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

Can You Get Solar Panels for Hot Water Only?

Yes. Panels designed to heat water and nothing else exist, they work, and they have done for decades. They are called solar thermal collectors, and they are a genuinely different product from the solar panels most people picture.

If that is what you came here for, here is the honest answer up front: solar thermal works perfectly well in Northern Ireland, but for most NI homes the money no longer stacks up. A solar thermal system costs £4,000 to £5,500 installed and saves a typical oil-heated NI home somewhere near £190 a year. That is a payback of roughly 21 to 29 years on equipment that lasts 20 to 25 years.

The reason is not the weather. It is that solar thermal does exactly one job, and in Northern Ireland the job it does is displacing heating oil, which is comparatively cheap per unit of heat. Meanwhile the alternative, solar PV with an immersion diverter, does the same job for a few hundred pounds as an add-on to a system that also cuts your electricity bill.

This guide covers both properly, with NI figures rather than UK averages, so you can see exactly where the “hot water only” route still wins.

The short version

	Solar thermal (hot water only)	Solar PV + immersion diverter
What it produces	Hot water only	Electricity, plus hot water from surplus
NI installed cost	£4,000-£5,500	£6,500-£8,500 (PV) + £300-£600 (diverter)
Cost if you already have PV	Not applicable	£300-£600
Annual hot water saving (oil-heated NI home)	~£190	~£110
Total annual saving	~£190	£500-£900
Maintenance	Annual service, fluid changes	None
Grid application needed	No	Yes (NIE Networks G98)
Typical payback	21-29 years	8-13 years (PV), 3-5 years (diverter alone)

What Solar Thermal Actually Is

A solar thermal collector absorbs the sun’s heat directly and moves it into your hot water cylinder through a fluid-filled loop. No electricity is generated at any point. It is a heating appliance, not a power source.

The two collector types

Flat plate collectors are a dark absorber plate inside an insulated, glazed box. A water and glycol mix circulates through copper tubes bonded to the plate, picks up heat, and carries it to a coil in your cylinder. They are robust, cheaper, and typically 2m x 1m per panel.

Evacuated tube collectors use rows of glass tubes, each holding an absorber inside a vacuum. That vacuum is excellent insulation, so the collector loses far less heat to cold air and wind. They cost more and are more fragile, but they hold their output better in exactly the conditions Northern Ireland specialises in: cool, breezy, overcast days where the air temperature sits well below the target water temperature.

For NI, evacuated tubes are the technically better choice, and most of the small number of systems still going in here use them.

How the system runs

1. Sun heats the collector on the roof.
2. Heat transfer fluid circulates through it, absorbing that heat.
3. A pump pushes the hot fluid to a coil inside your cylinder.
4. The coil warms the water; the cooled fluid returns to the roof.
5. A controller runs the pump only when the collector is hotter than the cylinder.

The loop is sealed and the fluid never touches your domestic hot water. An expansion vessel and pressure relief valve handle the thermal expansion.

What it costs in Northern Ireland, 2026

Component	Cost
Flat plate system (2 collectors)	£3,500-£4,500
Evacuated tube system (20-30 tubes)	£4,500-£5,500
Twin-coil cylinder, if yours is single-coil	£600-£1,000
Controller, pump station, pipework, labour	Included above
Typical NI total	£4,000-£5,500

Zero-rated VAT applies until 31 March 2027, so those figures carry no VAT.

The cylinder line matters more in NI than most guides admit. Solar thermal needs a twin-coil cylinder: one coil for the solar loop, one for your boiler. The single-coil cylinder sitting in most NI airing cupboards will not do, so a replacement is a real cost for most households rather than an edge case.

The Alternative: Solar PV With an Immersion Diverter

Solar PV panels generate electricity. An immersion diverter watches your generation and your household demand, and the moment you are producing more than you are using, it sends the surplus to the immersion heater in your cylinder rather than exporting it.

It does this proportionally and continuously. If you have 700W spare, it sends 700W. Switch the kettle on and it backs off instantly, then resumes. This happens hundreds of times a day with no input from you.

Crucially, it drives the immersion element your cylinder already has. There is no plumbing, no roof work, no new cylinder. An electrician fits the unit beside your consumer unit and clips a sensor around your meter tails, in one to two hours.

The common units in NI are the myenergi eddi (£350-£500 installed, best app, integrates with Zappi EV chargers), the Solar iBoost+ (£300-£400, simpler and cheaper), and Catch Power (£300-£400). Our immersion diverter guide covers the choice in detail.

