Solar Panels for Hot Water: PV vs Solar Thermal in Northern Ireland

Solar Panels for Hot Water: Which Approach Is Right for Your NI Home?

Heating water accounts for a significant chunk of energy costs in Northern Ireland homes. With many properties still using oil boilers and electricity costs remaining high, using solar energy to heat your water is one of the most effective ways to cut your bills.

But there are two fundamentally different ways to use solar energy for hot water, and they are often confused. Solar thermal panels heat water directly using the sun’s warmth. Solar PV panels generate electricity, which can then be diverted to your immersion heater to heat water. The two technologies are quite different in how they work, what they cost, and how practical they are for NI homeowners.

The short version: for most homes in Northern Ireland in 2026, solar PV panels with an immersion diverter offer better value, lower maintenance, and greater flexibility than solar thermal. But the full picture is worth understanding, so this guide compares both approaches in detail.

How Solar Thermal Works

Solar thermal technology has been around for decades. It uses rooftop collectors to absorb heat from the sun and transfer it to your hot water cylinder via a fluid-filled loop.

The two types of solar thermal collector

Flat plate collectors are the simpler and cheaper option. They consist of a dark absorber plate inside a flat, insulated box with a glass or polycarbonate cover. A heat transfer fluid (usually a water-glycol mix) circulates through copper tubes bonded to the absorber plate. When sunlight hits the plate, it heats the fluid, which is then pumped to a coil inside your hot water cylinder.

Flat plate collectors are robust, relatively affordable, and work well in moderate climates. A typical domestic system uses one or two panels, each measuring approximately 2m x 1m.

Evacuated tube collectors are more efficient, particularly in colder or cloudier conditions. They consist of a series of glass tubes, each containing a smaller tube with a dark absorber coating. The space between the inner and outer tubes is evacuated (a vacuum), which provides excellent insulation and reduces heat loss.

Evacuated tubes are better suited to Northern Ireland’s climate because the vacuum insulation means they lose less heat on cold, windy days. They also perform better at higher temperatures and in lower light conditions. However, they cost more than flat plate collectors and are more fragile.

How the solar thermal system heats water

1. Sunlight heats the collector on the roof.
2. A heat transfer fluid circulates through the collector, absorbing heat.
3. A pump (usually electric, powered by a small controller) pushes the heated fluid to a coil inside the hot water cylinder.
4. The coil transfers heat to the water in the cylinder.
5. The cooled fluid returns to the collector to be reheated.
6. A controller monitors temperatures and only runs the pump when the collector is hotter than the water in the cylinder.

The system is a closed loop. The heat transfer fluid stays in the system permanently and never mixes with your domestic hot water. An expansion vessel and pressure relief valve are included for safety.

Solar thermal costs in Northern Ireland

Component	Cost Range
Flat plate collector system (1-2 panels)	£2,500-£4,000
Evacuated tube collector system	£3,000-£5,000
Hot water cylinder (if not already present)	£400-£800
Controller and pump station	Included in system price
Installation labour	Included in system price
Total installed cost	£3,000-£5,000

These prices are for a complete, professionally installed solar thermal system suitable for a typical 3-4 bedroom NI home with an existing hot water cylinder.

How Solar PV Heats Water (With an Immersion Diverter)

Solar PV panels, as explained in our guide to how solar panels work, generate electricity from daylight. That electricity can power anything in your home, including your immersion heater. The key piece of technology that makes this work efficiently is the immersion diverter (also called a solar diverter or power diverter).

What an immersion diverter does

An immersion diverter is a small device installed next to your consumer unit (fuse board). It monitors two things in real time:

1. How much electricity your solar panels are currently generating
2. How much electricity your home is currently consuming

When your solar generation exceeds your household consumption (i.e. you have surplus electricity), the diverter automatically routes that surplus to your immersion heater. Instead of exporting the surplus to the grid for little or no payment, you are using it to heat water for free.

