PV Solar Geyser vs Solar Thermal — What Is the Difference?

“Solar geyser” is often used as a general term, but it can describe two very different technologies.

One approach uses photovoltaic panels to produce electricity for a heating element. The other uses solar collectors to transfer heat directly into the water or into a heat-transfer fluid.

Both can reduce reliance on conventional electrical water heating. Neither is automatically the correct answer for every property.

How PV water heating works

A PV geyser system uses solar panels similar in principle to those used in a solar-power installation. The panels produce DC electricity, which is managed by a dedicated water-heating controller and delivered to a suitable element.

There is no solar-heated water circulating between the roof and geyser. The energy moves through electrical cables.

A PV system can be used with certain existing geysers when the cylinder, element arrangement and selected equipment are compatible. It can also be supplied as part of a complete new water-heating system.

Examples in Sustainable’s current range include the Geyser Gecko PV Solar Water Heating System, Geyserwise Offgrid PV Solar Water Heating System, and capacity-specific Geyserwise PV systems.

How solar thermal water heating works

A solar thermal system uses a collector to absorb heat from the sun. That heat is transferred to the stored water.

Common collector types include evacuated tubes and flat plates. The system may use natural circulation, known as thermosyphon, or a pump to move fluid between the collector and cylinder.

In a direct system, household water generally circulates through the collector. In an indirect system, a separate heat-transfer fluid circulates through a heat exchanger. The correct choice can depend on climate, water quality, frost risk and the product design.

Sustainable offers direct solar geysers, indirect solar geysers, and complete solar water-heating kits.

The practical differences

How energy reaches the geyser

PV systems transfer electrical energy through cable. Solar thermal systems transfer heat through water or heat-transfer fluid and plumbing.

Use of an existing geyser

A PV conversion may be possible with a serviceable conventional geyser, subject to compatibility. Solar thermal installations often use a purpose-designed solar cylinder or require additional plumbing and components.

Roof-to-geyser route

PV requires an appropriate cable route. Solar thermal requires insulated pipework between the collector and cylinder. The property layout can make one route easier than the other.

Freeze and water-quality considerations

The selected thermal system must suit local conditions. An indirect design may be appropriate where freezing or water conditions are a concern, but the final selection must follow the product specification and installer’s assessment.

Controls and moving parts

PV systems use electrical control equipment. Pumped thermal systems use a controller, sensors and circulating pump, while thermosyphon systems rely on natural circulation. Each approach has different components to inspect and maintain.

Backup heating

Both technologies can be designed with backup heating. This may be electrical or, in a suitable configuration, gas. The backup strategy matters because it affects convenience, energy use and operating behaviour.

Which system performs better?

A universal answer would be misleading. Performance depends on the complete design and the site.

The following factors are usually more important than the technology label:

• Correct system sizing
• Collector or panel orientation and shading
• Quality of installation
• Pipe or cable route
• Cylinder insulation
• Household demand and timing
• Controller settings
• Backup behaviour
• Climate and seasonal conditions
• Ongoing inspection and maintenance

A poorly located or incorrectly sized system will disappoint regardless of whether it is PV or thermal.

When PV may be attractive

A PV approach may be worth considering when:

• You want to retain a compatible existing geyser
• A cable route is easier than new solar plumbing
• You prefer dedicated solar panels and electronic control
• You want to avoid circulating water through a roof collector
• The modular placement of PV panels suits the available roof space
• You need a water-heating system separate from the home battery

This does not guarantee that a PV conversion is suitable. The controller operating range, element, panels and geyser must still be matched.

When solar thermal may be attractive

A thermal system may be worth considering when:

• You are replacing the geyser and can install a purpose-designed system
• The roof and plumbing layout are suitable
• Direct solar heat collection fits the project
• A complete pumped or thermosyphon system is preferred
• An installer has assessed the climate, water quality and freeze requirements

The choice between flat-plate and evacuated-tube collectors is another design decision and should be based on the specific products and site rather than a simple rule.

What if the property already has rooftop solar panels?

An existing household PV system does not automatically mean the geyser should be placed on the inverter. Check whether there is surplus daytime production, whether the inverter can support the element, whether the system has suitable load control and whether batteries would be used unnecessarily.

In some cases, controlling the existing geyser to operate during periods of surplus generation may be practical. In others, dedicated PV water heating or solar thermal will provide a clearer separation.

This must be designed around the actual inverter, array, battery, loads and municipal requirements.

Compare complete solutions, not one component

Avoid comparing only the controller price or collector price. A useful quotation should make clear what is included and what remains to be supplied or installed.

Check for:

• Cylinder or compatibility with the existing cylinder
• Panels or collectors
• Controller and sensors
• Element
• Pump where required
• Mounting equipment
• Electrical and plumbing protection
• Pipework or cable
• Installation labour
• Compliance and commissioning requirements
• Backup-heating method

The cheapest-looking component can become an expensive project if the remainder of the system has not been considered.

In Part 4: How Solar and Gas Can Work Together, we examine a hybrid approach that uses available solar heat first and gas when additional heating is needed.

Compare solar water-heating systems

Browse Sustainable’s solar water-heating kits and solar geysers. For product guidance, send us your geyser details, location, roof type, household size and preferred backup method.