UPS Power and Runtime Calculator (Watt, VA, Wh)

Use the tool below to estimate the UPS power (Watts and VA) and energy (Wh) needed to keep your computers, servers, security cameras or other devices running for a set time during a power outage. Pick devices from the ready list, and if needed enter a custom device power and the runtime you want.

How is UPS power calculated?

When choosing a UPS, two different values matter: power and energy. Power is the total load the UPS can feed at the same time; energy determines how long it can feed that load. This tool estimates both. The total Watt (W) value of the connected devices gives the active power. UPS units, however, are mostly labeled in VA (Volt-Amps); the relationship between W and VA is set by the power factor: VA = W / power factor. If the power factor is unknown, 0.80 is a typical starting value.

For runtime, an energy calculation is needed. The approximate energy need is found with Wh = Total power (W) × time (hours). In reality, because of battery and inverter losses, this value is divided by the system efficiency and an energy safety margin is added. The tool shows the result as two cards: the calculated minimum need and the safe selection value rounded up to a practical capacity class. In addition, a safety margin (for example 25 percent) is added on top of the power for sudden load surges.

Caution with motor and cooling devices

Refrigerators, compressors, pumps, air conditioners and motor devices can draw several times their normal running power at the moment of start-up. Therefore, for these devices, do not decide based only on the normal Watt value in the table; the start-up (surge) power must also be considered. This tool does not recommend any UPS, battery model or exact battery count; it only helps you see your approximate power and energy needs. For a final choice, the device label values and manufacturer technical data should be taken as the basis.

UPS sizing formulas: W, VA, kVA and Wh

Choosing an uninterruptible power supply from a single number is misleading. A UPS normally carries two limits on its nameplate: apparent power (VA or kVA) and active power (W or kW). A UPS is adequate only when it satisfies both limits.

The most common mistake in three-phase systems is the voltage. Using 230 V instead of 400 V in the three-phase formula makes the result about 43% too low: you see 12.7 kVA instead of 22 kVA and select a UPS at half the required capacity.

The second common mistake is the motor nameplate. A motor marked “4 kW” does not draw 4 kW from the supply — that figure is shaft output power. With an efficiency of 0.88 and a power factor of 0.85, the same motor requires 4 / (0.88 × 0.85) ≈ 5.35 kVA of apparent power, roughly 34% above the nameplate value.

UPS types under IEC 62040-3: VFD, VI and VFI

Commercial names such as “online UPS” or “line-interactive” vary between manufacturers. The international standard IEC 62040-3 classifies a UPS by how far its output depends on the input, using a three-letter code. This is the figure to compare when reading quotations.

In a VFI unit the load is always fed through AC → DC → AC double conversion; because no transfer takes place when the mains fails, the output sees no interruption at all. In a VFD unit the load is fed directly from the mains while it is present, so there is a gap of a few milliseconds before the inverter takes over. That gap is harmless for most computers, but it can be enough to alarm a CNC control, an industrial PC or certain drives.

Inrush current and crest factor: the two things that break the calculation

Even when the total wattage is correct, a UPS can still fall short. There are two typical reasons.

1) Inrush (starting) current. Devices with motors and compressors — refrigerators, air conditioners, hydraulic units, compressors, pumps — draw three to seven times their rated current for a fraction of a second at start-up. If the UPS cannot supply that brief pulse it raises an overload alarm and transfers to bypass. The practical approach is to allow for the starting surge of the largest motor; there is no need to add every motor as if they all started together, because in practice they do not.

2) Crest factor. Computers, servers, LED drivers and NVRs use switch-mode power supplies that draw current in short pulses at the peaks of the waveform rather than as a sine wave. Peak current can be two to three times the RMS value, which is why a UPS chosen for IT loads is expected to have a crest factor of at least 3:1. A unit with a low crest factor rating can distort the output voltage or trip early on overload even when the wattage calculation is correct.

These two effects are also why the calculated figure is not used directly but rounded up to the next standard class. The “safe selection value” column in the tool above already contains that margin.

