Solar Thermal vs Solar PV for Hot Water Ireland – Which Works Best?

Many Irish homeowners ask whether they should install solar thermal panels for hot water or solar PV panels with an immersion diverter. Solar thermal panels heat water directly and cost €3,000–€5,500 installed (€1,800–€4,300 after the €1,200 SEAI grant). Solar PV panels generate electricity for all household uses and cost €6,000–€8,000 for a typical system after the €1,800 SEAI grant. For most Irish homes in 2026, solar PV with a power diverter like the Eddi delivers better value: it provides hot water from surplus electricity while also powering lights, appliances, EV chargers, and heat pumps, with a faster payback period of 5–7 years compared to 8–15 years for solar thermal.

What Is Solar Thermal and How Does It Work?

Solar thermal panels (also called solar hot water panels or solar collectors) use sunlight to heat water directly. The system circulates a heat transfer fluid, typically a glycol-water mixture, through panels mounted on your roof. When sunlight heats the fluid, it flows through a coil inside your hot water cylinder, transferring heat to your stored water. A controller monitors temperatures and activates the circulation pump when the panels are hotter than the cylinder.

The process works in six steps:

• Sunlight heats the solar collector panel on your roof
• Heat transfer fluid circulates through the panel, absorbing heat
• The hot fluid flows to a coil inside your hot water cylinder
• Heat transfers from the fluid to your stored water
• The cooled fluid returns to the panel to be reheated
• A controller manages the system automatically

Solar thermal systems typically provide 50–70% of a household's annual hot water needs. In summer months, they can supply up to 95% of hot water requirements. In winter, output drops to 15–20%, requiring your boiler or immersion heater to top up. Spring and autumn months deliver approximately 50% of hot water needs.

What Is Solar PV with an Immersion Diverter?

Solar PV panels generate electricity that can be used for any purpose in your home: lights, appliances, EV charging, heat pumps, and hot water. When paired with an immersion diverter (a device like the myenergi Eddi), solar PV can heat water using surplus electricity that would otherwise be exported to the grid at lower rates.

Here's how it works: your solar panels generate electricity during daylight hours. Your home uses what it needs immediately for appliances, lighting, and other electrical loads. Any surplus electricity that would normally export to the grid is automatically diverted by the Eddi to your immersion heater. The immersion heater converts that surplus electricity into hot water, storing it in your cylinder for later use.

This approach gives you the best of both worlds: electricity for all household uses during the day, plus free hot water from surplus generation. On cloudy days or in winter when surplus is limited, your boiler or standard immersion provides backup, exactly as it would with solar thermal.

The Two Types of Solar Thermal Panels

Irish homeowners can choose between two solar thermal technologies:

Flat Plate Collectors

Flat plate collectors cost €3,000–€4,500 installed and are the most common choice in Ireland. They consist of a flat glass panel with copper pipes running through an insulated box. Efficiency is 60–70% for converting sunlight to heat. These panels are robust, require minimal maintenance, and have a lifespan of 25+ years. They perform well in direct sunlight and are suitable for most Irish homes with reasonable south-facing roof space.

Evacuated Tube Collectors

Evacuated tube collectors cost €3,500–€5,500 installed and offer higher efficiency of 70–85%. They consist of rows of glass tubes, each containing a copper heat pipe in a vacuum. The vacuum insulation makes them more efficient in overcast and cloudy conditions, which suits Ireland's climate. They are lighter in weight than flat plates but can be more fragile. Lifespan is typically 15–20 years. These are best for homes with limited roof space where maximum output per square metre is essential.

Solar Thermal vs Solar PV: The Complete Comparison

*Figures correct as of August 2026. Costs are typical ranges for Irish installations.

Why Solar PV Wins for Most Irish Homes

For the vast majority of Irish homeowners in 2026, solar PV with an immersion diverter is the better investment. Here's why:

Electricity Is More Valuable Than Heat

Electricity costs approximately €0.32–€0.38 per kWh from the grid in Ireland. Heating water with gas or oil costs approximately €0.08–€0.12 per kWh-equivalent (accounting for fuel prices and boiler efficiency). Every unit of solar electricity you generate saves money at the full electricity rate, whether you use it for lighting, appliances, EV charging, or hot water. Solar thermal saves money only at the lower fuel-equivalent rate.

