It isn’t enough setup sensors and measure energy consumption to reduce waste of energy. You should have a clear visual representation of those measurements.That roundup might help you to choose correct tool or at least gives some idea for that. Personally I like The Energy Detective aka TED. Because it gives you ability to use it standalone or integrate into some smarthome system. Also don’t forget Google PowerMeter which isn’t mentioned by some reason in that roundup.
So, choose right energy monitoring tool for you and be green!