You can trial the site on your production deployment (or test lab) by contacting your sales representative. You will be asked to fill out a trial form and then you can access the site.
You can grab a few computers in a lab somewhere and install the trial version of BES, which will let you try BES Power Management on up to 30 computers for 30 days: http://support.bigfix.com/bes/install/beseval.html
The trial version of BES will give you an option to try any Fixlet site, but I am not sure if the eval list has been updated to include Power Management yet. If it has not, it should be up shortly.