The clever part is that the diverter works proportionally. If you have 500W of surplus solar power, it sends 500W to the immersion heater. If you then switch on a kettle (2kW), the surplus drops, and the diverter reduces the power to the immersion accordingly, or stops it entirely if there is no surplus. When the kettle finishes, the surplus returns and the immersion resumes heating.

This happens automatically, hundreds of times per day, with no input from you.

Popular immersion diverter models

Three models dominate the NI market:

Eddi (by myenergi): The Eddi is the most feature-rich diverter available. It can manage two heaters (e.g. immersion heater and a radiator or towel rail), integrates with myenergi’s Zappi EV charger and Libbi battery system, and provides detailed energy monitoring through the myenergi app. It supports programmable boost schedules and can be set to heat water at specific times using grid electricity if solar is insufficient. Price: £350-£450 installed.

iBoost / Solar iBoost+: The iBoost was one of the first immersion diverters on the UK market and remains popular due to its simplicity and reliability. The latest Solar iBoost+ model includes a wireless sender (clamp sensor) and a receiver unit that controls the immersion. It includes a manual boost button and a basic display showing how much free energy has been diverted. Price: £250-£350 installed.

Catch Power (Green Catch / Blue Catch): An Australian brand gaining popularity in the UK market. The Green Catch diverts surplus solar to a single load, while the Blue Catch supports multiple loads. Good app-based monitoring and simple installation. Price: £250-£350 installed.

The cost of adding a diverter to a solar PV system

Component	Cost Range
Immersion diverter unit	£200-£400
Installation (if done with solar panel install)	£50-£100
Installation (retrofit to existing solar system)	£100-£200
Total	£250-£600

Compare this with £3,000 to £5,000 for a solar thermal system. The diverter is an add-on to a solar PV system you may already be installing (or may already have), making it one of the most cost-effective energy-saving devices available.

PV + Diverter vs Solar Thermal: The Full Comparison

This is the comparison that matters most. Let us put the two approaches side by side across every factor that counts.

Cost comparison

Factor	Solar PV + Diverter	Solar Thermal
Solar panel/collector cost	£6,000-£8,000 (4kW PV system)	£3,000-£5,000
Hot water diverter/controller	£250-£600	Included
Total cost for hot water only	N/A (PV does much more)	£3,000-£5,000
Additional cost to add hot water to PV	£250-£600	N/A
Hot water cylinder required	Yes	Yes

The cost comparison is nuanced. Solar thermal is cheaper if you only want hot water and nothing else. But if you are considering solar PV for electricity generation (which most NI homeowners are), adding a diverter for £250-£600 is dramatically cheaper than installing a separate solar thermal system.

In practice, the comparison is usually between:

• Option A: Solar PV system (£6,000-£8,000) + diverter (£300-£500) = £6,300-£8,500 for electricity AND hot water
• Option B: Solar thermal (£3,000-£5,000) for hot water only, PLUS you still need to pay full price for electricity from the grid

Option A provides far more total value because the PV system generates electricity for all your needs, with the diverter adding hot water capability for a fraction of the cost.

Hot water performance comparison

How do the two approaches compare specifically for heating water?

Factor	Solar PV + Diverter	Solar Thermal
Summer hot water coverage	80-100%	80-100%
Winter hot water coverage	20-40%	30-50%
Annual hot water coverage	50-70%	55-75%
Heating speed	Moderate (3kW immersion)	Fast (direct heat transfer)
Performance on cloudy days	Lower (needs surplus electricity)	Better (heats from diffuse warmth)
Maximum water temperature	60-65°C (immersion thermostat)	60-90°C (controller limited)

Solar thermal has a slight edge in raw hot water performance, particularly in winter and on overcast days. Evacuated tube collectors can extract useful heat even when light levels are too low for PV panels to generate meaningful surplus electricity.

However, the gap is smaller than many people assume. A 4kW PV system in NI generates enough surplus on most days between March and October to fully heat a 150-200 litre cylinder. In winter, the PV system generates less surplus, but the diverter still captures every available watt and sends it to the immersion.

