Permaculture design allows for elements to support eachother. 'A house with a solar hot water system may also contain a back-up wood burning stove with a water jacket to supply hot water when the sun is not shining.' Mollison and Slay.
All elements have a purpose and the permaculture design process can allow for multiple elements to support the main element and its function.
Photo: Centre for Alternative Technology on Flickr used under CC BY 2.0.