In general, yes, your electricity consumption will go up a bit.
Nature of the beast. Your CPUs/GPUs will likely heat up a bit more which would probably require more cooling (especially if you live in the temperate/tropics zones) during the summer months.
Like most people here have said, if you have a watt-meter, you can fairly easily calculate or at least estimate what your electricity cost is going to be.
If you want to be energy efficient, there are ways of doing so, but they're not necessarily going to be come cheap (especially for initial capital cost). If you want examples of "Green" machines, go to
http://www.green500.org and you can see what some of the supercomputers are doing these days.
I've ran a check on all of my systems that's currently running F@H and they only average about 40-50 MFLOPS/Watt (which pales in comparison with the Green500 leader of 536.24 MFLOPS/Watt).
My next major system is targetted to be at least 100 MFLOPS/Watt in order to increase my overall computational efficiency.