When deciding whether to use any energy source, three components need to be considered: the energy source itself, the reason energy is needed, and the technology that will best match the available energy source to the needs of the user.
As mentioned earlier, solar power can be captured either as light, electricity or as thermal power (heat). Discussions of solar energy systems are often divided into the categories of passive systems and active systems. In the case of solar water heating systems, the additional categories of direct and indirect are also used. However, before we describe solar energy systems further, we need to consider the importance of energy efficiency.