I am using apache2 and some cron jobs, but not much else is on the Raspberry Pi. I am not using a separate powered hub for the Aeon Lab ZStick S2. I run Debian 6 headless and have nothing else plugged in other than Aeon Lab S2 and my ethernet cable.
So far it has averaged about 2.45 watts an hour, putting me at 1.13ish kilowatts since I started monitoring it 18 days go. Based off this data, I think it will cost about $4.00 a year. I think I will get some small savings from having the lights on timers but nothing close to the cost of the equipment.
TL;dr – while the footprint for the Pi is really small, this is a project you do for the lols as you will not save a substantial amount of money.
Maybe Jim can share some EtherRain 8 power consumption specs and water saving ideas.
Update On Raspberry Pi Power Usage
I tested both a UK and Chinese made Raspberry Pi Rev B 512mb RAM with nothing plugged in (no HDMI, Ethernet, USB) and they both hovered at about 1.9 watts with the latest Rasbian install. Plugging in the ethernet to allow it to be useful in a headless fashion, drove it to 2.2 watts. Logging in via SSH and doing a simple ping didn’t change anything. Running Raspbmc was a little more taxing – an unplugged board would hover around 2.1 watts.