Dave Liquorice wrote:

To avoid disappointment, the required capacity of UPS should be
calculated before purchase, otherwise its likely to fail to deliver
the run time wanted. UPS capacity is rated in kWh (kilowatts x hours),
so for example a 0.2kWh UPS could run a 100w (0.1kW) load for 2 hours.

Not convinced that is correct. My APC Smart UPS 700 is rated at a nominal
700W, ie it will deliver 700W but the runtime at that power level is at
best 10 minutes.

Yes, I see that thar rating in the wiki is kWh but I've not seen that used
as the headline rating of a UPS.

Most UPS' will have a rating in VA or KVA. This indicates the maximum
load[1] rather than the run time. The Capacity of the batteries in Ah
will be proportional to the run time. More professional units allow the
run time to be increased with additional battery boxes. It is worth
noting also that the internal inverter in the UPS will have some load
anyway and hence there will be a maximum run time even without any load
on the UPS. This can be as little as 20 mins on a small one.

[1] More significantly combined real and reactive load - so with much
computer related kit (especially older stuff) you need to allow for the
non unity power factor.

UPSes are also rated by their max continuous power output. The load
appliance(s) should not exceed this rating. Note its the load's VA
that matters rather than watts. For some loads VA = watts, and for
some their VA rating is greater than watt rating.

Ah, I'd swap these to around as the VA is more often quoted along with a
runtime at various power levels rather than an overall kWh rating,
certainly for smaller UPS units. Large industrial ones may well have their
ratings quoted differently.

IME, they often do it the same way on bigger boxes.

Finally UPSes are also rated by peak power output. Some appliances
draw well above running power for a brief period at startup. It would
be wrong to assume that if the UPS meets an appliance's run current it
would also meet the startup current requirement.

Add a warning that switching something on that takes a surge can trip the
UPS protection circuit when running on battery power. That same appliance
probably won't when the UPS is on mains power.

Even if you don't trip the UPS, you may glitch the power for long enough
to upset something else already powered. I deliberately put my big CRT
monitor a separate UPS from anything else that might not appreciate
being glitched.

(If you have a power cut while the computer is on, but the monitor is
off, and you find you need to turn the monitor on to close down the
system gracefully and get yourself out of whatever docs you left open.
it can be quite annoying if the monitor switch on surge resets the
computer! DAMHIK)


--
Cheers,

John.

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