UPS Size Calculator (kVA)

How to use the UPS size calculator

Add up the load of everything the UPS will power, in watts. Enter the UPS’s output power factor and a margin for growth. You get the minimum kVA and kW rating; choose the next standard size up.

For more background, read How to Calculate Battery Backup Time.

UPS size formula

UPS kVA = Load W × (1 + margin) ÷ PF ÷ 1,000

Typical loads

Equipment	Typical power
Desktop PC with monitor	150–300 W
Laptop	45–100 W
Network switch (24-port)	30–60 W
Wi-Fi router	10–20 W
1U server	250–500 W
Laser printer (printing)	400–800 W
PLC control panel	100–500 W
LED lighting per room	30–60 W

Use measured or nameplate input power. Avoid connecting laser printers, heaters and motors to small UPSs — their inrush current can overload them.

Standard UPS ratings

1, 2, 3, 5, 6, 10, 15, 20, 30, 40, 60, 80, 100, 120, 160 and 200 kVA are common sizes.

Choosing the topology

Type	Best for
Offline (standby)	Home PCs, small loads
Line-interactive	Offices, network racks — handles voltage swings
Online double-conversion	Servers, industrial control, medical, sensitive loads

Backup time

The UPS rating sets how much load it can carry; the battery sets how long. See the battery backup calculator to size the batteries.

Frequently asked questions

How do I calculate UPS size?

Add up the load in watts, add a margin of 20–30% for growth, and divide by the UPS power factor. 4 kW × 1.25 ÷ 0.9 ≈ 5.6 kVA, so choose a 6 kVA UPS.

Should I size by kVA or kW?

Both. The load must not exceed either the kVA or the kW rating of the UPS. Check the datasheet for both figures.

How much margin should I add?

20–30% is common. It allows for future loads and keeps the UPS from running near full load, which improves reliability and battery runtime.

Does UPS size decide backup time?

No. Backup time depends on the battery bank. See the battery backup calculator.

Related calculators

• Inverter Battery Backup CalculatorFind how many hours your inverter battery will run your load — or the battery Ah you need for a target backup time.
• kVA to kW Converter (and kW to kVA)Convert kVA to kW and kW to kVA using power factor — for generators, UPS, transformers and DG set sizing.
• Amps to Watts Calculator (and Watts to Amps)Convert amps to watts for DC, single-phase and three-phase circuits — with charts for 12 V, 120 V and 230 V.
• Electricity Cost Calculator (Appliance Running Cost)Calculate how much an appliance costs to run per day, month and year from its wattage, hours of use and electricity rate.
• Transformer Size Calculator (kVA Rating from Load)Calculate the transformer kVA rating needed for your connected load from kW, power factor, diversity factor and future growth — and pick the next standard size.
• Generator Size Calculator (DG Set kVA)Calculate the diesel generator (DG set) kVA rating needed for your connected load, power factor, future margin and largest motor start.

Embed this calculator