Most UPS sizing errors are not arithmetic errors. They are scope errors: the load was measured from nameplates, the runtime was assumed, and the site voltage was taken from the utility’s nominal value rather than the actual feeder. The arithmetic then produces a precise answer to the wrong question.
This guide uses the same method we apply to project inquiries: measure the load, convert to kVA, add a margin, check the motor start, size the battery, and adjust for the site. The result is a capacity range and a battery target, not a single model number.
What does “size a UPS” actually mean?
Sizing a UPS means finding the smallest standard frame that can carry the real load, the largest step change, and the required runtime, with enough margin for growth and battery aging. It is a constraint problem, not a single multiplication.
There are four constraints. The continuous kW sets the inverter size. The largest motor or compressor start sets the overload requirement. The runtime sets the battery bank. The site voltage and frequency set the topology and the input range. A design that satisfies three of the four constraints can still fail on the fourth.
Start with a load worksheet. List each load, its running watts, its quantity, and whether it has a motor or a high inrush. Add the loads that run at the same time, not every load in the building.
| Load | Running W | Quantity | Simultaneous? | Notes |
|---|---|---|---|---|
| Server | 350 | 4 | Yes | Measured at the PDU |
| Network switch | 60 | 2 | Yes | PoE budget separate |
| Storage array | 400 | 1 | Yes | Spinning disks at start |
| Monitor | 30 | 4 | Yes | Often omitted |
| Laser printer | 600 | 1 | No | High cyclic load |
| Air conditioner | 1,800 | 1 | No | Motor start; separate circuit |
The worksheet is more valuable than a rule of thumb. It shows which loads are continuous and which are occasional, and it prevents the printer and the air conditioner from being added to the UPS load by mistake.
How do I convert kW to kVA?
The UPS is rated in kVA, but the load is usually measured in kW. The conversion uses the output power factor:
kVA = kW ÷ power factor
For an online UPS with a 0.9 output power factor, a 7.2 kW load requires 8 kVA. A 10 kVA UPS delivers 9 kW, so it carries the load at 80% of its real-power rating. That is a healthy design point.
If the load is 10 kW at 0.9 PF, the required UPS is 11.1 kVA before margin. The next standard size is 15 kVA, not 10 kVA. A 10 kVA UPS cannot deliver 10 kW; it delivers 9 kW at full load and should not be run at 100% continuously.
| Load | PF | Required kVA | With 25% margin | Standard size |
|---|---|---|---|---|
| 3 kW | 0.9 | 3.3 kVA | 4.2 kVA | 5 kVA |
| 7.2 kW | 0.9 | 8.0 kVA | 10.0 kVA | 10 kVA |
| 14.4 kW | 0.9 | 16.0 kVA | 20.0 kVA | 20 kVA |
| 36 kW | 0.9 | 40.0 kVA | 50.0 kVA | 60 kVA |
| 72 kW | 0.9 | 80.0 kVA | 100.0 kVA | 100 kVA |
Use the UPS output power factor, not the load’s displacement power factor, for this step. The load PF matters for the real-power demand; the UPS PF tells you how much real power the frame can deliver at its kVA rating.
How much design margin should I add?
Add 20–25% for a normal commercial or IT load. Add 30% for a motor-heavy load, a load with uncertain growth, or a site where the actual draw is not yet measured.
The margin is not “extra for the salesperson.” It covers four real effects. Load growth adds capacity over the life of the UPS. Battery aging reduces the runtime, not the UPS capacity, but a lightly loaded UPS runs cooler and has more headroom for the battery recharge. Harmonic current from non-linear loads increases the RMS current for the same kW. And the difference between nameplate and actual draw is often large, especially for servers and HVAC equipment.
| Margin | When to use | Risk |
|---|---|---|
| 10–15% | Measured load, stable, short life | Little room for growth |
| 20–25% | Normal commercial / IT | Balanced |
| 30% | Motor-heavy, uncertain, or critical | Higher capital cost |
| 40%+ | Very uncertain load | Likely oversized; re-measure |
Do not add 50% “to be safe” without a reason. An oversized UPS runs at low load, which can reduce efficiency and, on some modular frames, waste module capacity. The right margin is the one you can explain.
