Many modern computers can enter low power-usage states when they detect they are idle. This mostly involves powering down the monitor, stopping the hard drive, and allowing the CPU to enter a slower idle state that does not produce as much heat. Running the client on a normally idle should not affect its ability to power down the monitor, which is a significant part of the power usage.

However, the hard drives of a power-saving machine may be prevented from spinning down if the client continues to periodically save or load blocks to disk. If you have multiple hard drives in your machine, you may want to consider ensuring that your client buffers and logs are on the hard drive that is most likely to have other activity as well (such as your OS swap file, or OS System directory), allowing the other less frequently accessed drives to spin-down unaffected. You might also want to consider enabling the Client's "nodisk" mode so that it only uses RAM for its operations, but be aware that your work may be lost if your computer crashes or loses power (wasting the power and idle cycles that the client could have used for productive work if it wasn't lost). You might also want to be aware of the fact that spinning up/down your hard drives can actually reduce its lifetime.

Additionally it is true that the Client will also probably prevent your CPU idle from entering its reduced power consumption idle cycle mode and keep it running at it's maximum power state. Depending on the computer, this may amount from a very minor portion of total usage power to a significant amount. You should also be aware that sometimes computer fans run only when excessive heat is detected (such as from a continuously operating CPU or hard drive). These cooling fans are an additional source of power usage.

Overall, the actual difference in power consumption by computers that are running the client during periods of time when they are normally left on (for unrelated purposes) is varied depending on the hardware. On much of the hardware of a few years ago, the difference was very minor. More modern hardware has focused heavily on optimizing performance per watt and as such the client can consume much more power than an idle system. Please keep that in mind.