Electricity is pretty cheap. The kind of gear required to make the electric bill a significant problem would be a bankrupting level of capital expense. I would estimate it would take a pretty large rack and tens of thousands of dollars to consume several kilowatts at which time hundreds of dollar energy bills would be a rounding error.

Also you need to correct for HVAC.

If its winter a KWh of BTC is the same heat as a KWh equiv of natgas, although the natgas is a little cheaper than electrical, by about a factor of 2. So spend $10 on a absolutely roaring system (About a KW continuously 24x7 which you're not going to be doing on non-dedicated hardware) and save about $4 on my gas bill so the NET is an expense of $6.

In the summer you have to correct for the overall system coefficient of performance, and people who have no idea what a COP is usually assume its "1" (legally mandated by local building code to be a minimum of 10 where I live, your experience may vary). So spend $10 on electricity and add an extra buck or so for HVAC cooling capacity.

What it comes down to is whether you will ever break even with what you put into your rig + all the ongoing expenses for what you get out.

> Electricity is pretty cheap. The kind of gear required to make the electric bill a significant problem would be a bankrupting level of capital expense.

The problem is that often Bitcoins are cheaper.

http://www.bitcoinx.com/profit/ is an example.

Note that the initial values (60,000MHash/sec, and a very tidy profit in 3 months) are appropriate only for one of the specialised hardware options. If you start filling it in with realistic values for a PC with a graphics card (see, e.g., https://en.bitcoin.it/wiki/Mining_hardware_comparison) you'll likely find the calculations running rather less in your favour.

Profiting is also much easier when bitcoins are $188 each than it is when they are $10 each.