Which UPS?

[James Knott]

Scott Allen wrote:
> With a 48VDC output UPS powered by mains AC:
>    - While on mains (most of the time):
> UPS needs to convert 120VAC to 48VDC at, let's say, 85% efficiency.
> The 48VDC PC power supplies above are 70% efficient but let's say we
> find one that's 85%. UPS 85% and PC 85% is 72.25% overall.

Whether AC or DC supply to the power supply, there are two stages 
involved, conversion of AC to DC and then converting that DC to the 
required voltages through a switching converter/regulator.  I'd have to 
question why having both in the same package results in higher 
efficiencies of the order you claim.  Many "server farms" run on about 
200V DC or so and get better efficiencies than AC powered systems.  The 
rectified DC inside a typical power supply would be in the range 
approaching 170V.  The conversion of the high voltage DC to the voltages 
required by the computer should have about the same efficiency, no 
matter what the source, all else being equal.
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