The major part of worldwide energy production concerns fossil energy. Therefore, concomitant carbon diox- ide production is a problem. What are suitable alternatives? Here we present considerations for selection of energy sources needed for future worldwide energy consumption. Plants are collectors and converters of sunlight and are indispensable in the carbon cycle. This cycle converts sunlight to chemical energy, and next frees this energy by biochemical oxidation in humans and other organisms, or by chemical oxidation.