To put it as simply as possible, it's finding out how a house loses it's energy after the sun has gone down, and stopping that from happening. The less it loses the less you have to put back into it! 

We use the 5 principles of Passive House to achieve this: thermal insulation, windows, air tightness, adequate ventilation strategy, and thermal bridging.

With an energy efficient house, there will also be a whole range of economical ways of creating energy and heating the home, solar panels are just one of those.