Maintenance comparison

This is where solar PV with a diverter wins decisively.

Factor	Solar PV + Diverter	Solar Thermal
Annual maintenance needed	None	Yes (recommended annually)
Typical annual service cost	£0	£75-£150
Heat transfer fluid	N/A	Needs replacing every 5-10 years (£100-£200)
Pump	N/A	Can fail (£150-£300 to replace)
Collector/panel lifespan	25-30 years	20-25 years
Frost protection	N/A (no fluid)	Required (glycol mix or drain-back system)
Risk of leaks	None (no fluid loop)	Possible (fluid loop, connections, pump seals)

Solar thermal systems have more moving parts and more things that can go wrong. The pump circulates fluid continuously when the sun is shining, pump seals can fail, the heat transfer fluid degrades over time and needs replacing, and the system needs checking annually to ensure the glycol concentration is correct for frost protection.

Solar PV panels have no moving parts. The diverter is a solid-state electronic device with no maintenance requirements. The immersion heater element is the only component that wears out, and it would wear out regardless of whether you use solar or grid electricity to power it.

Over a 20-year period, the maintenance cost difference can be significant:

• PV + diverter maintenance: approximately £0 (panels are maintenance-free; diverter has no serviceable parts)
• Solar thermal maintenance: approximately £1,500-£3,000 (annual servicing, fluid changes, potential pump replacement)

Roof space comparison

System	Roof Area Required	Orientation
Solar PV (4kW, 10-12 panels)	17-20 m²	South, east, or west
Solar thermal (2 flat plates)	4-5 m²	South (ideally)
Solar thermal (evacuated tubes)	3-4 m²	South (ideally)

Solar thermal uses less roof space for the same hot water output. If roof space is severely limited (a small terraced house, for example), solar thermal can deliver meaningful hot water savings from just 3-5 square metres of roof area, whereas a PV system needs considerably more space to generate enough surplus for effective immersion diverting.

However, for most NI homes, roof space is not the limiting factor. A typical semi-detached or detached house has 20-40 square metres of suitable roof area, more than enough for a 4kW PV system.

Why Most Installers Now Recommend PV + Diverter

If you speak to solar installers in Northern Ireland today, the overwhelming majority will recommend solar PV with an immersion diverter over solar thermal for hot water. Here is why the industry has shifted.

Versatility

Solar PV generates electricity that can be used for anything: lights, appliances, cooking, EV charging, and hot water (via a diverter). Solar thermal only heats water. If you spend £4,000 on solar thermal, you have a system that does one thing. If you spend £7,000 on PV plus a £400 diverter, you have a system that powers your entire home AND heats your water.

Simpler installation

Solar PV installation is a well-established, standardised process that most NI installers handle routinely. Solar thermal involves plumbing, fluid filling, pressure testing, and controller configuration. It requires a different skill set and takes longer. Many solar installers who started as thermal specialists have transitioned fully to PV because the installation is simpler and customer satisfaction is higher.

Better financial returns

The total financial return on a PV system vastly exceeds that of solar thermal. A 4kW PV system in NI saves £500-£900 per year across all electricity uses. Solar thermal saves £100-£200 per year on hot water costs alone. Even accounting for the higher upfront cost of PV, the payback period is shorter and the lifetime savings are much greater.

For a full breakdown of PV system costs and returns, see our solar panel costs in Northern Ireland guide.

Battery compatibility

Solar PV integrates naturally with battery storage. A battery stores surplus solar electricity for evening use, covering lighting, appliances, and (via the immersion diverter’s timer or boost function) hot water. Solar thermal has no interaction with battery storage at all.

Future-proofing

As homes move towards full electrification (electric vehicles, heat pumps, induction cooking), having a solar PV system that generates electricity becomes increasingly valuable. Solar thermal, which only heats water, does not contribute to these future energy needs.

NI-Specific Considerations

Northern Ireland has some unique characteristics that affect the solar hot water decision.

