OS X quite often even thinks the battery is dying and needs to be serviced or replaced, because the energy pattern is so weird running a Windows VM.
It's Windows alright, you can run any number of Linux machines and there will be nothing like the energy pattern from a Windows VM.
Same also for Parallels and Fusion, I've run both extensively.
My take: it's a lack of architecture in Windows that makes it impossible for Virtual Hosts to plug in a limited number of energy managers (mostly I think it's i/o processes for different applications that run on their own tracks in Windows, that affects energy consumption in Parallels) - you'd need specific ones to handle e.g. Office (which has its own architectural components for indexing, cacheing etc), and if you have e.g. an enterprise managed machine, which again adds a number of services with their own control points.
So we'll have to live with it until Windows gets architectured in a way that won't allow specific applications to run their own low-level services. My guess is as good as yours as to when that might occur 🙂