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)?
I began my answer with the original thread on the Raspi forums: http://www.raspberrypi.org/phpBB3/viewtopic.php?f=63&t=6050&p=291334&hilit=watts+power#p291334
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 almost certainly more than 10% of that if you can get the power down to 0.4 W.
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 to note the release of the model B+, A+, and then Pi 2 Model B. According to measurements by Alex Eames 
The A+ and B+ really offer huge improvements in the power circuitry. Wow!
Note that the Pi 2 Model B is a 4-core machine with each core idling near 0 and maxing near 0.25 W power demand.
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.
 For comparison, a reasonable battery bank with 5 V output, similar in size to a lipstick case, contains a little over 11 Wh (3.2 Ah*3.7V) and a 12 V car battery stores a total energy, fully charged, of about 1000 Wh.
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.