How do I size the battery for the runtime?
Runtime is a battery calculation, not a UPS feature. The simplified formula is:
Ah = kW × 1,000 × hours ÷ (DC bus × inverter efficiency × usable depth of discharge)
For a 7.2 kW load, a 1-hour runtime, a 192 VDC bus, 0.85 inverter efficiency, and 0.6 usable depth of discharge, the result is about 73.5 Ah. A string of 16 × 12 V 100 Ah blocks gives 100 Ah nominal, so it covers the target with a small margin. At 2 hours, the required Ah doubles to about 147 Ah, so two parallel strings of 100 Ah are needed.
The formula is a starting point. At short runtimes, the battery delivers less than its 20-hour rating, so the actual bank is larger. Use the manufacturer’s discharge table at the expected end voltage.
| Runtime | Load | DC bus | Estimated Ah | 12 V 100 Ah strings |
|---|---|---|---|---|
| 5 min | 7.2 kW | 192 V | 6 Ah | 1 (minimum string) |
| 15 min | 7.2 kW | 192 V | 18 Ah | 1 |
| 30 min | 7.2 kW | 192 V | 37 Ah | 1 |
| 60 min | 7.2 kW | 192 V | 74 Ah | 1 |
| 120 min | 7.2 kW | 192 V | 147 Ah | 2 |
| 240 min | 7.2 kW | 192 V | 294 Ah | 3 |
The numbers are indicative and assume a healthy battery at 25 °C. A hot battery room reduces the available capacity and the life. A cold room reduces the capacity at discharge. Both change the bank size.
What changes when the grid is unstable?
An unstable grid changes the topology decision before it changes the capacity. A line-interactive UPS transfers in 4–6 ms and corrects voltage with an AVR stage. An online UPS runs the load through a double conversion and transfers in 0 ms. If the grid regularly sags to 160 V or swings to 260 V, the line-interactive unit may spend too much time on battery, and the battery may not recover between events.
The input window is the specification to check. Many online UPS models accept 85–300 VAC, which covers deep sags and generator swings. If the input falls below that window, the UPS goes to battery or bypass. A servo stabilizer upstream can keep the UPS on mains, but it adds cost and a maintenance item.
| Grid condition | Recommended topology | Why |
|---|---|---|
| Stable, occasional outage | Line-interactive | Lower cost, adequate transfer |
| Frequent sags, short outages | Online, wide input | 0 ms transfer, battery reserved |
| Deep brownouts, generator | Online + servo AVR | Stable input, clean output |
| Motor loads, industrial | Industrial online | High overload, wide input |
| Data center, N+1 | Modular online | Redundancy and growth |
The site voltage should be measured, not assumed. A week of logging at the distribution board will show the lowest and highest phase-neutral voltage, the phase imbalance, and the frequency. That log is more useful than a utility statement.
What should I send to the supplier?
Send the load worksheet, the largest motor or compressor, the required runtime, the phase and voltage, the site country, the ambient temperature, and the documentation requirement. If you have a single-line diagram or a consultant’s specification, include it.
The supplier should return the kVA, the kW, the output PF, the input window, the battery configuration, the dimensions, the weight, the heat rejection, and the runtime basis. If the quotation only says “10 kVA UPS,” ask for the rest. The battery and the input range can change the price by a large factor.
A good quotation also states what is excluded: civil works, cable, installation, duty, and clearing. That clarity prevents the surprise that arrives after the equipment lands.
The sizing checklist
- Measure the running kW, not the nameplate.
- Add 20–25% margin for a normal load, 30% for a motor-heavy load.
- Check the largest motor start against the UPS overload curve.
- Size the battery from the runtime, the DC bus, and the discharge curve.
- Confirm the input window against the measured site voltage.
- Choose the topology by the load sensitivity and the grid, not by the price alone.
- Ask for the model sheet, the battery sheet, and the drawing before the order.
The calculator on this site gives a first-pass capacity and battery estimate. Use it to frame the inquiry, then confirm the final configuration with the model datasheet and the site log.
