Peak Power Consumption = 1* 66 + 4 * 2.3 + 1 * 7.9
= 66.0 + 9.2 + 7.9
= 83.1 W
Power Supply Wattage = 83.1 / 0.8
= 103.9 W

How much power will the 7200 rpm disk drive consume if it is idle roughly 50% of the time?
HD Power Consumption = 0.5 * 7.9 + 0.5 * 2.9
= 5.4 W

Given that the time to read data off a 7200 rpm disk drive will be roughly 75% of a 5400 rpm disk, at what idle time of the 7200 rpm disk will the power consumption be equal, on average, for the two disks?
Let the 7200disk busy time = x
7200disk power = 5400disk power
4.0 * (1x) + 7.9 * x = 2.9 * (1x/0.75) + 7.0 * x/0.75
4.0 + 3.9 x = 2.9 – 3.87 x + 9.33 x
1.1 = 5.47 x – 3.9 x
Hence x = 0.70
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