Oil boiler integration

Approximately 68% of NI homes use oil central heating, the highest proportion in the UK. Most oil-heated homes have a hot water cylinder that the boiler heats via a coil. This is ideal for solar hot water integration, because:

• The cylinder is already there. No need to install one (saving £400-£800).
• The solar system pre-heats the water. During the day, solar (whether PV via diverter or thermal) heats the water in the cylinder. When you need hotter water, the oil boiler only needs to top up the temperature rather than heating from cold. This significantly reduces oil consumption for hot water.
• Summer oil savings are dramatic. Between May and August, a well-sized solar system can provide virtually all your hot water, meaning the oil boiler does not need to fire for hot water at all. Given that heating oil costs around 60-70p per litre, the savings are meaningful.

No mains gas in rural areas

Many rural NI properties are not connected to the gas network and rely entirely on oil or LPG for heating and hot water. Solar hot water is particularly valuable for these homes because the alternative fuel costs are higher than mains gas. Every litre of oil or LPG displaced by solar energy saves more money than displacing mains gas would.

Existing immersion heaters

Most NI homes with a hot water cylinder already have an immersion heater fitted (it is typically included when the cylinder is installed). This means adding a solar PV diverter requires no additional plumbing work. The diverter simply controls the existing immersion heater, routing surplus solar electricity to it when available.

If your immersion heater element is old or low-powered, replacing it with a new 3kW element costs approximately £80-£120 including a plumber’s visit. This is a worthwhile investment to ensure the diverter can heat water efficiently.

Climate considerations

NI’s maritime climate (mild, cloudy, and wet) slightly favours evacuated tube solar thermal over flat plate thermal collectors, because evacuated tubes retain heat better in cool, windy conditions. However, this advantage has to be weighed against the overall case for PV, which is stronger for the reasons outlined above.

NI’s climate is perfectly adequate for solar PV hot water. The key months for solar hot water (April to September) deliver enough PV generation for effective immersion diverting, and the diverter ensures none of that generation is wasted.

Installation Considerations

What you need for PV + diverter

• Solar PV system. Any grid-connected PV system works with a diverter. System size of 3kW or above is recommended to generate meaningful surplus for water heating.
• Hot water cylinder. A vented or unvented cylinder of 150-200 litres is ideal for a family home. If you have a combi boiler with no cylinder, one will need to be installed.
• Immersion heater. A standard 3kW immersion element in the cylinder. Most cylinders have one already.
• Diverter unit. Installed next to your consumer unit by an electrician. Typical installation time is 1-2 hours.
• CT clamp. A small sensor that clips around your meter tails to measure generation and consumption. This is how the diverter knows when surplus power is available.

What you need for solar thermal

• Solar thermal collector. Flat plate or evacuated tube, mounted on the roof.
• Hot water cylinder with twin coils. The cylinder needs two coils: one for the solar thermal loop and one for your boiler. If your existing cylinder only has one coil, it will need replacing (£400-£800).
• Pump station and controller. The pump circulates the heat transfer fluid; the controller manages when the pump runs.
• Pipework. Insulated copper or stainless steel pipes running from the roof collector to the cylinder.
• Expansion vessel and safety valves. Required for the pressurised fluid loop.

Solar thermal installation is more disruptive than adding a PV diverter. It involves roof work (mounting the collector), running pipes through the building, and potentially replacing your hot water cylinder. Typical installation time is 1-2 days.

Making the Right Choice for Your Home

Choose solar PV + diverter if:

• You want electricity generation as well as hot water
• You value low maintenance and simplicity
• You already have (or are installing) a solar PV system
• You have adequate roof space for PV panels (17+ square metres)
• You want the option to add battery storage in the future
• You plan to get an electric vehicle or heat pump in coming years
• Your budget allows for a full PV system

Choose solar thermal if:

• Hot water is your only objective and you have no interest in electricity generation
• Your roof space is extremely limited (less than 10 square metres available)
• You have a large hot water demand (large family, multiple bathrooms) and want maximum hot water output from minimum roof area
• You already have solar PV and want to add thermal to a different roof section for additional hot water capacity (rare, but possible)

Consider combining both (rare cases)

In theory, you could install both solar PV (for electricity) and solar thermal (for hot water), using different sections of your roof. In practice, this is rarely cost-effective. The additional hot water from solar thermal on top of a PV system with a diverter is marginal, and the combined installation and maintenance costs are hard to justify.

