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A UPS always shows two numbers, for example 1000VA / 600W. Many people look only at the first and are then surprised when the UPS overloads. Here is what they mean.
What VA and W are
- W (watts) – real power, the energy a device actually uses.
- VA (volt-amperes) – apparent power, voltage multiplied by current.
The ratio between them is the power factor (PF): W = VA × PF. Older and budget UPS units have PF ≈ 0.6 (1000VA = 600W); modern online UPS systems have 0.9–1.0 (1000VA = 900–1000W).
Which number to use
Choose a UPS by watts: the total consumption of your devices (W) plus a 20–30% reserve must not exceed the UPS watt rating. For example, a 450 W load is at the limit of a 1000VA/600W UPS – a 1500VA/900W model is the better choice.
How much typical devices use
| Device | Typical consumption |
|---|---|
| Office computer | 60–150 W |
| Powerful / gaming computer | 300–600 W |
| Monitor | 20–40 W |
| Router, Wi-Fi access point | 10–20 W |
| Switch (without PoE / with PoE) | 10–30 W / 50–400 W |
| IP camera (PoE) | 5–12 W |
| Video recorder (NVR) with disks | 20–50 W |
| Server | 200–600 W |
| Gas boiler (pump, controls) | 80–150 W |
These are averages – check the device documentation or measure with a power meter for exact figures.
What not to plug into a UPS
Laser printers, heaters, ovens, kettles and large motors draw a very high current when they start or heat up and will easily overload an office UPS. Plug them into the UPS surge-only outlets or power them separately.
Finally
Power is one thing, runtime is another. Two UPS units of the same power can run for 5 minutes or for hours depending on their batteries – see how many batteries does a UPS need and the UPS calculator. To choose the type read line-interactive vs online UPS, and browse models in the UPS category.