I have also tried to measure the power consumption of a PC with a 400 W power supply, and the real power consumption was about 10 persent or 35 watt. I then calculated what 35 w of extra heating of my apartment would cost, and I concluded that I could afford it.
There is a very easy way to find out if a PC consume a lot of power or not. This is to feel with your hand on the main board controller, around the processor, the graphic adaptor, and the blow out from the power supply. If there is a lot of heat, it runs with some power consumption. If most areas are mainly cold, it consumes rather litle energy. In general: If the fan blow out from the power supply feels rateher cold, and not to hot, power consumption will normally be 50 Watt or less. (I actually as measured a number of PC's for power consumtion, and this is my general practical conclusion.)
I have also tried to run the sme server at different platforms, like old laptops and even a HP thin client and it all worked. The HP thin client did not perform well enough, but I rebuildt it to be a Asterisk telephony server, and it has been running for thas purpose for more then a year now in my friends apartment.
Generally I agree with the posts above the a laptop or a sub laptop is not a practical "hosing" for a server and it will generally not perform to well. I once tried to install SME server on my Shuttle X-Box and this worked very well. If it was not used as my desktop now, It would have been my SME server.