Which UPS?

[Scott Allen]

On 11 June 2012 13:16, Lennart Sorensen <lsorense-1wCw9BSqJbv44Nm34jS7GywD8/FfD2ys at public.gmane.org> wrote:
> What you would want is:
>
> http://www.powerstream.com/DC-PC-48V.htm
>
> (or something similar)

Perhaps something similar but more efficient. From the above link:
Efficiency 70% typical @48V DC Full Load

If you're feeding this from a DC output UPS that provides 48V
converted from AC, your going to waste a lot of power when the mains
are up, for the short time that you'd be running off the battery. In
addition to the low efficiency of the above supplies, you have to
factor in the UPS's efficiency for continuously converting the full
power requirements of the PC, in addition to charging and maintaining
the batteries.

Standard AC input power supplies in PCs are typically 80% efficient or better.

On 11 June 2012 10:31, Lennart Sorensen <lsorense-1wCw9BSqJbv44Nm34jS7GywD8/FfD2ys at public.gmane.org> wrote:
> Yes, using a UPS to generate AC just to convert it back to DC is very
> very stupid.  Yet everyone (almost) keeps doing it.

If the primary power that you have for your PC is 120VAC (we're
talking the typical North American home here) then an AC output UPS
isn't so stupid.

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.
  - While mains are out (rarely):
DC from the battery is converted to AC at maybe 75% efficiency or
better. The PC converts this back to DC at 80% or better. UPS 75% and
PC 80% is 60% overall.

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.
  - While mains are out (rarely):
Battery supplies 48VDC directly to the PC. No power loss in the UPS.
PC power supply is 85% efficient, which is the overall efficiency of
the system.

So, given the above two systems, I'd rather use the AC output UPS,
which is at least 80% efficient most of the time but only 60% during
the short periods when the mains are out, as opposed to the DC output
UPS which is only about 72% efficient most of the time except briefly
85% while mains are out.

Note that I didn't factor in power required to charge and maintain the
battery, since it should be about the same in both cases.

-- 
Scott
--
The Toronto Linux Users Group.      Meetings: http://gtalug.org/
TLUG requests: Linux topics, No HTML, wrap text below 80 columns
How to UNSUBSCRIBE: http://gtalug.org/wiki/Mailing_lists