It is assumed that the costs of exhaustible energy increase linearly with the cumulative consumption, whereas the costs of renewable energy increase as the annual amount of renewable energy consumed grows. Special attention is paid to the possible role of nuclear energy under two different CO2 emission constraints, and different nuclear cost conditions related to the available amount of plutonium and whether or not the nuclear non-proliferation treaty prevails. A large set of different factors contribute to the current level of security of energy supply.