A solar water heater turns free sunlight into the hot water your home uses every day — showers, washing up, cleaning — usually covering most of that demand without burning gas or running an electric immersion heater. In a sunny country like Spain it is one of the fastest-paying pieces of solar equipment you can buy, often cheaper to install than a rooftop PV array and simpler to maintain. This guide explains how solar water heaters work, the main types (including thermosiphon and heat-pipe vacuum tubes), how they compare with gas and electricity, and what payback looks like for a Spanish home.
How a solar water heater works
Every solar water heater does the same basic job: a collector on the roof captures sunlight and turns it into heat, and that heat is stored in an insulated tank of water until you open a tap. The clever part is how the heat moves from the collector into the tank — and the most elegant answer needs no pump and no electricity at all.
The thermosiphon principle (no pump, no power)
A thermosiphon system relies on a simple law of physics: warm water is less dense than cold water, so it rises. The storage tank sits above the collector. As the sun heats the water in the collector, it becomes lighter and floats up into the top of the tank, while cooler, denser water from the bottom of the tank sinks down to take its place. This sets up a slow, continuous loop that runs entirely on natural convection — no circulation pump, no controller, no grid connection. It is the reason a thermosiphon unit keeps making hot water during a power cut, and why it has so little to go wrong.
The trade-off is that the tank has to sit above the collector, which is why thermosiphon systems are the familiar "panel with a horizontal cylinder mounted just above it" you see on roofs all over the Mediterranean. Pumped (forced-circulation) systems put the tank indoors and use a small pump instead, but they add electrical parts and complexity. For most homes in Spain, the passive thermosiphon is the sweet spot of cost, simplicity, and reliability.
Evacuated (vacuum) tubes vs flat-plate collectors
There are two mainstream collector designs:
- Evacuated tube collectors use rows of double-walled borosilicate glass tubes with a vacuum sealed between the two walls. That vacuum is a near-perfect insulator — like a Thermos flask — so very little of the captured heat leaks back out, even on a cold, breezy, or partly cloudy day. They keep performing well in winter and at low sun angles, which matters in northern and inland Spain.
- Flat-plate collectors are an insulated glazed box with a dark absorber plate. They are robust, lower-cost, and excellent in warm, sunny conditions, but they lose more heat to the surrounding air when it is cold, so their winter output is lower than vacuum tubes for the same area.
For year-round domestic hot water in Spain, evacuated tubes are the most common choice because the better cold-weather performance keeps the system useful in the months when you most want a hot shower.
Heat-pipe vacuum tubes — how the heat actually gets out
The best evacuated-tube systems use a heat pipe inside each tube. A sealed copper pipe holds a tiny amount of fluid; when the sun warms the tube, that fluid vaporises and the vapour rushes up to a small condenser bulb at the top of the tube. The bulb sits in a manifold where the heat is transferred to the water (or to an antifreeze loop), the vapour condenses back to liquid, runs down, and the cycle repeats — hundreds of times an hour. This "dry" connection has two big advantages: each tube is independent, so if one ever cracks the rest of the system keeps working, and the heat pipe starts moving heat at very low light levels, squeezing useful energy out of dull mornings.
Types of solar water heating systems
Beyond the collector, systems differ in how they handle water pressure and how they protect against freezing and scale. These choices decide which model fits your home.
Pressurised vs non-pressurised
- Non-pressurised (open-vented) systems hold water in the tank at atmospheric pressure, fed by a float valve. They are the simplest and cheapest, but the hot water reaches your taps by gravity, so the pressure is gentle — fine for many homes, less ideal if you want a powerful shower.
- Pressurised systems run at mains pressure, so the hot water comes out with the same force as your cold tap. They pair naturally with heat-pipe vacuum tubes (the dry connection keeps the high-pressure water out of the glass tubes) and are the better choice where strong, even pressure matters across multiple bathrooms.
Direct vs indirect (and freeze protection)
- Direct (open-loop) systems circulate the actual drinking water through the collector. They are simple and efficient and work well along the frost-free Spanish coast.
- Indirect (closed-loop) systems circulate a non-toxic antifreeze fluid through the collector and pass its heat to the water through a coil inside the tank. The water you use never enters the collector, which makes these the right pick for inland areas with winter frost or for hard-water regions where scale would otherwise build up in the tubes.
With or without electric backup
No solar system makes 100% of your hot water on the shortest, greyest days of the year. That is what a backup heat source is for. A model with electric backup has a resistive heating element built into the storage tank that tops the water up to your set temperature when the sun has not done enough — automatically, only when needed. It guarantees hot water 365 days a year while the sun still does the heavy lifting most of the time. A model without backup is cheaper and ideal if you already have a gas boiler or another heat source to cover the gaps. The Enera PRO solar water heater comes in both a standard thermosiphon version and a version with electric backup so you can match it to your setup.
