Yes, so that means the power consumption of HDDs is pretty much stable; when doing read/write work is will only slightly increase. The biggest consumption comes from maintaining the spindle speed; which decreases due to friction with the air there.
5400rpm drives generally use only half the power 7200rpm drives use.
As suggested, the power will be pretty much identical whether you have two drives in a RAID configuration or are just using them as stand-alone drives.
The one thing that would increase power consumption would be if you were using a dedicated RAID controller card. If you're just using the RAID capabilities built into the motherboard chipset then there'd be no increase.