Which UPS?

[Molly Tournquist]

A few side issues here may be muddling the picture and making complex internal PSUs seem clunky, contrasting positive plain statistics.

Doing all the conversion inside the computer case adds more heat to the system; of course, this matters more with smaller form factors. There's the prospect of what if power outage rates were a lot worse. If the power was already out, DC input would facilitate being able to plug a simple battery to power the system. Perhaps these sorts of things just make a PC with a DC plug just intuitively appear less obtuse.

Also, having joint DC production for many devices should be good for efficiency. This is probably something that contributes a bit in the datacenters. Another thing that could influence it would be having some of the power conversion further away, even if every stage of conversion is exactly as efficient, air conditioning power may still be getting saved.

On 12 June 2012 11:41, Lennart Sorensen
<lsorense at ...> wrote:
> On Tue, Jun 12, 2012 at 11:30:31AM -0400, Scott Allen wrote:
> > 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
> 
> Yeah it could certainly be made more efficient.
> 
> > 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.
> 
> Efficiency wise, yes. But it ought to be much cheaper to build a UPS on
> the DC side for a computer. I have once seen a PC that had a UPS built
> in to the power supply system on the DC side. I think it provided about
> 10 minutes of run time.
> 
> > 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.
> 
> Of course many high end UPSs always run the PC on generated power since
> it is cleaner (supposedly) than what comes from the wall. I wonder how
> much efficiency that costs.
> 
> -- 
> Len Sorensen

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