An idle CPU uses minimal energy, whether it has a clock rate of 800Mhz or 125Mhz, at a given OPP level. This was discovered some time ago with the original attempt at a low power setting: the clock speed used to get turned down to 14Mhz, but this had detrimental effects on the clock and other live systems. Upon experimentation, it was found that the battery level when put into "low power" at 14Mhz changed by the same amount as if it were clocked down to 125Mhz, so now the low power mode only clocks down to 125Mhz. It has to be 125Mhz because that's the maximum guaranteed clock speed at OPP1. After that change was added, further experimentation found it to be basically the same across all OPP levels: clocked at 500, 600, or 800, it didn't matter: if it sat idle on the desktop the battery drain was the same at OPP3.