Solar PV delivers 3–5 times more value per unit of solar energy captured than solar thermal for most Irish households.

Solar PV Does Everything Solar Thermal Does (and More)

A solar PV system with an Eddi immersion diverter heats your water using surplus electricity, exactly as solar thermal would. The difference: when your panels generate more electricity than your home is using, the Eddi automatically diverts that surplus to your immersion heater rather than exporting it to the grid. You get free hot water from solar, just like solar thermal, but you also get electricity for everything else.

Solar thermal gives you only hot water. Solar PV gives you hot water plus electricity for lights, appliances, EV charging, and heat pumps.

Export Income from Surplus Generation

Under the Clean Export Guarantee (CEG), surplus solar PV electricity earns €0.18–€0.24 per kWh when exported to the grid. Most suppliers pay approximately €0.19 per kWh. For a typical 5 kWp system, annual export income is €100–€300, depending on household consumption patterns.

Solar thermal has no equivalent. Excess heat in summer simply raises water temperature until the cylinder's thermostat cuts out, wasting energy rather than earning income.

Future Flexibility

Solar PV electricity can power any future electrical loads: a heat pump if you upgrade your heating system, an EV charger when you buy an electric car, a home battery for energy independence, or new appliances. Solar thermal heat can only ever go into your hot water cylinder. There is no flexibility as your household's needs change.

The Irish government's climate targets mean more homes will install heat pumps and EVs in coming years. Solar PV supports both. Solar thermal supports neither.

Faster Payback and Higher Return

At the time of writing August 2026, a typical 5 kWp solar PV system costs €6,000–€8,000 after the €1,800 SEAI grant and pays back in 5–7 years. Annual savings are €500–€1,200 depending on household consumption and export levels.

Solar thermal costs €1,800–€4,300 after the €1,200 grant and pays back in 8–15 years. Annual savings are €200–€400 depending on current fuel type and hot water consumption.

Over a 25-year lifespan, solar PV delivers significantly higher total savings. The difference compounds when you factor in CEG export income, EV charging savings, and avoided grid electricity price increases.

The Solar Thermal Maintenance Reality

Solar thermal systems require ongoing maintenance that solar PV does not. The heat transfer fluid (glycol) degrades over time and must be checked every 3–5 years and fully replaced every 5–7 years. Glycol replacement costs €100–€200 per service. The circulation pump typically needs replacement after 10–15 years at a cost of €150–€300.

Annual visual inspections are recommended to check for leaks, corrosion, and panel condition. If glycol leaks occur, repairs can be expensive and require a specialist solar thermal installer. Many installers have moved away from solar thermal in favour of solar PV, making it harder to find qualified technicians for servicing.

Solar PV systems have no fluids, no pumps, and no regular servicing requirements. Occasional panel cleaning (once every 1–2 years) is the only maintenance, and many homeowners find Irish rain does the job naturally.

When Does Solar Thermal Still Make Sense?

Despite solar PV's advantages, there are specific situations where solar thermal remains a strong choice:

Very High Hot Water Demand with Limited Roof Space

If you run a B&B, guesthouse, or have a large family (5+ people) with very high hot water consumption, and your roof space is limited to 6–8 square metres of usable south-facing area, solar thermal captures more heat energy from that small area than solar PV would. Thermal collectors convert 60–70% of solar irradiance to useful heat, compared to solar PV's 18–22% conversion to electricity (though PV's electricity is worth more per kWh).

For these high-demand, space-constrained scenarios, solar thermal may deliver better hot water performance from the available roof area.

Replacing a Failed Immersion-Heated System

If your existing hot water system relies entirely on an electric immersion heater and your cylinder has failed and needs replacement anyway, solar thermal directly substitutes your most expensive energy use at a lower upfront cost (€1,800–€4,300 after grant vs €6,000–€8,000 for PV).

This only makes financial sense if you genuinely cannot accommodate solar PV due to roof orientation, shading, or structural issues.

You Already Have Solar PV

Adding solar thermal alongside an existing solar PV system is unusual but can make sense for very high hot water demand households where the PV system with Eddi isn't meeting all hot water needs. This is rare, and most installers would recommend adding more PV capacity or a home battery before considering solar thermal.