The only scenario where combining both makes sense is a very large property with very high hot water demand (e.g. a farmhouse with multiple bathrooms, a B&B, or a property with a swimming pool). For these situations, consult a specialist installer who can model both systems together.

Cost Savings: PV + Diverter vs Solar Thermal

Let us put some real numbers on the savings for a typical NI household.

Assumptions

• 3-4 bedroom house with 4 occupants
• 200-litre hot water cylinder
• Oil boiler for central heating and hot water backup
• Current hot water cost: approximately £400-£600 per year (oil or electric immersion)
• 4kW solar PV system (if choosing PV route)

Annual hot water savings

System	Annual Hot Water Saving	Total Annual Saving (all energy)
Solar PV (4kW) + diverter	£150-£250	£500-£900 (electricity + hot water)
Solar thermal (evacuated tubes)	£150-£250	£150-£250 (hot water only)
Solar thermal (flat plate)	£120-£200	£120-£200 (hot water only)

The hot water savings are broadly similar between PV + diverter and solar thermal. The massive difference is that PV provides electricity savings on top, making the total financial benefit three to four times greater.

Payback period

System	Installed Cost	Annual Saving	Simple Payback
PV (4kW) + diverter	£6,500-£8,500	£500-£900	8-13 years
Diverter only (added to existing PV)	£250-£600	£150-£250	1-3 years
Solar thermal (evacuated tubes)	£3,500-£5,000	£150-£250	14-25 years
Solar thermal (flat plate)	£2,500-£4,000	£120-£200	13-25 years

The standout figure is the diverter-only payback period. If you already have solar PV panels, adding a diverter for £250-£600 is one of the best investments you can make. It pays for itself within 1-3 years and continues saving you money for decades.

Solar thermal payback periods are longer because the system only saves on hot water, while the upfront cost is substantial. With maintenance costs factored in, the effective payback stretches further.

What About Heat Pumps?

If you are considering renewable heating options, you may also be weighing up heat pumps. Our guide on solar panels vs heat pumps covers this comparison in detail.

In the context of hot water specifically:

• Air source heat pumps heat water very efficiently (COP of 2.5-3.5 for hot water production) and can work alongside solar PV.
• Solar PV + heat pump + diverter is an increasingly popular combination. The PV generates electricity, the diverter heats water with surplus, and the heat pump provides space heating. The PV helps offset the heat pump’s electricity consumption.
• Solar thermal + heat pump is a less common combination. Both systems involve separate roof or outdoor hardware, making installation more complex and expensive.

For most NI homeowners, the recommended progression is: solar PV first, then a diverter for hot water, then consider a heat pump for space heating if and when you plan to move away from oil.

Getting Started

If you are ready to explore solar hot water for your NI home, here is what to do:

1. If you already have solar PV panels: Ask your installer about adding an immersion diverter. This is a simple, low-cost addition that can usually be done in a single visit. If your original installer is not available, any qualified electrician can fit a diverter.

2. If you are planning a new solar PV installation: Include an immersion diverter in your specification when comparing quotes from NI installers. Most installers will include it as an optional add-on. Having it fitted during the initial installation saves on the installation cost compared to retrofitting later.

3. If you are specifically interested in solar thermal: Ask installers for a solar thermal quote alongside a PV + diverter quote so you can compare the options directly. Make sure the thermal quote includes the cost of any cylinder replacement and ongoing maintenance.

Whichever route you choose, using solar energy to heat your water is one of the smartest ways to reduce your energy costs in Northern Ireland. The technology is proven, the savings are real, and the combination of solar PV with an immersion diverter offers remarkable value for money.