With the help of fundamental considerations regarding the extension of power station systems it is shown that the energetic amortization period presents a boundary value for an energy-positive extension of power stations. It is only, if the extension is slower than with a doubling within an energetic amortization period, that the power station system can make positive energy contributions available and relieve other energy sources. Otherwise. it will be an additional energy consumer, for whose supply possibly additional power stations may have to be built. As the energetic amortization period for solar power stations is system-immanently much greater than for coal and nuclear power stations, this results practically in a serious restriction in the extension feasibility, whilst the limit is beyond the realistic range for coal and nuclear power stations.