British Thermal Units... :-)
(For continental Europeans: 1 BTU is the amount of heat (energy) that warms up 1 pound of water by one degree Fahrenheit. :-)
Many people still remember kilocalorie: 1 kcal = amount of heat that raises the temperature of 1 kg water by 1 deg Centigrade.
BTU is then a "British kilocalorie", only it's 4 times less (0.2522 kcal). In the SI system, it's 1055 J (Joules).)
[edit]:
Wolfram Alpha also says it's about
0.25 × energy released by explosion of one gram of TNT. :-)
yeah def remember the 9.5th/s! blah!
BTC causes quite a lot of BTU!
Well BTC on GPU causes alot of BTU, the ASICs aren't so bad (although don't go grabbing a block erupter whiles its running, jesus those things get toasty).
Of course my GF didn't know what BTUs were... She just used the wrong initials in an attempt to ask me how my mining was going. I give her credit for caring at all... part of the reason I love her

. She does, however, very much like the things I buy her with my mined "BTUs" haha...