In addition to the calculator, there are some deeper links you can reach (mostly .gov sites) as well as a disclosure about the math behind the calculator. Pretty decent PSA - interesting that Google devoted home page real estate to it.
Energy conservation is one of Google's big initiatives. Eric Schmidt gave an extremely thought-provoking speech about it recently at the Commonwealth Club in San Francisco, which I heard on public radio.
(Streaming audio available on the Commonwealth Club site [radiotime.com]. Unfortunately, there apparently is no permalink).