Solar vs gas vs electric water heating
Heating water is one of the largest energy uses in a typical home — often a quarter of the household energy bill. Here is how the three common options compare:
- Electric (immersion / heat-pump tank): simple to install but you pay full grid price for every kWh of heat. An electric immersion is the most expensive way to make hot water in most Spanish tariffs.
- Gas (mains or bottled): cheaper per kWh than electricity, but you are exposed to a volatile fuel price, the system burns fossil fuel, and a combustion appliance brings flue, servicing, and carbon-monoxide considerations.
- Solar thermosiphon: the fuel — sunlight — is free and price-stable forever. Once installed, a well-sized system supplies roughly 60–80% of a Spanish household’s annual hot water with essentially no running cost, no combustion, and very little maintenance. The remaining 20–40% on cloudy winter days is covered by your backup (electric element or existing boiler).
The headline figure to remember: solar does not usually replace your backup entirely, it slashes how often the backup has to run. That is where the savings come from.
Is a solar water heater worth it in Spain?
Spain is close to the ideal climate for solar water heating: much of the country sees 2,500–3,000+ hours of sun a year, and demand for hot water is steady all year round. A 150-litre thermosiphon system is sized for a household of roughly two to four people.
The economics are straightforward. A family heating water with electricity can spend several hundred euros a year on that alone. A solar thermosiphon covering ~70% of that demand removes most of the cost, typically saving on the order of €200–€350 a year depending on your tariff, family size, and how much hot water you use. Against the installed cost of a domestic thermosiphon unit, that usually means a payback of around three to six years — after which the hot water is close to free for the rest of the system’s 15–20+ year life. Many Spanish regions and municipalities also offer subsidies or IBI (property-tax) reductions for solar thermal installations, which shorten the payback further; it is worth checking what is available in your comunidad autónoma before you buy.
Ready to see the models? Browse the Enera PRO 150 L thermosiphon range — including the standard version and the version with electric backup for guaranteed year-round hot water.
Maintenance and lifespan
A thermosiphon system has almost no moving parts, so maintenance is light. The main item is the magnesium anode — a sacrificial rod inside the tank that corrodes in place of the steel, protecting the tank from rust. Checking and occasionally replacing it (every few years) is the single most important thing you can do to reach the full lifespan. Beyond that: keep the tubes free of heavy dust or leaf litter, and on closed-loop systems have the antifreeze fluid checked periodically. A quality system lasts 15–20 years or more, and on evacuated-tube models individual tubes are inexpensive and replaceable if one is ever damaged.
The Enera PRO solar water heater
The Enera PRO 150 L is a pressurised thermosiphon system built around heat-pipe evacuated tubes — the configuration that gives strong tap pressure, reliable cold-morning performance, and the resilience of independent tubes. It is sized for a typical Spanish household of two to four people and ships in two versions: the standard thermosiphon, and a version with electric backup for households that want guaranteed hot water on every grey winter day without keeping a separate boiler. Both run passively on sunlight for most of the year, so most days your hot water costs nothing at all.
See full specifications and pricing on the water heaters page.
Frequently asked questions
Does a solar water heater work in winter and on cloudy days?
Yes. Evacuated (vacuum) tubes capture diffuse light and insulate the captured heat extremely well, so they still produce hot water on cold and overcast days, just less than in full sun. For the shortest, greyest days a model with electric backup tops the tank up automatically so you never run out.
Do thermosiphon systems need electricity or a pump?
No. A thermosiphon works purely on natural convection — warm water rising into the tank above the collector — so it needs no pump, no controller, and no grid connection. It even keeps making hot water during a power cut. Only the optional electric backup element uses any power, and only when the sun has not done enough.
How much hot water does a 150-litre system provide?
A 150 L tank is sized for a household of roughly two to four people for everyday use — showers, kitchen, and washing. Larger households or homes with very high hot-water use would step up to a bigger tank.
What is the difference between pressurised and non-pressurised systems?
A non-pressurised system delivers hot water by gravity, so the pressure at the tap is gentle; it is the simplest and cheapest option. A pressurised system runs at mains pressure, giving the same strong flow as your cold tap — better for powerful showers and homes with several bathrooms. The Enera PRO is a pressurised heat-pipe system.
How long does a solar water heater last, and what maintenance does it need?
A quality system lasts 15–20 years or more. Maintenance is light: check the sacrificial magnesium anode every few years and replace it when worn, keep the tubes clean, and on closed-loop systems have the antifreeze checked periodically. Individual evacuated tubes are inexpensive to replace if one is ever damaged.
How long until a solar water heater pays for itself in Spain?
For a typical household replacing electric water heating, payback is usually around three to six years, after which the hot water is close to free for the rest of the system’s life. Regional subsidies or IBI reductions can shorten that further.
Will solar cover all my hot water, or do I still need a backup?
A well-sized system supplies roughly 60–80% of a Spanish home’s annual hot water. It is not designed to replace your backup entirely — it dramatically reduces how often the backup (electric element or existing boiler) has to run, which is where the savings come from.