternator's regulator hunts, other equipment on the set complains, and the UPS itself, seeing an input outside its window, drops to battery and stays there until the battery is flat, at which point the whole point of the arrangement is lost. The traditional rule is to size the set at two to three times the UPS rating, or 1.5 times for a UPS with a modern low-distortion rectifier; the generator sizing guide has the electronic-loads section. Frequency is the other issue: a generator's frequency moves as its load steps, and a UPS set to a tight input window will not synchronise to it. Most online units have a generator mode that widens the window and slows the rate at which they will follow a changing frequency, and a charger walk-in that ramps the recharge current up over some seconds so the set is not hit with the battery-charging load the instant it takes over.
Decide the sequence before buying either. The UPS bridges the start; the generator carries the load and refills the UPS; when the mains returns, the changeover switch retransfers and the set cools down and stops. Sizing the UPS for the generator's start time plus a margin, rather than for a long runtime, is what keeps the battery small and the shutdown clean.
The example in numbers
A small office server room in Karachi: two 1U servers, a PoE switch, a NAS, the router and the fibre terminal, measured at 1 200 W at 0.9 power factor. The building generator takes about a minute to come on, so 15 minutes of runtime bridges it and, if it fails, shuts the servers down. Online double-conversion, 25 % margin, 48 V bus, 60 % usable at the 15-minute rate, UPS output power factor 0.9, 5 A charger.
- 1 200 ÷ 0.9 = 1 333 VA; with margin 1 667 VA and 1 500 W. The next standard rating is 2 kVA, 1 800 W at 0.9: both limits cleared, loading 66.7 % at the real load.
- Energy from the battery 1 200 × 0.25 ÷ 0.92 = 326 Wh; bank to install 326 ÷ 0.6 = 543 Wh, or 11.3 Ah at 48 V.
- Four 12 V blocks in series; the next standard block is 12 Ah, which only just covers it and gives about 15.9 minutes. An 18 Ah block gives 23.8 minutes and room for ageing.
- Discharge current 1 200 ÷ 0.92 ÷ 48 = 27.2 A, which is 2.26 C on 12 Ah and 1.51 C on 18 Ah: high-rate UPS blocks either way, not general-purpose ones.
- Recharge from flat at 5 A: about 2.9 hours for the 12 Ah bank. Two cuts an hour apart get a short second runtime.
The UPS sizing calculator reproduces these figures. Change the runtime to two hours and it asks for four 100 Ah blocks and a day of charging, which is the calculator's way of saying that hours are the inverter's job.
Installation details that decide the lifetime
- Temperature. VRLA batteries are rated at 20–25 °C and lose roughly half their life for every 8–10 °C above it. A UPS in a closed cabinet or a store room in a Karachi summer runs its batteries at 40 °C; give it ventilation or air conditioning, and never stand it against a wall in the sun.
- A dedicated circuit. A 2 kVA UPS draws up to 9 A at 230 V for the load and more while charging; give it its own breaker, and do not plug a UPS into another UPS or into a power strip with a surge filter that trips on the inverter's waveform.
- Earthing and the neutral. The UPS needs the installation earth for its filter leakage current and for touch safety, not a separate rod. Check in the manual whether the output neutral is bonded to earth on battery, because an RCD downstream of the UPS depends on it.
- The shutdown signal. Connect the USB or network card and configure the servers to shut down at a set remaining runtime; a UPS that carries a server for fifteen minutes and then drops it has only delayed the crash.
- Replacement and testing. Plan on replacing VRLA blocks every three to five years in a cool room and every two in a hot one, and run a test discharge, under real load, at least once a year and after every replacement: a battery that has failed silently is found by the outage otherwise. IEEE 1188 sets out the maintenance and test practice for VRLA batteries.
- Bypass. A maintenance bypass switch lets the UPS be replaced without dropping the load; on anything that matters, specify it with the UPS. Ahmedonics designs and integrates power protection with the control systems and IT it protects, including the sequence between UPS, generator and inverter; the sizing above is where that design starts.
References
- IEC 62040-3:2021, Uninterruptible power systems (UPS) — Part 3: Method of specifying the performance and test requirements — the VFI, VI and VFD classification, transfer-time and output-performance codes
- IEEE Std 1184-2006, IEEE Guide for Batteries for Uninterruptible Power Supply Systems — battery selection, high-rate sizing, temperature and end-of-life capacity
- IEEE Std 1188-2005, IEEE Recommended Practice for Maintenance, Testing, and Replacement of Valve-Regulated Lead-Acid (VRLA) Batteries for Stationary Applications
- IEC 61000-3-2, Electromagnetic compatibility — Limits for harmonic current emissions (equipment input current ≤ 16 A per phase) — why modern power supplies above about 75 W have active power-factor correction
- EN 50171:2001, Central power supply systems — battery-backed central power systems for emergency lighting; a parallel approach to runtime and battery design