If you've noticed - on a UPS the load capacity is specified in so-called Volt-Amperes (VA). But we're used to Watts (W). And it's no wonder - the power of power supply units is specified precisely in Watts.
Why do they do this, as if - confusing us?
We won't even go roughly into electrical engineering and recall words like "reactive power," but will explain it simply.
The thing is that the values "power supply unit power" and "power consumption through the power supply unit" are quite different. Power supply units have losses, and rather significant ones - up to 30%.
Thus, if a power supply unit delivers 500W of power, then it will draw from the mains during this time up to 500*1.3=650W. 150W in this case is the loss.
Cool or simply good power supply units have smaller losses - their efficiency is up to 90-95% (as manufacturers claim). Then when delivering 500W they will draw 500*1.05=525W from the mains.
I.e. we see that consumption from the mains is always higher than what the power supply unit delivers to the electronics inside the system unit (or other device). Hence the decision to use VA power characteristics in UPS units.
So, a UPS honestly says that it will deliver 500VA, and the power supply unit may consume these 500VA, but actually provide only 350W to the computer.
Conversion
So, to convert VA to W you need to know the efficiency of your power supply unit (or power supply units). For a typical computer, an efficiency of 0.7 is commonly assumed, so we calculate as follows:
W = VA * 0.7
Where
W - is the unknown value of Watts,
VA - is the value written in the UPS specifications
0.7 - is the efficiency
If only the system unit is powered from the UPS (without a monitor and other devices), or system units, and you are sure that the efficiency of their power supply units is, for example, 0.9 (90%) - then multiply, accordingly, by 0.9 instead of 0.7
That's the math.
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