One consideration for the implementation of solar hot water systems is the seasonal nature of the solar energy.

With a daily average solar irradiation in the summer months that is six to seven times higher than in the winter months and domestic hot water demand that is relatively constant throughout the year, the designer of a solar hot water system faces a choice - should the system be specified to meet hot water demand in winter, spring or summer?

A system that is sized to meet a household's requirements in winter will be six to seven times larger than it needs to be in summer. The solar panels are sitting idle for large parts of the year. The utilization of the system is poor and the cost-effectiveness of the over-sized system reduced compared to a system with higher utilization.

On the other hand, a system sized to match demand in summer will use more auxiliary heating in winter. The compromise that is usually chosen maximises the ratio of useful energy collected to the capital investment. Normally this works out to be a system that just meets demand in the summer, and provides around 50-60% of the hot water throughout the year.

The challenge for using solar to provide space heating is that although both are seasonal, their seasonality is out of phase – when solar energy availability is at a maximum, space heating is not required, when space heating is most needed, solar energy is at a minimum. The graph shows the timing and relative sizes of energy demand for space heating and domestic hot water for a well-insulated home of average size.

To make a meaningful contribution to space heating, the area of solar panel needs to be significantly increased compared to a system that satisfies domestic hot water only.

A system with larger solar panel area is likely to produce a large excess of energy in the summer months compared to covering the hot water demand, so will spend long periods in summer not running because the heat load is satisfied early in the day. Outside of summer, the larger panel area gives a greater coverage of hot water demand, and there is sufficient energy to shorten the heating season and make a contribution to space heating in spring and autumn (shown in orange). The additional energy from targeting space heating can be modest, but on the other hand, the larger solar thermal system will have a lower cost per square metre than the smaller system due to costs that do not scale linearly with area, costs such as the controller, connecting pipes and hot water store.

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