A question you always ask yourself when buying new hardware is how much power do I really need?
In most cases you end up with an oversize to be on the safe side, rather than facing the consequence of having to little power. Due to all the attention for the environment and the fasion to be green, oversize is not an option.
Calculating the correct amount manually is tricky, and you will probably forget something. Every single bit of hardware consumes power. The RAM for example, one unit doesn’t use much but when you install a bunch that number could be the difference.
Having this issue myself, I stumbled upon a great website, eXtreme Outer Vision, making this issue non existent!
For personal use, this service is free. You’ve got some limited options regarding hardware, but it mainly covers all your needs as long as you don’t go berserk. For commercial use you have to pay a very small price, at this moment priced at $1.99. Extremly useful when dimensioning servers where the load should be just right.
Apart from the calculator they have some great tools for calculating Overclock, Airflow, etc. Have a look!