Solar Water Heating: The Overlooked Half of Solar Energy

For most homes the biggest energy consumer after heating and cooling is hot water. Heating water with electricity is expensive. Heating it with gas is cheaper but emits carbon. Heating it with sunlight is almost free once the equipment is installed, and yet solar thermal systems get a fraction of the attention that photovoltaics receive.

A solar hot water system is refreshingly simple. Panels on the roof, called collectors, absorb sunlight and heat a fluid that flows through them. That fluid transfers its heat to a large insulated tank, either directly or through a heat exchanger. When you turn on the tap in the morning, the water in the tank is already hot.

There are two collector types you will see in catalogues. Flat plate collectors look like solar panels but are darker and have a glass cover trapping heat inside. Evacuated tube collectors are rows of glass tubes with a vacuum between the outer and inner surfaces, which insulates the hot fluid inside so effectively that they keep working on cold sunny days when a flat plate collector would struggle.

Circulation is either active or passive. Active systems use a small pump controlled by a differential thermostat that only runs when the collector is hotter than the tank. Passive or thermosiphon systems use the natural rising of hot fluid, which is elegant but requires the tank to be mounted above the collectors and is impractical for most European roof designs.

Freeze protection is essential in any climate that sees frost. The two common approaches are drainback systems, where the collector empties itself into a small tank whenever the pump stops, and glycol systems, where a mix of water and propylene glycol circulates permanently. Both work well, but glycol needs to be tested and replaced every five to seven years to prevent breakdown that can damage the collectors.

In terms of payback, solar thermal systems in sunny climates like southern Europe or the Middle East pay for themselves in three to five years by displacing electric or gas water heating. In cooler climates like the United Kingdom the payback is longer and the case is weaker, particularly compared with a heat pump.

Storage sizing follows a simple rule. You want enough tank capacity to store roughly two days of hot water demand so that a cloudy day does not force the backup element to kick in. For an average family of four that means about three hundred litres of storage, which is a large but not unusual tank size for solar retrofits.

Backup heating is required in every climate. The tank has an electric element, a gas coil or a heat pump loop that maintains a minimum temperature when solar alone cannot. A well designed backup system only runs when necessary and is invisible to the household, but a badly configured one can silently heat the tank overnight and eat all the savings the collectors provide.

Legionella management is a genuine safety concern. Bacteria that cause legionnaires disease thrive in warm stagnant water between twenty and forty five degrees, which is exactly the temperature range in a lightly used solar tank on a cool day. A weekly high temperature cycle to sixty degrees or higher is standard practice and is automated by most modern controllers.

The reason solar thermal has lost ground to photovoltaics is that PV has become so cheap that you can heat water with electricity from your own panels using a simple diverter switch. That approach uses the same roof space for two jobs and avoids the plumbing complexity of a dedicated thermal system. In practice, however, dedicated solar thermal still delivers more usable heat per square metre of collector than PV plus diverter.

For new builds and major renovations, seriously consider a hybrid approach. A modest solar thermal array for hot water and a larger PV array for electricity often outperform an all-PV solution on total energy delivered per euro spent. Ask your architect to model both options honestly before you commit.

One caveat worth stating out loud. Solar thermal has more moving parts than photovoltaics. Pumps, controllers, expansion vessels and heat exchangers all need occasional service. If you value a truly maintenance free system above every other consideration, PV plus a diverter is the simpler answer even if it is slightly less efficient in absolute terms.