Why the NI Numbers Look Different

This is where most UK guides mislead Northern Irish readers, because they assume you heat water with mains gas or electricity. Most of you do not.

The oil-boiler catch

61% of NI households use oil central heating as their primary heat source (NISRA, 2024/25), down from 70% in 2015/16 but still by far the highest proportion in the UK. That single fact changes both sides of this decision.

Heating oil at the Consumer Council’s recent average of roughly £948 per 900 litres works out at about 10p per kWh of raw energy. Allow for the fact that a boiler firing in summer purely for hot water is inefficient, perhaps 70%, and you are displacing heat worth around 14p per useful kWh.

Now compare that with grid electricity. After the 6.2% rises applied by Power NI in July 2026 and SSE Airtricity in August 2026, NI standard tariffs sit around 27p to 31p per kWh.

So the value of solar hot water depends entirely on what it is replacing:

Your current hot water source	Value of each solar kWh
Oil boiler	~14p
Immersion heater on grid electricity	27-31p

An NI home on oil is displacing the cheaper of the two. Every “save £300 a year on hot water” headline you read is quietly assuming you are on the expensive one.

What that does to the diverter maths

A diverter has a hidden cost that solar thermal does not: the export payment you give up. NI suppliers pay roughly 5p to 8p per kWh for exported solar, and they do so voluntarily, because the GB Smart Export Guarantee legislation does not extend to Northern Ireland.

So each unit you divert nets you the displaced fuel cost minus the export you forfeited:

Scenario	Displaced	Lost export	Net per kWh	Annual saving*
Oil-heated NI home	~14p	~6p	~8p	~£110
All-electric NI home	~29p	~6p	~23p	~£299

*Assuming roughly 1,300 kWh diverted per year from a 4kW system.

Solar thermal has no export to forfeit, so in an oil-heated home it captures the full ~14p per useful kWh, around £190 a year. Per unit of energy, solar thermal genuinely does better here. It just costs around ten times as much to install.

The subsidy that vanished a decade ago

Here is the NI-specific history that explains why you rarely see solar thermal on NI roofs any more.

Northern Ireland ran its own Domestic Renewable Heat Incentive, introduced in 2014, which paid homeowners for renewable heat including solar thermal. It closed to new applicants on 29 February 2016, and nothing replaced it.

Great Britain’s domestic RHI, by contrast, kept running until March 2022. GB homeowners had six more years of solar thermal subsidy than NI homeowners did. When GB’s scheme ended it was succeeded by the Boiler Upgrade Scheme, which does not cover solar thermal and does not apply in Northern Ireland anyway.

The result is that the NI solar thermal market has had no financial support since early 2016. The installer base moved to PV, the supply chain thinned out, and prices never fell the way PV prices did. This is a Northern Ireland problem specifically, not a UK-wide one.

There is no NI grant that funds domestic solar of any kind today. The Affordable Warmth Scheme (up to £7,500, household income under £23,000, running to March 2028) funds insulation and heating measures, not solar panels or solar thermal. Our NI solar grants guide sets out what is and is not available.

One genuine advantage for hot water only

Solar thermal needs no grid connection application. It is a plumbing job.

Solar PV in Northern Ireland requires notification to NIE Networks under G98 for inverters up to 3.68kW, or a full G99 application for anything larger, which is approved subject to local network capacity. In parts of NI where the network is constrained, that adds delay and occasionally a cap on system size.

If you are in a constrained area, or on a listed building or in a conservation area where a discreet 3 square metre collector is easier to get past planning than 20 square metres of PV, hot water only becomes a legitimate answer rather than a compromise.

Climate

NI’s maritime climate is mild, cloudy and windy. That favours evacuated tubes over flat plates, because vacuum insulation limits the heat the collector loses to cold moving air.

It does not, however, rule out PV. Between April and September a 4kW NI system produces enough daytime surplus on most days to bring a 200 litre cylinder up to temperature. The diverter’s job is simply to make sure none of that surplus leaks to the grid at 6p.

Head to Head

Performance

	Solar PV + diverter	Solar thermal
Summer hot water coverage	80-100%	80-100%
Winter coverage	20-40%	30-50%
Annual coverage	50-70%	55-75%
Cloudy day performance	Weaker (needs surplus)	Better (uses diffuse heat)
Max water temperature	60-65°C	60-90°C
Roof area needed	17-20 m² (4kW)	3-5 m²

Solar thermal wins on raw hot water performance, particularly in winter and on dull days, and it wins decisively on roof space. Those are real advantages, not marketing.

