> I am thinking of getting a UPS that will take external batteires. I'd like
> to calculate how long a 40 watt 12 VDC battery holds a typical computer
> system (PII with colour monitor) and cosequently calculate how many
> batteries to keep for each hour I intend to increase. How do I
> calculate all
> this ?

     Unfortunately this is not as easy as it sounds, the reason being that
your devices use AC. You mention you have a 40 watt battery, for how long is
this rating? You need to get the amp-hour rating of each battery you use.
Then you must determine how many amps you system will use, the wattage
marked on the back of each device is a start but is usually higher than
normal use. Now the AC fun! :( In DC, multiplying volts by amps gives you
watts, if you computer were DC then you could just match ratings. However
with AC watts equals voltage x amps x power factor. The power factor is a
number between 0 and 1. If you want a technical explanation of why this
power factor exists let me know. The power factor of every AC device is
theoretically different, so if you pump in 400VA (DC watts) and the power
factor is 0.5, you actually only get 200W of power out of it. Usually the
power factor of PCs is around 0.8, sometimes better (larger) sometimes worse
(smaller). I recommend you use a power factor of around 0.8 unless you have
the equipment to measure it (an oscilloscope can be used for this although
there are meters specifically designed to determine power factors). Now, if
this wasn't enough! The inverter in the UPS is not perfect, see if you can
find the efficiency of the inverter in the UPS and factor (there's that word
again!) that in as well. There are other factors that come into play (the
non-linearity of battery power curves) but this info should get you started.
Good luck. TTYL

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                      Bob Wright<[log in to unmask]>
                       Drew Dunn<[log in to unmask]>