We spend a lot of time studying backup to removable disk. Would it surprise you to learn that 3.5 inch hard drives use less power than 2.5 for backup on a per Gigabyte stored basis?
3.5″ disks have around twice the surface area of 2.5″ disks. They also typically have 1.5 to 2 times as many platters. No surprise that total storage per drive runs about 3-4 times larger for 3.5″ compared to 2.5″. For the past 2 years (2010-2012) the largest available 2.5″ drive has been 1TB. The largest 3.5″ drives have been 3TB, recently going to 4TB.
Let’s compare the power usage of a Seagate Barracuda 3TB 3.5″ drive running at 7200 RPM to a 1TB Seagate Constellation (ST9100640NS) running at 7200 RPM. For this comparison we assume neither drive spins down or goes to standby mode:
1TB 2.5″ drive – Idle – 3.31 Watts, Typical operating 5.21 Watts, PowerChoice mode 1.2 W
3TB 3.5″ drive – Idle – 5.4 Watts, Typical operating 8.0 Watts, Sleep Mode .75 Watts
Let’s assume our backup job takes 8 hours per night and we need to backup 3TB:
Using 2.5″ drives – 3 drives *(5.21W*8Hrs + 3.31W*16Hrs) = 283.9 Watt*Hours per day
Using 3.5″ drives – 1 drives *(8W *8Hrs + 5.4W*16Hrs) = 150.4 Watt*Hours per day
Conclusion: Backing up to a 3.5″ hard drive uses half the power
(assuming the drives don’t go into standby mode).
Obviously there are a lot of other scenarios we can imagine. If both drives were in use 24 hours:
2.5″ drives – 3 drives *(5.21W*24Hrs) = 375.1 Watt*Hours per day
3.5″ drive1 – 1 drive * (8W*24) = 192 Watt*Hours per day
Still about a 2 to 1 power savings in favor of 3.5″
What if the drives spin down (sleep) when not in use?
Using 2.5″ drives – 3 drives *(5.21W*8Hrs + 1.2W*16Hrs) = 182.6 Watt*Hours per day
Using 3.5″ drives – 1 drives *(8W *8Hrs + .75W*16Hrs) = 76 Watt*Hours per day
Now it’s 2.4 to 1 power savings in favor of 3.5″
What if the 2.5″ drives spin down but the 3.5″ drives don’t?
Using 2.5″ drives – 3 drives *(5.21W*8Hrs + 1.2W*16Hrs) = 182.6 Watt*Hours per day
Using 3.5″ drives – 1 drives *(8W *8Hrs + 5.4W*16Hrs) = 150.4 Watt*Hours per day
So even if the 2.5″ drives spin down, we use less power with the 3.5″ drive.