I have a MacBook Pro 5,1 running Yosemite 10.10.5.

After replacing the battery, it has had excellent runtime on battery for about 6 months. Then the runtime had suddenly dropped by a factor of 2x-3x. The battery still has 95% capacity remaining as indicated by coconutBattery.

Nothing seems out of place in Activity Monitor, there are no apps using significant energy, yet the idle power consumption as indicated by coconutBattery is never below 15W.

The runtime reduction did not coincide with any Apple updates.