Maintenance

This is where the gap opens up.

	Solar PV + diverter	Solar thermal
Annual service	Not needed	Recommended, £75-£150
Heat transfer fluid	None	Replace every 5-10 years, £150-£250
Pump	None	Can fail, £200-£350
Frost protection	Not applicable	Glycol or drain-back, needs checking
Leak risk	None	Fluid loop, seals, connections
Lifespan	25-30 years	20-25 years

Over 20 years, solar thermal servicing and consumables realistically total £1,500 to £3,000. A PV array and a solid-state diverter total approximately nothing. Factor that in and solar thermal’s payback stretches past the life of the equipment.

Real numbers for an NI household

Assumptions: 3-4 bedroom NI home, four occupants, 200 litre cylinder, oil boiler, roughly 2,200 kWh of useful hot water demand a year.

Option	Installed cost	Annual saving	Simple payback
Diverter added to existing PV	£300-£600	~£110	3-5 years
Diverter added to existing PV (all-electric home)	£300-£600	~£299	1-2 years
Full 4kW PV + diverter	£6,500-£8,500	£500-£900 (all energy)	8-13 years
Solar thermal, evacuated tube	£4,500-£5,500	~£190	24-29 years
Solar thermal, flat plate	£3,500-£4,500	~£165	21-27 years

The standout is the first row. If you already have solar PV on your roof, adding a diverter is close to free money. The standout in the other direction is solar thermal, where payback exceeds the warranty on the equipment.

Full PV costings are in our NI solar panel costs guide, and the fuel comparison in solar vs oil heating costs.

So When Is Hot Water Only Still the Right Call?

Solar thermal is not obsolete. It is just narrow. Choose it if:

• Your roof space is genuinely tight. Under about 10 square metres of usable south-facing roof, PV cannot generate enough surplus to matter, but 3 to 5 square metres of evacuated tubes still delivers useful hot water.
• Your hot water demand is unusually high. Large families, multiple bathrooms, a B&B, a farmhouse. The more water you heat, the more a system that only heats water earns its keep.
• You have a G98/G99 constraint or a planning restriction that makes a PV array difficult but a small collector acceptable.
• You have no interest in electricity generation at all and will not consider it later.
• You already have PV filling your best roof and have a second aspect you want to put to work. Rare, but it happens on farm buildings.

Choose solar PV with a diverter if none of those apply, which for most NI homes is the case. You get hot water plus electricity, no maintenance, no fluid, no pump, and a route to adding a battery or EV charger later.

Should you fit both?

Almost never. The extra hot water solar thermal adds on top of a PV system with a diverter is marginal, while you pay twice for installation and take on the thermal system’s maintenance. The exception is a property with genuinely large hot water demand and plenty of roof, where a specialist should model both together.

What About Heat Pumps?

If you are rethinking hot water, you may also be weighing heat pumps. In hot water terms specifically, an air source heat pump produces hot water at a coefficient of performance around 2.5 to 3.5, so it is efficient, and it pairs naturally with solar PV.

The sensible NI progression for most homes is: solar PV first, then a diverter for hot water, then a heat pump for space heating if and when you move off oil. Our solar panels vs heat pumps guide covers the full comparison.

What To Do Next

1. If you already have solar PV: get a diverter quoted. It is the single best-value upgrade available to you, and any qualified electrician can fit one. Start with our diverter guide.
2. If you are planning a PV installation: specify the diverter up front. Fitted alongside the panels it costs £50 to £100 in labour rather than £100 to £200 as a retrofit. Compare quotes from NI installers and ask for it as a line item.
3. If you specifically want hot water only: ask for a solar thermal quote and a PV plus diverter quote, and make sure the thermal quote includes the twin-coil cylinder and the annual service cost. Compare them over 20 years, not on sticker price. Be aware that MCS-certified solar thermal installers are now thin on the ground in Northern Ireland, so allow time to find one.

Heating water with the sun is still one of the most sensible things you can do to an NI home. For a small number of houses that means solar thermal. For most, it means a £400 box beside the fuse board doing quietly what a £5,000 roof array used to.

Figures reflect NI market conditions as of August 2026. Oil prices are volatile: check the Consumer Council price checker for current rates.