Revised 24 May 2019

Steady state means a constant level of capital stocks with some fluctuations in the form of buildings, transport systems, and machinery etc. which all  depreciate over time due to the laws of entropy. Continued flows of energy and materials are therefore necessary to maintain and replace capital stock. As humankind transitions from fossil fuels to harnessing solar energy, the flow rate of solar energy and the size of the solar "net" or solar energy converter used to capture that solar energy will ultimately set a limit to the possible inflows of energy required to sustain capital stock. 

While maintaining a steady state level of capital stock, control of the outflows of depreciation determines the level of capital stocks as much as does the inflows. It is therefore in our interests to use capital stock which has low depreciation with high durability and high recycling potential. By doing so we are able to maintain a larger total capital stock for the same inflow of energy and materials. Alternatively, we are able to sustain the same level of capital stocks with smaller inflows. By using capital stocks, such as transport systems which are more energy efficient than the systems we currently use, we are also able to sustain a larger total capital stock. Maximising energy use efficiency, recycling, and producing goods of long durability with high recycling efficiency potential will all help to sustain an economic system at a higher consumer level than would be otherwise possible.