Which UPS?

[Scott Allen]

On 12 June 2012 21:14, James Knott <james.knott-bJEeYj9oJeDQT0dZR+AlfA at public.gmane.org> wrote:
>  How can two stages be more efficient than one, particularly if the
> converting/regulating stage is essentially identical either way?

Who said anything about two stages being more efficient than one? Go
back and reread the post you are questioning. I'm saying that two
stages is more efficient than three.

Note that I was talking about the specific case where the user is
limited to a 120VAC source, which is on line, and either a 120VAC
output UPS is used, or a 48VDC output UPS is used.

My statement, that you quoted and questioned, was for the 48VDC output UPS:
-----
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.
-----

There are three stages involved:
1. Rectification of 120VAC to DC in the UPS.
2. Conversion/regulation to 48VDC in the UPS.
3. Conversion/regulation to multiple voltages in the PC.

My statement for a 120VAC output UPS, which I claim is more efficient was:
-----
With a standard 120VAC output UPS:
 - While on mains (most of the time):
AC is fed to the PC from the mains by the UPS (usually through a
mechanical relay). There's no power loss in the UPS. PC efficiency is
80% or better, which is the overall efficiency of the system.
-----

There are two stages involved:
1. Rectification of 120VAC to DC in the PC.
2. Conversion/regulation to multiple voltages in the PC.

I was even generous and gave the DC input PC power supply 85%
efficiency. I gave the AC input PC power supply only 80% efficiency.

I agree that DC UPSs in data centre type environments can likely be
more efficient but, again, you quoted and questioned the statements
and math I gave for a home 120VAC environment.

-- 
Scott
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