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ITFrequently Asked Questions · General
What is the power factor?
The power factor is the ratio of real power to apparent power and shows how efficiently electrical energy is used. Ideally it is close to 1. In UPS systems a high power factor is critical, because it makes better use of the UPS's available power and reduces the risk of overload.
Some devices do not draw current fully in step with the voltage, either because of coils and motors or because of power supplies without correction. More current then flows through the wiring than the useful power actually requires. Apparent power (VA) is voltage multiplied by current, while real power (W) is what is actually converted into work or heat. For example, a 700 W load with a power factor of 0.7 needs 1000 VA.
A UPS has two limits, in VA and in W, and the load must stay below both. Their ratio is the UPS's output power factor: a 1000 VA UPS with a factor of 0.9 delivers up to 900 W. Modern computer power supplies with active correction (PFC) have a power factor close to 1, so their watts are almost equal to their VA, and the watt limit is usually the first one reached.
In practice, when comparing UPS units, always look at the watts, not just the VA. Two models with the same VA can deliver noticeably different real power, and a load that "fits" in VA may still overload the UPS in watts.
