Re: [MLUG] Determining Power Supply Size

[Mark Haidekker <EMAIL:PROTECTED>]

On Wednesday 03 July 2002 01:23 pm, you wrote:
> Hey everyone,
>
> Does anybody know exactly how to figure out just how big of a power supply
> any particular computer needs? There has to be some science to this. It
> can't be that incredibly difficult looking up wattage and adding it all
> up, right?
>
> Regards,
> Neil

Maybe a few figures. There are two important specs for an UPS, the wattage 
and the run time. Usually, the run time is not well publicized, because it's 
quite pathetic in cheaper supplies.

Let's assume the typical PC, which consumes 80 watts, plus a monitor (19"), 
which also rates at around 80 watts. It may be somewhat lower, but these are 
safe values. If you want to measure your individual PC, you need a watt meter 
(that's what I did).

A simple 350 W cheapo UPS can handle this load, but...

There is a battery in the UPS, whcih has a certain capacity, measured in Ah 
(Ampere-hours). Typical values are 7-12 Ah. Let's stay with 7 Ah.

Due to limitations of the converter, you can use only something around 30% of 
the capacity. This would be about 2 Ah. The converter does not have 100% 
efficiency. Here, I have to guess, but I think a switching converter can do 
70%. Now we are down to 1.4 Ah usable capacity. This also assumes a fresh 
battery, because they lose capacity _rapidly_.

Our PC runs at 110 volts, and PC+monitor use 160 watts. Thus, they draw 1.45 
amperes. With 1.4 Ah capacity, this would last for slightly less than one 
hour.

This means, for a subsecond outage, almost any UPS would do. If you want to 
bridge lengthy blackouts, you'd want to look at the battery capacity. 

Mark

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