Listed Buildings or Conservation Areas

In rare cases where planning permission is difficult to obtain for solar PV due to visual impact concerns on listed buildings, solar thermal panels may be more acceptable to planning authorities because they look different from PV panels. Always check with your local planning office before proceeding.

Why Solar Thermal Fell Out of Favour in Ireland

Solar thermal was popular in Ireland during the 2000s and early 2010s when SEAI grants were more generous and solar PV was significantly more expensive. Three factors changed the market:

The cost of solar PV panels fell by approximately 80% between 2010 and 2025, making PV installations affordable for most homeowners. At the same time, PV panel efficiency increased from 15% to 22%, meaning smaller roof areas could generate more power.

The introduction of immersion diverters (Eddi, Solar iBoost) in the mid-2010s allowed solar PV to deliver the hot water benefit that previously required solar thermal. This eliminated solar thermal's unique selling point.

The SEAI solar thermal grant remained at €1,200 while the solar PV grant increased to €1,800 (and was previously as high as €2,400 for larger systems). The grant structure now favours PV, reflecting government recognition that PV delivers better value for public investment.

As a result, solar thermal installations in Ireland have declined by approximately 85% since 2015, while solar PV installations have grown exponentially. Most Irish solar installers no longer offer solar thermal as a primary product.

Can I Have Both Solar PV and Solar Thermal?

Yes, both systems can be installed on the same property, and both SEAI grants can be claimed separately (€1,800 for PV and €1,200 for thermal, total €3,000). However, this is rarely the optimal approach.

The roof space that solar thermal panels occupy is almost always better used for additional solar PV panels. More PV capacity means more electricity generation, more surplus to divert to hot water via the Eddi, and more export income. The complexity and maintenance cost of running two separate systems rarely justifies the modest additional hot water output.

The exception is if you genuinely have more south-facing roof space than your solar PV system can use (unusual for most homes) and very high hot water demand. In that case, filling remaining roof space with thermal collectors rather than more PV panels may make sense, given PV's diminishing returns as you export more surplus.

For the vast majority of Irish homes, a single well-sized solar PV system with an Eddi immersion diverter is simpler, cheaper to maintain, and delivers better overall value than a combined PV and thermal installation.

The Eddi Immersion Diverter Explained

The myenergi Eddi is the most popular immersion diverter in Ireland and is the product WattCharger recommends for solar PV installations. It costs €400–€600 installed and integrates seamlessly with your solar PV system.

The Eddi monitors your solar generation and household consumption in real time. When your panels generate more electricity than your home is using, the Eddi automatically diverts that surplus to your immersion heater. It can modulate power delivery from 100W to 3,680W (a full 16A circuit), meaning even small amounts of surplus are captured for hot water rather than exported at lower rates.

The Eddi works with any solar PV system and any immersion heater. It requires no changes to your existing hot water cylinder, provided you have a standard immersion element. Installation takes 1–2 hours and is typically included in your solar PV installation quote.

For homes with both an immersion heater and a heat pump or second immersion (for example, an Aga or range cooker), the Eddi can manage two heating loads, prioritising hot water first and diverting any remaining surplus to the second load.

See our guide to solar and immersion diverters for detailed information on how the Eddi works with WattCharger solar installations.

Solar Thermal Costs and Payback in Ireland

At the time of writing August 2026, solar thermal system costs in Ireland are:

Running costs are minimal, approximately €10–€20 per year for the circulation pump's electricity consumption. However, glycol replacement every 5–7 years adds €100–€200 per service, and pump replacement after 10–15 years costs €150–€300.

Payback period depends on your current hot water fuel source. For homes using an electric immersion heater (the most expensive option), payback is 5–10 years. For oil-heated hot water, payback extends to 7–14 years. For gas-heated hot water, payback is 8–16 years.

These payback periods are significantly longer than solar PV's 5–7 year payback, and the total lifetime savings are lower.

