According to Introduction to Permaculture, Bill Mollison and Reny Slay, 1991, the second law of thermodynamics states that energy is constantly degraded into less and less usable forms. This is especially true of energy derived from fuel.
Energy derived from the sun arrives afresh each day and if not captured is radiated out into space, to rejoin the solar energy which missed the earth in the first place. The total power of solar radiation entering the earth's atmosphere is 174 billion megawatts. This is also the power of radiation leaving the earth's atmosphere. If not, the temperature would change rapidly. For the period of time the energy remains on earth, a portion can be captured.
Permaculture seeks to design sustainable "permanent" systems by:
a. Minimising the fuel requirements of the system. Elements in a design (for instance, food plants, animals, native habitat) are placed in zones depending on how often they are used or need to be serviced. High usage elements are placed closest to the centre of activity, which is usually the home.
b. Biological resources are utilised where ever possible rather than tools and products that emobdy or consume large amounts of fossil fuel (for instance machinery and inorganic fertilisers). Green manures (legumes), which add nitrogen to the soil using energy from the sun, are used in preference to fertiliser produced using fossil fuels, and pigs and chickens are used to cultivate the land rather than tractors.
c. Each element is integrated into the system in such a way that it serves many functions.
d. Energy is "cycled" so that the output of one element becomes the input to another and it also trapped, stored and used as many times as possible within the system before it is released in the form of heat, which finds its way out into space.
Energy captured from renewable sources is also sustainable, provided the energy costs of manufacturing the the equipment which captures the energy is accounted for. Equipment which wears out quickly might never produce as much energy as was used to create it.