He proposes as an example a single USB device that does not support Selective Suspend. "That USB device itself may have very low power consumption (e.g., a fingerprint reader), but until that device enters the suspend state, the processor and chipset must poll the device at a very high frequency to see if there is new data. That polling prevents the processor from entering low-power idle states and, on a typical business-class notebook, reduces battery life by 20 to 25 percent," he writes.
He presents a second example, this time of an application that increases platform timer resolution, which means the processor won't be able to use low-power idle modes efficiently. "We have observed a single application that keeps the timer resolution increased to 1ms can have up to a 10 percent impact on battery life on a typical notebook PC," writes DeWhitt.
To address these types of issues, the Windows 7 team has developed a new inbox utility that provides an HTML report of energy efficiency issues. "If you want to try it out on Windows 7, run powercfg /energy at an elevated command prompt," DeWhitt writes. "Be sure to close any open applications and documents before running powercfg; this utility is designed to find energy efficiency problems when the system is idle. powercfg with the /energy parameter can detect USB devices that are not suspending and applications that have increased the platform timer resolution."