People are focusing on the power usage but that would honestly be the least of my concerns. The power usage would be minor but as soon as you start getting a decent number of visitors bandwidth wil be a real big problem.

For a while (a matter of weeks) I hosted a single page on a home machine, that was just a redirect to catch the few members we missed when we changed URLs. But it seemed like people always managed to find a way to crash my machine and it was off as much as it was on because of that!

If it's a long term site it's just not worth it.