ups recommendation

hideki · 4 Feb 2009 at 03:25

Im wondering what sort of ups should i be looking to get.

how can i tell which one has sufficient power to allow a graceful shut down?

my spec:
750watt bequiet
asus p6t deluxue
intel i7 920
samsung 320gb
wd 500gb
3x120mm sharkoon
xfx 8800gts
dell 30" lcd

what puzzles me more is my psu has a strange external power cable connector, square shaped. im not sure how i'd connect it to a ups, as they use stanard atx power cable connections

bremen1874 · 4 Feb 2009 at 04:55

The APC website has a fairly comprehensive UPS selection wizard that's worth a look. It probably hasn't been updated for i7 yet but you should be able to pick something close. Even if you don't want an APC UPS it'll give to a guide to the required VA.

The square plug you've got is the 15 Amp version of the 10 Amp kettle type leads normally used.

In areas where the mains is 110 Volts (USA etc.) the higher capacity power supplies can pull too much current for the normal 10 Amp cables.

Over here with our 230/240 Volt supply it's completely pointless as the current pulled will be more than halved and would easily be handled by a standard 10 Amp cable.

To connect you machine to a UPS either modify an extension lead by replacing the normal three pin plug with a 'cold' kettle type plug, or alternatively modify your existing lead by replacing the plug in the same way. The plug you'll need is known as a C14, a rewireable one should only cost a couple of quid.

If you don't want to modify the cables yourself you should be able to find a pre-made C14 to C19 cable reasonably easily.

EsaT · 4 Feb 2009 at 10:34

Combined peak draw of those components (with PSU losses and display) would be around 400-450W. (idle/desktop draw still around 250W because of power hogging display)

So 500W would be good requirement for UPS which would handle at least few minutes under full peak load and 10-15 minutes of idling.

hideki · 4 Feb 2009 at 11:43

thanks for the help guys, and for the name of that cable i need, i wasnt sure there was such a thing

bremen1874 · 4 Feb 2009 at 13:17

Watch out for the fact that UPSs are rated in VA~ rather than Watts. For example APC rate 800 VA~ UPSs as only providing 500 Watts.

With many modern power supplies having active PFC the VA~ and Wattage figures should be much closer but it's still worth adding a bit of extra capacity to allow for the difference.

hideki · 4 Feb 2009 at 21:39

is there a direct conversion from va to watts on ups or is that dependent on model? they seem to quote the va more freely

bremen1874 · 4 Feb 2009 at 22:09

Okay PF (Power Factor) varies between 0 and 1.

If PF of the load on the UPS is 1 (perfect) then the VA and Wattage figures would be the same.

I believe that the standard PF used when quoting UPS ratings is 0.6, which is a very conservative figure and explains the big difference between the claimed VA and Wattage figures.

I've just checked the specs on my Corsair PSU and they're claiming a PF of 0.99 which is near as matters perfect. An older PSU or one not using active PFC would probably be less.

I'd guess that for computer use if you took the available Wattage as 90% of the claimed VA~ then you'd be okay.

EsaT · 4 Feb 2009 at 22:12

hideki said:
they seem to quote the va more freely

What else can you expect from BS... PR departments?
It's bigger!

Every credible UPS maker tells real output/watt rating in specs immediately after VA-rating.
Good UPSes have power factor of ~0.66 and I've kept 0.6 as general rule but some cheaper UPSes like Belkin go down to lowly 0.55.

bremen1874 said:
With many modern power supplies having active PFC the VA~ and Wattage figures should be much closer but it's still worth adding a bit of extra capacity to allow for the difference.

PFC or lack of it doesn't affect to consumed real power in any way and you can't use volt-amperes for substituting watts regardless does load have perfect power factor.
So forget whole VAs because they don't mean anything. If UPS can provide required real watts it more than surely can handle that small additional reactive power needed by power factor corrected devices.

bremen1874 · 4 Feb 2009 at 22:56

Having checked manufacturer's information you shouldn't exceed the rated VA~ OR the rated Wattage.

The PF does has an influence as it sets the ratio between the VA~ and Wattage consumed for a particular load. This doesn't (as I first thought) mean the available Wattage increases with a better PF. It does however mean that an Active PFC PSU could exceed the rated Wattage whilst not exceeding the rated VA~.

So to be safe either lookup the manufacturer's rated Wattage and work to that, or use a PF of 0.6 to get a reasonable Wattage figure from the quoted VA~.

I'll now go and do some real work rather than spreading disinformation.