Now that "green" is all the rage I am wondering just what kind of savings they really are. Is it worth replacing older hard drives with newer, "green" hard drives? When is the point that they pay for themselves in the cost of electricity. I have some drives in a nas that basically sit idle most of the day but are powered on 24/7. They are not very large and are a couple of years old.
Green drives use only half the power normal 7200rpm drives use; 3-4W while normal drives use 6-8W. This adds up as you have more drives in your system.
If its online 24/7, perhaps spinning down the HDDs when they are inactive for over 20 minutes is a good idea.
Each idle watt that you have on 24/7 costs you about 2 euro. So a normal HDD would consume 16 euro of electricity per year; that means that in 4 or 5 years, the electricity costs are higher than the initial cost of the unit.