What Size Solar Thermal system do I need?

This article explains what size solar thermal system you will need. The will change if your are installing a solar system in a new or existing property.


Determining the right size solar thermal system is a much different process from determining the right size oil, gas or electric hot water system.

The more conventional systems are designed according to the maximum hot water requirement of the system.


How is a Solar Thermal system sized?

The size of a solar thermal system is designed in order to ensure it does not provide a greater amount of energy than is necessary to recharge the hot water store during times of low demand, like in warmer summer months.

By using a larger store, a greater solar fraction may be achieved. But using a store too big for the system may result in low temperatures during times of low solar availability. This would waste money and space.

According to current Building Regulations, the hot water store should provide no less than 80% of the daily hot water requirement, or 25 litres per square metre of the collector area.

Determining the necessary Size of your Solar Thermal system

The required size of a solar thermal system is determined differently for new and existing buildings. Where a solar thermal system is being retrofitted to an existing structure the system capacity can be figured out according to measurements of the demand.  These are noted during low consumption periods in the warmer summer months.

Where a solar thermal system is installed in a new building, the system should be sized according to the consumption of similar existing buildings. Experience in the industry is of great importance in determining the size of your solar thermal system.

Backup heat sources

There will be times when the solar thermal system is unable to meet the demand placed upon it. In order to ensure enough hot water is available, the solar hot water is used to preheat water which then flows into a continuous flow water heater, or a conventional water cylinder.

A purpose-built cylinder is often used in places where the solar coil utilises the lower space in the cylinder and a conventional primary heating coil is located within the top half of the cylinder.

Design tips

The energy required to pump the water around the system can be great. Therefore it can be advantageous to keep the pressure drops within the system to a minimum by careful planning of the pipework and the possible use of solar powered pumps.

Image: Sustainable Building Solutions

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