No app can tell you that as there aren't any sensors.
There are voltage sensors for most components(GPU/Motherboard/CPU) but current usage increases according to load, so I don't think its possible to know the power consumption in realtime.
You may want to try this though: http://research.microsoft.com/en-us/downloads/fe9e10c5-5c5b-450c-a674-daf55565f794/