If you own a house on the Spanish coast, the case for solar hot water is not really a physics question. The physics is settled and we explain it on the water heaters page and, step by step, in the explainer on how solar water heaters work. What decides whether it happens is a shorter, more awkward list: where the tank can legally and physically go, whether your comunidad has a say, whether your town hall gives anything back, and what the thing needs from you over fifteen years. This guide is that list, written for a home in Spain rather than for a search engine.
The one constraint that decides most of it: the tank has to sit above the collector
A thermosiphon has no pump. That is its great virtue — nothing to fail, no electricity in the basic version — and it is also its one hard constraint. Gravity does the circulating, which means the tank must sit physically above the collector, in the same assembly, out in the open. It cannot be tucked into a plant room or dropped into a cupboard the way a pumped system’s cylinder can.
In Spanish housing that produces three very different situations:
- Villa, townhouse or anything with its own azotea. The straightforward case. There is room, the assembly is anchored for the roof’s wind loading — the unit is rated to 30 m/s, force 11, and 35 mm hail — and it ties into the hot-water circuit you already have.
- An apartment in a comunidad de propietarios. This is a governance problem, not an engineering one. The roof is normally a common element, so putting a visible 150-litre tank on it needs the community’s agreement. Owners are often surprised by this; we check it before quoting, because quoting for something the junta will refuse helps nobody.
- An old town centre or a protected setting. Some municipalities impose conditions on what may be visible on a roof. That is a planning question for the ayuntamiento, and it belongs at the start of the process rather than at the end.
What it displaces: mains gas, a bottle, or an immersion heater
Hot water is one of the larger energy loads in a Spanish home, and the honest observation is that what a solar thermosiphon saves you depends far more on what you are replacing than on how sunny your town is.
A coastal villa running an electric immersion heater on a standard tariff is the strongest case. A house on bottled butane — still common in rural and older coastal properties — is the next: the saving there is not only money but the errand. Mains gas is the weakest case of the three, and we will say so rather than build a spreadsheet around it. Nobody can tell you your number from an article; it is on your own bill, and the calculator reads it straight off your invoice.
The house that is empty for half the year
A large share of the properties we work on are second homes occupied in bursts. A thermosiphon does not mind sitting idle, but the manufacturer gives one instruction that almost nobody follows: if the heater will be unused for more than 72 hours, close the cold-water supply to the system. It is in the Enera PRO installation manual, and it is the difference between returning in September to an untouched unit and returning to a tank that has cycled at high temperature all August with nobody drawing a drop.
The other figure worth knowing for a second home is thermal inertia: the tank holds its temperature for up to 72 hours without sun. Two overcast days do not mean a cold shower. It is not, however, a system that comes up to temperature the same afternoon you open a house that has been shut for three months.
What actually goes wrong — and what voids the warranty
Upkeep on an evacuated-tube thermosiphon is minimal but not zero, and the two things that matter are both counter-intuitive.
The magnesium anode. It is a sacrificial part: it corrodes so the tank does not. It is replaced roughly every 6 years on the solar-only model and every 3 years on the version with the electric element, because that one works the tank harder. Doing it on schedule is a condition of the 2-year warranty. It is worth weighing when you choose between the two versions — the one that gives you certainty in January is also the one that asks for attention twice as often.
Filling the tank in full sun with the tubes empty. The manual excludes this from the warranty explicitly. A dry evacuated tube in July sun reaches very high temperatures, and introducing cold water into it is the classic consequence of a commissioning done at three in the afternoon instead of at dawn or dusk. It is precisely why this is not sold as a self-assembly kit: ENERA installs it turnkey.
What you will never have to do is descale the inside of the tubes. They are sealed and dry — water never enters them — which in a region with heavily mineralised water is exactly the maintenance that gave older solar thermal its bad name. In dusty areas an occasional rinse of the outside is the whole job.
"I already have panels — doesn’t that heat my water for free?"
This is the most common objection from owners who have already gone solar, and it deserves a real answer. Spain runs a net-billing scheme, compensación de excedentes: the electricity your system sends back to the grid is credited against what you draw from it on the same monthly bill. Your surplus is not thrown away — it becomes a discount. But it is a discount on the energy portion of the bill, valued below what that same kilowatt-hour costs you to buy, and the fixed network charges stay where they are.
Applied to hot water, that means heating a tank with electricity you exported and then re-imported is the expensive route. Per square metre of roof, a thermal collector covers the hot-water load considerably better than panels feeding an immersion heater. The two are complements, not alternatives: let the thermosiphon take hot water off the bill and your PV output is freed for cooling, appliances and the car — the loads photovoltaics are genuinely good at.
Municipal tax relief: which towns actually name solar thermal
Foreign owners are usually the ones paying the IBI on the properties we install on, and they are the least likely to know that a bonification may exist at all. The facts below were read on the ordenanzas fiscales and on the Agencia Tributaria manual themselves, not on comparison sites, and they were checked on 5 August 2026. Ordenanzas are re-approved every autumn and take effect on 1 January, so confirm before you commit.
The Valencian regional income-tax deduction. The most relevant one for solar thermal, because the tax office’s own manual heading covers self-consumption and thermal generation alike: 40 % of the amount invested at a main residence, or 20 % at a second home, on a deduction base of up to €8,800 per dwelling per year. Payment has to be traceable — card, transfer, cheque or bank deposit — and anything covered by a public grant does not count.
