Zuchinni, it's pretty easy to figure out, so long as you don't take any powersaving features into account. I've got a 250W power supply, so every hour that it's on I use .25 kWh. I pay $.1258/kWh for electricity, so it costs $.03145 to run my computer for an hour. If I left it on 24/7, it would cost about $23 for 30 days. Do the same using the power requirements for your monitor and anything else that is on when the computer is, and you get a good picture of what it costs to run it.