My searching efforts have failed when trying to find this. On average, how much energy does the raspberry pi consume in 24 hours ( minimal usage vs max usage in a day AND usb vs micro usb powered)?
So, yeah, probably not more than:
4 W*24 h = 96 Wh (I guess that's 345.6 kJ) (note this is energy, not a power)
and likely not less than half that if you get the power down to 2 W. This is for a Raspberry Pi model B, the model A consumes even less power.
Even at relatively high electricity prices you are looking at < $0.02 per day of running the box.
Note, power consumption has been changing (for the better) with improvements in the software, because it's possible for the OS to power down some blocks of the machine when they are idle or perhaps if explicitly turned off.
Updated in Sept 2014 to note the release of the model B+: Less power! According to measurements by Alex Eames (see http://raspi.tv/2014/how-much-less-power-does-the-raspberry-pi-b-use-than-the-old-model-b )
A lightly-loaded model B with keyboard = 1.89 W -> daily 45.36 Wh
The B+ really offers huge improvements in the power circuitry. Wow!
I hope I'll be able to offer a more complete update with regard to the Raspberry Pi 2 Model B, but so far all I can say is that it's expected to use about the same amount of power as the B+ at idle, and at max CPU, not much more than the B. We shall see!
Also please realize that although it's very little power, if you are buying a solar panel, battery, charge regulator, etc. just to run the pi, you are spending a lot more on power hardware than you are spending on computing.
Regarding shutting off USB/LAN i/o for sleep/processing and then re-enabling it, see Disable LAN9512
I am running a Raspberry Pi2 with a 2TB WD element HDD mounted as root disk (and apm set to 254 - effectively disabling standby) via a Y-connector. Using a USB VA meter (see http://www.ebay.com/itm/371062446562?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT ) I measure (with both the USB HDD and pi2) 5V and about 0.7 A. Measured separately the disk and Pi2 each uses about 50% of this power...but that measurement may not be accurate because of the common USB 5V source...
In any case this means 3.5W and running 24/7 it gives about 31 kWh of energy per year. The highest rate for electricity is about 15 cents per kWh. So the cost of running my Pi2 server + 2TB hdd is about $5 per year.