I think this discussion is worth sharing from Energy DSM.

Energy efficiency involves technology that produces the same end product while using less energy. For example, an energy efficient air conditioner produces the same level of cooling capability while using less energy than the average air conditioner on the market.

Conservation involves cutting waste of energy whereas energy efficiency does not. For example, I can replace my old air conditioner with an energy efficient one, but can still waste energy by running it while I’m not home.

Here’s dictionary.com’s opinion:

Dictionary.com says conservation is “the careful utilization of a natural resource in order to prevent depletion” whereas efficiency means “the ratio of the work done or energy developed by a machine, engine, etc., to the energy supplied to it, usually expressed as a percentage.”

This reminds me of the “recycle” logo –> Don’t forget there are two other legs: Reduce and Reuse.