What the calculator does
A UPS has two ratings that are sized separately and confused constantly. The power rating, in VA and in watts, says how much load it can carry; the battery says for how long. The calculator takes the load in watts and its power factor, adds a margin, and picks the next standard UPS rating that satisfies both the VA and the W limit, since a modern UPS is rated at 0.9–1.0 output power factor and can be watt-limited before it is VA-limited. It then works out the energy the battery must deliver for the runtime you need through the inverter, corrects for how little of a lead-acid battery's printed capacity is available at a fast discharge, and turns the result into a bank of standard 12 V blocks at the UPS's bus voltage. It checks the discharge rate in C, warns when a general-purpose battery would be out of its depth, estimates the runtime the chosen bank actually gives, and says how long the UPS charger takes to refill it.
It is for the office UPS under the desk, the rack UPS behind a switch stack and a couple of servers, and the small server room; in Pakistan it is also the way to tell a true UPS (minutes of ride-through and a clean shutdown) from the "UPS" that is really an inverter with a tubular battery for hours of load-shedding, which the battery backup runtime calculator sizes.
Formula
UPS rating ≥ S × (1 + margin) and UPS rating × PFUPS ≥ P × (1 + margin) → next standard kVA
Loading = max(S ÷ rating VA, P ÷ rating W) × 100 %
Energy from the battery E = P × t ÷ 60 ÷ ηinv (Wh, t in minutes)
Bank to install EC20 = E ÷ usable share Ah = EC20 ÷ Vbus
Blocks in series = Vbus ÷ 12 block size = next standard Ah ≥ Ah (parallel strings above 200 Ah)
Discharge current I = P ÷ ηinv ÷ Vbus (A) C-rate = I ÷ AhC20
Runtime with the chosen bank = AhC20 × Vbus × usable × ηinv ÷ P × 60 (min)
Recharge from flat ≈ AhC20 × 1.2 ÷ Icharger (h)
where ηinv is the inverter efficiency on battery (0.92 online, 0.95 line-interactive, 0.96 standby, typical of a 50–80 % loaded unit), the usable share is the fraction of the 20-hour capacity a lead-acid battery gives up at this discharge time, and the standard ratings are 0.65, 1, 1.5, 2, 3, 5, 6, 8, 10, 15, 20, 30 and 40 kVA with blocks of 7, 9, 12, 18, 26, 40, 65, 100, 150 and 200 Ah. The factor 1.2 on recharge is the charge that must be returned for every ampere-hour taken out of a lead-acid battery.
Worked example
A small office server room in Karachi: two 1U servers, a 24-port PoE switch, a NAS, the router and the fibre terminal, measured together at 1 200 W with a power factor of 0.9. The building has a generator that takes about a minute to come on, so 15 minutes of runtime is enough to bridge it and, if it fails, to shut the servers down. Online double-conversion, 25 % margin, a 48 V battery bus, 60 % usable at the 15-minute rate, UPS output power factor 0.9, 5 A charger.
- Apparent power = 1 200 ÷ 0.9 = 1 333 VA; with margin 1 667 VA. Watts with margin: 1 500 W.
- The next standard rating is 2 kVA, which at 0.9 output power factor is 1 800 W: 1 667 ≤ 2 000 VA and 1 500 ≤ 1 800 W, so it fits. Loading at the actual load is 66.7 %, in the healthy range.
- Energy from the battery = 1 200 × 15 ÷ 60 ÷ 0.92 = 326 Wh. At 60 % usable the bank to install is 326 ÷ 0.6 = 543 Wh, which at 48 V is 11.3 Ah.
- Four 12 V blocks in series. The next standard block is 12 Ah, so the bank is 4 × 12 V 12 Ah; it only just covers the 11.3 Ah needed and gives an estimated 15.9 minutes. The next block up, 18 Ah, would give 23.8 minutes and margin for ageing.
- Discharge current = 1 200 ÷ 0.92 ÷ 48 = 27.2 A, which on 12 Ah is 2.26 C. That is well above 1 C: a general-purpose VRLA block will not deliver its 60 % at that rate, so the bank must be UPS high-rate blocks (the type rated in watts per cell for 15 minutes), or 18 Ah blocks, which bring it to 1.51 C.
- Recharge from flat at 5 A ≈ 12 × 1.2 ÷ 5 = 2.9 h; a second outage within that time gets a shorter runtime.
Ask the same question for two hours instead of fifteen minutes and the numbers change character: 2 609 Wh from the battery, 3 261 Wh to install at 80 % usable, 68 Ah at 48 V, four 100 Ah blocks, and a 5 A charger taking a day to refill them. That is an inverter-and-battery installation, not a UPS.
