Drive Specifications
For competitor drives, drive power usage data is derived from publicly available competitor web pages, data sheets, and specifications.* WD drives, drive power usage data is derived from actual test results and/or product specifications.

Calculating the Infrastructure Multiplier
The infrastructure “multiplier” which translates hard drive power usage to system power usage may be found in source document: “Report to Congress on Server and Data Center Energy Efficiency Public Law 109-431” issued on August 2, 2007 by the Environmental Protection Agency (EPA).

Duty Cycle Conversion
Duty cycle conversion methodology is based on a number of assumptions. The overall “duty cycle” number can be separated into discrete blocks of time spent in the various different operating modes. For instance, when running 100% duty cycle, all of the time is spent in what we call “RVA mode”, whereby the drive is always at full power. Alternatively, when running at 50% duty cycle, some of that time is spent in lower power modes.**

Cost
Cost is measured in dollars per kilowatt hour ($/KWh) and is based on regional electric cost averages gathered nationwide which may be found in source document: “Datacentre Energy Dynamics 2007” from the Lifecycle Energy Efficiency Conference / Expo 2007.

*Linear extrapolation was used in cases were required data was unavailable. The margin of error that this may introduce is negligible.
**Ratios used for various duty cycles are derived from logical deduction. Each ratio used for each duty cycle is applied consistently across all suppliers.