[OLPC-AU] Calculating total power draw of XO

[Richard A. Smith]

On 03/17/2011 12:59 AM, James Cameron wrote:
> On Thu, Mar 17, 2011 at 03:50:30PM +1100, Sridhar Dhanapalan wrote:
>> What I can tell you is that our individual power adapters have the
>> following characteristics:
>>
>>    * input 100-240V AC 50/60Hz
>>    * output 12V DC 2500mA
>
> These are maxima and minima specifications, and only give you a limit
> for how much power can be drawn.
>
> Why don't you pop down to Coles or Woolies and get an energy meter; $25
> or so unit that plugs into the wall, you plug the appliance into it, and
> it tells you how many watt hours are consumed.  Saw one in Coles the
> other day.

As Tony pointed out there are extensive discussions on this on the power 
mailing list.  I highly recommend reading the archives.  However, most 
of those numbers are for XO-1 not 1.5

http://lists.laptop.org/pipermail/power/

Some XO-1.5 numbers from some of my previous tests are here:

http://blog.laptop.org/2010/08/25/hard-xo-power-figures/

But, I'm going to recommend that you go with James suggestion.  The 
reason is that most of the numbers I've gathered don't apply to your 
setup.  All of the measurements I have were taken with the XO 1.5 single 
adapters.

You are using the XOP 5 unit charger.  That complicates your calcs a bit 
because the efficiency of that charger is going to depend on the load. 
ie if you use it to charge 1 XO rather than 5 in single session its 
going to be much less efficient. Probably 20 or 30 %

Many other things can cause the efficiency to vary and give you 
different results.  I think that a power meter connected to a bank of 
XO's is your best bet for a number that does not involve a bunch of 
calculations and some guesswork.

Oh and when you use the meter make sure you take your readings on a bank 
of XO's, at least 10.  None of the cheap power meters are designed be 
accurate down in the range that the XO's run at.  You need 10 or 20 of 
them to get the power draw up enough to where its accurate.  Then just 
divide your results by the number of XOs to get per XO.

-- 
Richard A. Smith  <richard at laptop.org>
One Laptop per Child