Municipal relief is far less even, and not every ordenanza that bonifies "solar panels" includes thermal. Among the towns where we have solar hot-water installations and have read the official text:
- Altea: 25 % off the IBI for three tax periods on predominantly residential property, with the total bonified capped at 50 % of the installation cost, plus a 95 % ICIO bonification. Solar thermal is named expressly.
- Torrevieja: 50 % off the IBI for three years, for solar thermal and/or electric systems, and not available where the installation is compulsory. Thermal has to show at least 4 m² of collector area, alongside the technical report, assembly certificate and municipal authorisation.
- El Campello: 50 % off the IBI for five years, effective the year after the application and evidenced with invoices from an approved installer. It also asks for 4 m² of useful collector area and for the property to have been the applicant’s habitual residence for at least a year. A 50 % ICIO bonification comes with it.
- Dénia: up to 50 % off the IBI for three periods and up to 25 % for two more — but on a sliding scale by cadastral value, so the headline rate applies only to the lowest band and falls to 20 % and 10 % above it. Minimum 4 m² of collector area. ICIO is bonified at 95 %, on the solar share of the works budget only.
- Elche: 50 % off the IBI once only and for at most three tax periods. It is the only one on this list that sets a performance condition for thermal: the system must cover at least 70 % of annual hot-water demand under CTE DB HE-4.
And the part nobody publishes: some towns give nothing. We read the ordenanzas fiscales of Finestrat, La Nucía and Benidorm — three towns where we have installations — and none of the three has any solar bonification on either the IBI or the ICIO. That is a finding, not a gap in our data. These bonifications are discretionary, so never infer your town’s from the one next door.
Where we have installed them
In the anonymised installation set we publish — 583 installations as of 20 August 2026 — 231 are solar hot water rather than photovoltaic. That is close to 40 %, and it is why this article exists: thermal is not a sideline for us.
The towns with the most thermal installations, in order:
- Torrevieja — 16
- Finestrat — 11
- Altea — 10
- Orihuela — 9
- El Campello — 7
- La Nucía — 6
- Calpe — 5
- Dénia — 4
- Elche — 4
- Santa Pola — 4
- Benidorm — 2
- Lloret de Mar — 2
That is the set we can publish without identifying anyone, not the company’s full history. Each town page linked above carries its PVGIS irradiation figures and the verified incentives for that municipality.
What we install, and what we need from you
The unit is the Enera PRO 150 L, either solar-only or with a 1500 W electric backup element and a 619-F controller. Stainless-steel 316 inner tank with a copper heat-exchange head, Φ58 × 1800 mm borosilicate 3.3 evacuated tubes with CPC parabolic reflectors, 6 bar working pressure, 45–90 °C service water, design life of fifteen years or more. Prices — VAT included, supplied and installed — live on the water heaters page, where they update themselves. That is deliberately why this guide contains no euro figure typed by hand.
To size it we need three things: how many people live in the house permanently, what your roof looks like, and what hot-water system you have now. Start with the calculator or go straight to contact. If 150 litres is not enough, the manufacturer allows several units to be linked in series or in parallel on one shared circuit, so a large household is not excluded.
Frequently asked questions
Can I fit a solar water heater to an apartment in Spain?
Technically yes, but the tank has to sit above the collector and in the open, normally on the roof. In a building under horizontal-property rules the roof is usually a common element, so the comunidad de propietarios has to agree. We check that before quoting.
Do Spanish town halls give a discount on the IBI for solar hot water?
Some do and some do not, and it is never automatic. Altea, Torrevieja, El Campello, Dénia and Elche all bonify the IBI for solar thermal systems, each with its own conditions, duration and ceiling. In Finestrat, La Nucía and Benidorm we read the ordenanzas fiscales and found no solar bonification at all. Never assume your town matches its neighbour — these are discretionary.
What should I do with the system if the house is empty for months?
The manufacturer says to close the cold-water supply to the system if it will be unused for more than 72 hours. It is a simple precaution and particularly relevant for a second home. The tank itself holds temperature for up to 72 hours without sun, so a couple of overcast days will not leave the house without hot water.
What voids the warranty on a thermosiphon?
Two things above all. Filling the tank in full sun with the tubes empty — the classic result of commissioning in the middle of a summer afternoon — and failing to replace the magnesium anode on schedule: roughly every 6 years on the solar-only model and every 3 years on the electric-backup version. The equipment warranty is 2 years.
Is solar hot water still worth it if I already have PV panels?
Usually yes, because they do not compete. Under compensación de excedentes your PV surplus is credited against the energy portion of the same monthly bill, at less than it costs you to buy that electricity back, so heating water with exported-then-reimported power is the expensive route. With the thermosiphon carrying hot water, your PV output is free for cooling, appliances and an electric car.
Does a solar thermosiphon replace my gas boiler?
It can, and it does not have to. Plumbed as a pre-heater in front of an existing gas or electric water heater, the old appliance only has to make up the difference on the days there is one. The solar-only model needs no electricity at all; the version with the 1500 W element and the 619-F controller brings the tank to a set temperature on a schedule, which is what most people choose when it is to be the only hot-water source.
How much does a solar water heater cost in Spain, installed?
The current price, VAT included and supplied and installed, is on the water heaters page — we publish it there rather than in this text precisely so it can never go stale. What is fixed is what it covers: the unit, roof mounting with anchors calculated for the wind loading, and connection to your existing hot-water circuit.