SEAI Grant Requirements

The SEAI solar thermal grant is a fixed €1,200, regardless of system size or type (flat plate or evacuated tube). Key requirements include:

• Home must have been built and occupied before 31 December 2021
• Installation must be by an SEAI-registered solar thermal contractor
• System must include a new dedicated solar hot water cylinder (twin coil)
• BER assessment required before and after installation
• Minimum collector area of 2 square metres
• Grant application must be submitted before works begin

The SEAI solar PV grant is up to €1,800 for systems up to 4 kWp and above. Unlike solar thermal, the PV grant scales with system size, providing €700 per kWp for the first 2 kWp and €200 per kWp thereafter, capped at €1,800 for systems of 4 kWp or larger.

Both grants cannot be claimed for the same property unless they are for different technologies (i.e., you can claim both if you install both PV and thermal, but you cannot double-claim for a single system).

The Verdict: Solar PV + Eddi Wins for Most Irish Homes

For the vast majority of Irish homeowners in 2026, solar PV with an immersion diverter is the better investment. The combination delivers hot water from surplus electricity (replicating the solar thermal benefit) while also generating electricity for all other household uses. Solar PV qualifies for a higher SEAI grant (€1,800 vs €1,200), earns CEG export income, has a faster payback period (5–7 years vs 8–15 years), requires minimal maintenance, and provides flexibility for future EV charging and heat pump installations.

Solar thermal is not a bad product, it simply cannot compete with solar PV on financial grounds for most homes. The technology has been overtaken by cheaper, more versatile solar PV combined with smart diverters like the Eddi.

The only scenarios where solar thermal makes sense in 2026 are very high hot water demand with limited roof space, replacement of failed immersion-only systems where PV is not feasible, or as a supplement to existing PV for exceptional hot water needs.

Frequently Asked Questions

What is the difference between solar thermal and solar PV?

Solar thermal panels heat water directly using sunlight. Solar PV panels generate electricity that can be used for any purpose, including heating water via an immersion diverter. Solar thermal provides hot water only; solar PV provides electricity for all household uses plus hot water when paired with a diverter.

Is solar thermal more efficient than solar PV?

Yes, for converting sunlight to heat. Solar thermal converts 60–70% of solar irradiance to useful heat, while solar PV converts 18–22% to electricity. However, electricity is worth 3–5 times more per kWh than heat in Ireland (€0.32–€0.38/kWh vs €0.08–€0.12 fuel-equivalent), so solar PV delivers better financial returns despite lower conversion efficiency.

Can I heat water with solar PV panels?

Yes, using an immersion diverter like the myenergi Eddi. The diverter routes surplus solar electricity to your immersion heater, providing free hot water from solar generation. This approach gives you the same hot water benefit as solar thermal, plus electricity for all other uses.

Do I need a special cylinder for solar PV hot water?

No. A solar PV system with an Eddi works with your existing hot water cylinder and standard immersion heater. No cylinder replacement is needed. Solar thermal requires a twin-coil cylinder, which must be replaced if your existing cylinder is single-coil.

Is there still an SEAI grant for solar thermal in Ireland?

Yes. At the time of writing August 2026, the SEAI provides a fixed €1,200 grant for solar thermal installations. The solar PV grant is higher at up to €1,800, reflecting PV's better financial returns and greater contribution to Ireland's renewable energy targets.

How long do solar thermal panels last in Ireland?

Flat plate collectors last 20–25 years. Evacuated tube collectors last 15–20 years. The glycol fluid must be replaced every 5–7 years, and the circulation pump typically needs replacement after 10–15 years. Solar PV panels last 25–30 years with minimal maintenance.

Should I replace my old solar thermal system with solar PV?

Not necessarily. If your existing solar thermal panels are working well, keep using them until they fail. When replacement is needed, solar PV with an Eddi is the better long-term investment. If you're choosing between new solar thermal and new solar PV today, choose solar PV.

Final Thoughts

The choice between solar thermal and solar PV was once a genuine debate. In 2026, the answer is clear for most Irish homes: solar PV with an immersion diverter delivers better value, faster payback, higher lifetime savings, and future flexibility that solar thermal cannot match. Solar thermal made sense when PV was expensive and diverters didn't exist. Today, technology has moved on, and solar PV is the smarter investment.

Ready to Install Solar PV?

WattCharger designs and installs solar PV systems with Eddi immersion diverters across Ireland. Our SEAI-registered team handles site surveys, grant applications, installation, and commissioning. Every system includes an Eddi as standard, giving you free hot water from surplus solar generation.

Get your free site assessment and detailed proposal today.

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