Battery sizing: from Wh to Ah, ageing and temperature

Converting the Wh figure into battery capacity uses the bus voltage: Ah = Wh / (V_bus × η × usable fraction). That simple conversion, however, is more optimistic than real battery behaviour. The reasons are:

• End-of-discharge voltage. Battery voltage falls as it discharges. In practice an end-of-discharge value of 1.75 V per cell is used; since a 12 V block contains six cells, that is 10.5 V per block. With 16 blocks in series the bus is not the nominal 192 V but 168 V at the end of discharge. Calculating with the nominal voltage overstates the available energy by roughly 14% and undersizes the battery.
• The short-discharge penalty. The Ah figure on a battery label normally refers to a slow 20-hour discharge (C20). In a fast 10-minute discharge a VRLA battery delivers only about half of that capacity. This is why UPS manufacturers size batteries from a watts-per-cell table rather than from Ah.
• Ageing factor. At the end of its useful life a battery delivers about 80% of its nominal capacity. An ageing factor of 1.25 is therefore applied in the field: the battery selected is 25% larger than the day-one calculation.
• Temperature. The reference temperature is 25 °C. A colder room reduces capacity; a warmer one raises capacity slightly but shortens life — as a rule of thumb every 10 °C increase roughly halves VRLA battery life. Do not install a UPS in a boiler room or on top of a control cabinet.
• Parallel strings. Series connection raises voltage; parallel connection raises capacity. With two parallel strings you divide the calculated total Ah by two to select the batteries per string — your total requirement does not fall.

For these reasons the tool here does not propose an exact battery count. The Wh figure it produces is a pre-sizing value; the final selection must be made together with the battery manufacturer discharge curve and the DC bus voltage window of the UPS.

Worked example: an 8-camera CCTV system for 30 minutes

A site has 8 IP cameras, an NVR, a PoE switch and a modem that must keep running for 30 minutes after a power failure.

1. Power margin: 196 W × 1.25 = 245 W minimum output power.
2. Apparent power: assuming PF 0.80, 245 / 0.80 = 306 VA, which falls into the 500 VA standard class.
3. Theoretical energy: 196 W × (30 / 60) = 98 Wh.
4. Losses and margin: 98 / 0.85 × 1.20 ≈ 138 Wh.
5. Ageing factor (1.25): 138 × 1.25 ≈ 173 Wh of nominal battery energy.
6. Battery capacity: on a 24 V DC bus (2 × 12 V in series), 173 / 24 ≈ 7.2 Ah. Because of the short-discharge penalty, a 9 Ah or 12 Ah pair is normally selected in practice.

Result: a UPS in the 500 VA / 300 W class with a 24 V battery bus able to deliver 30 minutes of runtime. If uninterrupted recording matters, a VFI (online) unit is preferable to a VI (line-interactive) one.

Six common mistakes when selecting a UPS

1. Looking only at the VA rating. A UPS marked 1000 VA carries only 600 W if its power factor is 0.6. Ignoring the watt limit means an overload on the first real load.
2. Treating VA as runtime. VA is power, not time. Runtime is set by the battery energy in Wh.
3. Treating Ah as runtime. A 100 Ah battery carries very different energy at 12 V and at 48 V. Comparisons must always be made in Wh.
4. Adding motor nameplates directly. The kW on the plate is shaft power and cannot be used before dividing by efficiency and power factor.
5. Connecting a laser printer. The fuser in a laser printer draws very high power intermittently and forces the UPS to be oversized for no benefit. Common practice is to keep printers off the UPS output.
6. Running the UPS at full capacity. A unit permanently loaded at 90–100% runs hotter, shortens battery life and leaves no room for growth. A typical target is 70–80% loading.

If the site also suffers from voltage dips, fluctuation or flicker, the assessment must compare a UPS against a voltage regulator rather than considering the UPS alone. See the UPS and voltage regulator calculator, where three-phase installations, inrush current and CNC machine scenarios are covered in detail.