This way you can log in a database for example how long and how many times the TV was On. Simply check the current power usage, if it is larger then the Off power usage the device behind the circle is On.
I log these changes in a database and this way I know for all my devices how many times and how long each time they are used.
It's also nice to see How many times the fridge is going on and off.
When you have this data it's also very easy to make some charts.
Check this page (way down the page) to see the logging real time
http://www.bwired.nl/plugwise.asp
<i>below you can see some logging and how long a device was On or Off</i>
