Correct sizing of solar water heating systems is both a science and an art, developed over the last 40 years by scientists and engineers. There are three main steps involved in performing such an analysis: 1) Determine the monthly energy requirement for the application. This energy requirement is often referred to as the "load". 2) Determine the contribution of a solar system to satisfy a portion of the load. The contribution is also known as the "solar fraction". 3) Determine the economic value, such as rate of return or payback time.