UPS topologies (IEC 62040-3)
| Topology | Class | Transfer time | Output on mains | Protects against | Efficiency on mains | Suits |
|---|---|---|---|---|---|---|
| Standby (offline) | VFD | 2–10 ms | The raw mains, filtered | Outage, some surges | 95–98 % | A PC, a router, a till |
| Line-interactive | VI | 2–4 ms | Mains through a tap-changing autotransformer (AVR) | Outage, sags and swells | 94–97 % | Small offices, network racks, desktops |
| Online double-conversion | VFI | none | Made by the inverter: voltage and frequency independent of the input | Everything, including frequency drift and distortion from a generator | 90–95 % (higher in eco mode) | Servers, medical and laboratory equipment, industrial control, anything on a generator |
Typical real draws (measured, not nameplate)
| Load | Typical draw | Note |
|---|---|---|
| Desktop PC | 60–150 W | A 500 W PSU label says nothing about the draw; a gaming PC under load can reach 300–400 W |
| Monitor, 22–27 in | 20–40 W | Older CCFL-backlit monitors more, and at a poor power factor |
| Laptop with charger | 30–60 W | Has its own battery; often better left off the UPS |
| 1U server | 150–400 W | Depends on CPUs, disks and load; read the management interface |
| 24-port PoE switch | 50–300 W | The PoE budget dominates: count the phones, cameras and access points |
| Router, ONT or fibre terminal | 10–20 W | Cheap to keep up; the reason home users buy a small UPS |
| NAS, 2–4 bays | 30–60 W | Spinning disks; more during spin-up |
| Laser printer | never on a UPS | The fuser heater draws 1 000 W and more in pulses; it trips or damages a small UPS |
Assumptions and limitations
- VA and W are both limits. The load's VA is W ÷ its power factor; the UPS's W capacity is its VA × its own output power factor. Older UPSs rated at 0.7 deliver only 700 W from 1 000 VA, and a load of PCs with power-factor-corrected supplies is nearly all watts, so the W check governs. Enter the UPS output power factor from the datasheet, not a guess.
- The usable share stands in for Peukert's law and the cut-off voltage. Lead-acid capacity is printed at the 20-hour rate; at a 15-minute discharge a general-purpose VRLA block gives about half to two-thirds of it before the inverter's low-voltage cut-off. UPS-grade high-rate blocks are designed for this and are specified in watts per cell at 15 minutes; use their tables for a precise runtime. Lithium (LiFePO4) UPS batteries lose little at high rates: set 90–95 % usable.
- Temperature. Battery ratings are at 25 °C; capacity is lower when cold, and life halves for roughly every 8–10 °C above 25 °C. A UPS in a hot cupboard in Karachi in June will need new batteries in two years, whatever the arithmetic says.
- No inrush or motor loads. A UPS is for electronic loads. Compressors (air conditioners, refrigerators), pumps and laser printers draw several times their running current at start or in pulses and belong on an inverter sized for them, or on the generator.
- Generator compatibility is a separate check. A UPS rectifier on a generator distorts the alternator's voltage and may refuse to synchronise, staying on battery until it is flat. Size the set at two to three times the UPS rating, or specify a UPS with a low-distortion (IGBT) rectifier and a wide input frequency window; the generator sizing calculator and its guide cover the set.
- Transfer time against hold-up. A standby or line-interactive UPS breaks the output for 2–10 ms when the mains fails. Most switch-mode power supplies hold up for 10–20 ms at full load, so this is normally fine; some laboratory, medical and control equipment is not, and needs the online topology.
- Ageing. Batteries are usually replaced at 80 % of rated capacity, so a bank sized exactly for the runtime falls short in its third year. The margin on the UPS rating does not help the runtime; choose the next block up or a longer runtime if the minutes matter.
- Single-phase only. Above about 20–40 kVA, UPSs are three-phase and sized per phase with separate battery cabinets and a generator-compatibility study; the standard ratings and block sizes here do not apply.
Frequently asked questions
Why do UPSs quote both VA and W?
Because the inverter is limited by current (VA) and its power stage by heat (W), and the load's ratio between them depends on the load. A 1 000 VA / 900 W UPS can carry 900 W of servers with active power-factor correction, but only 700 W of old monitors at 0.7 power factor, because those draw 1 000 VA. Size to whichever limit you hit first.
Can I run an air conditioner on a UPS?
No. A 1.5-ton split draws about 1.5–2 kW running and, unless it is an inverter unit, several times that for a moment at start, and it runs for hours. A UPS is built for minutes of electronic load. For an air conditioner during load-shedding you need an inverter rated for the surge, a large battery (two hours of a 1.5 kW load is over 4 kWh at the battery, or about 200 Ah at 24 V after losses and depth of discharge), and ideally solar to charge it; the battery backup runtime calculator does that sizing.
How long will a 1 kVA UPS run a PC?
Do the arithmetic. A PC and monitor at 120 W on a line-interactive 1 kVA unit with two 12 V 9 Ah blocks (24 V bus): energy at the battery = 120 ÷ 0.95 = 126 W; the bank is 24 × 9 = 216 Wh at C20, of which about 70 % is usable at this rate, 151 Wh; runtime ≈ 151 ÷ 126 × 60 ≈ 72 minutes on new batteries, perhaps 50 on two-year-old ones. Enter your own load and blocks above.
Why does the UPS battery die in two years?
Three reasons, usually together. Heat: a UPS in a closed cabinet runs its batteries at 35–40 °C, which halves their life. Cycling: a UPS battery is designed for a few deep discharges a year; a UPS in Lahore doing two load-shedding cuts a day deep-cycles it hundreds of times, which a small VRLA block was never built for. Undercharging: a small charger that never quite completes the absorption stage before the next cut leaves sulphate on the plates. The charging guide explains the stages. Where load-shedding is daily, a deep-cycle battery on an inverter is the right tool and the UPS should only bridge the seconds.
Does a UPS need its own earth?
It needs the installation's earth, properly connected: the input filters leak a few milliamps to earth, and the metal case must be at earth potential for safety. It does not need a separate earth rod. What does need checking is the output neutral on battery: some online UPSs bond the output neutral to earth when the input is lost and some do not, which affects whether an RCD downstream of the UPS works as intended. The manual states it; ask the vendor if it does not.
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 and the transfer-time and output-performance codes
- IEEE Std 1184-2006, IEEE Guide for Batteries for Uninterruptible Power Supply Systems — battery selection, sizing at high rates, 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
- EN 50171:2001, Central power supply systems — battery-backed central power systems for emergency lighting; a parallel approach to runtime and battery design