Solar Water
Using the sun to heat water is the simplest, oldest, most obviously sensible solar technology there is — and it is the one solution in this whole framework that is currently shrinking. Not because it failed, but because two better things quietly overtook it.
The effect arrives across lifetimes. This is a gift to people you will not meet.
Project Drawdown classifies this as Keep Watching.
Origins
This is very nearly the oldest solar technology of all. A black barrel of water left in the sun is a solar water heater, and humanity has always known it.
The first commercial version was patented by Clarence Kemp in Baltimore in 1891 — the “Climax,” essentially black-painted tanks in a glazed box. Then in 1909 William Bailey’s “Day and Night” heater made the crucial improvement: it separated the collector from an insulated storage tank, so a household had hot water after sundown. It sold widely across sunny, gas-poor Southern California — until cheap natural gas arrived and killed the market. Solar water heating’s first death, like small wind’s, came from a fossil fuel undercutting it, a full century ago.
So it migrated to where fuel stayed dear and sun stayed plentiful. Israel began mandating solar water heaters on new buildings, and the rooftop tank became near-universal there. Cyprus reached the highest per-capita use in the world. Greece, Turkey, and Australia followed. And above all China, which became the overwhelming global manufacturer and installer of cheap evacuated-tube systems, putting hot water within reach of hundreds of millions.
For two decades it was one of the great quiet renewable success stories. Then, through the 2010s, photovoltaics and heat pumps began, gently and decisively, to eat its lunch.
What it actually is
Solar water heating does one humble thing supremely well. A dark collector on a roof warms water; the warm water rises into an insulated tank; you draw hot water from the tap. In its simplest form there are no electronics and no pump at all — a black tank and the fact that hot water rises will do it. It typically cuts the energy needed to heat water by seventy to eighty percent.
And heating water is not a small matter. In many countries it is a quarter or more of a household’s total energy use. Do it with sunlight and you displace an enormous amount of gas and electricity.
So why is this sensible, proven technology in decline? For two reasons — and both are, in the larger view, good news for the climate even as they are bad news for this particular machine.
First, rooftop solar electricity collapsed in price. A roof covered in cheap photovoltaic panels is more useful than the same roof covered in thermal collectors, because electrons are flexible: they can heat water, or run a heat pump, or do anything else the house needs.
Second, the heat pump arrived. An air-source heat-pump water heater, running on that increasingly clean electricity, delivers hot water at two to four times the efficiency of ordinary electric heating, works at night and in winter, needs no roof-facing collector, and cannot freeze.
Between them, solar panels plus a heat pump now do solar water heating’s job through the electric system, and do it more flexibly. The dedicated thermal collector is being leapfrogged — which is not a failure. It is a technology being retired by better ones, which is exactly what is supposed to happen.
The numbers
A solution in absolute decline. Global solar-thermal capacity in operation stood at roughly 544 gigawatts-thermal at the end of 2024 — and fell about two-and-a-half percent that year, as retirements outpaced new installations. The market for new collectors contracted by around fourteen percent. This is the rare climate solution whose graph now points down.
China leads the retreat. The largest market by far, China has seen sales fall in almost every year since 2014, as domestic buyers switched to heat pumps and rooftop photovoltaics.
Still a vast installed base. Systems operate in at least 134 countries and deliver on the order of 443 terawatt-hours of heat a year — the energy in roughly 260 million barrels of oil. The contribution remains large; it is simply no longer growing.
The savings are real. Against an electric-resistance heater, a solar system still cuts water-heating energy by seventy to eighty percent.
The competition, in numbers. Heat-pump water heaters run two to four times more efficiently than resistance heating, and rooftop photovoltaics have fallen to a few dollars a watt or less — together undercutting the case for a single-purpose thermal collector in any home with a decent grid.
The telling split. Decline is sharpest in complex pumped systems (China, much of Europe). Simple thermosyphon systems — no pump, no electronics — remain far more stable across Latin America, the Mediterranean, sub-Saharan Africa, and South Asia, because they are cheap and they compete against fuel that is expensive or absent.
Why it matters
We want to hold two things at once here, because the honest story is a little sad and entirely fine.
Solar water heating is beautiful in the way that genuinely simple technologies are beautiful. A thermosyphon system on a warm-country roof has no pump, no electronics, no grid connection, and almost no failure modes; it will make hot water for decades and ask for little more than a ladder and a rag. There is a purity to it that the movement should be able to admire out loud.
And it is being retired, and that is all right. A movement that only ever adds and never lets anything go is not being honest about progress. The point was never the thermal collector; the point was hot water without carbon. If panels and heat pumps do that job better and more flexibly, the collector has earned its rest.
But here is where the tidy “it’s obsolete” story goes wrong: it is only being leapfrogged where the electric system is cheap, clean, and reliable. In much of the world it is none of those things. A family in a sunbelt town with a flickering grid and no gas line does not have a heat pump humming on clean electrons. They have a roof and the sun. For them a two-hundred-dollar thermosyphon tank is not yesterday’s technology — it is the best answer that exists, today.
So the truthful verdict is neither “dead” nor “the future.” It is this: a mature technology gracefully ceding the rich, electrified world to better options, while remaining one of the highest-return, lowest-tech climate tools in the sunbelt Global South. Both are true at once. We should stop selling it as a growth story and start valuing it as an appropriate one — which, in a culture that over-rewards the new, is its own small act of honesty.
What it actually takes
Match the tool to the place. In a wealthy, grid-connected home going all-electric, the honest recommendation is usually photovoltaics plus a heat-pump water heater, not a thermal collector. Say so, even when it means selling less of the thing with your name on it.
Keep it simple where simplicity wins. In sunbelt regions with weak grids, dear fuel, and low incomes, cheap thermosyphon systems are the right answer — and the barriers are financing and trained installers, not the technology itself.
Fix the installer gap. One persistent, unglamorous obstacle everywhere is the shortage of competent installers and the absence of universal certification. That is a training and standards problem, entirely solvable, and it causes more disappointment than any flaw in the hardware.
Design for the failure modes that remain. Freezing in cold climates, heat loss on cold nights, scaling in hard water — all solved decades ago, all still producing bad installations where the lessons were skipped.
Value the appropriate, not only the novel. The climate conversation lavishes attention on the new. A boring tank that has worked since 1909 deserves support in the many places where it still quietly wins.
Where it matters most
The Sonoran, Mojave, and Chihuahuan deserts of the American sunbelt are where solar water heating first succeeded — Bailey’s “Day and Night” heaters sold across Southern California until cheap gas undercut them — and where, on off-grid and high-cost sites, it still makes quiet sense today.
The Western Ghats and the sun-rich plains of South Asia are classic thermosyphon country: fierce sun, an expensive or unreliable grid, and a vast population heating water — the exact conditions in which a cheap tank wins outright.
The North China Plain holds the technology’s rise and its retreat in a single frame: the world’s factory for evacuated-tube collectors, and now the market shrinking fastest as heat pumps and photovoltaics take over.
The East African Rift and the high, sunlit valleys of the Andes are where simple solar water heating is not a legacy at all but a live, underused opportunity — sun in abundance, grids that are thin, fuel that is dear.
Even across the Colorado Plateau and the interior West, off-grid homesteads and cabins remain a natural fit for a technology that asks for nothing but sunlight and a south-facing roof.
How to tell it’s being done well
Is the grid here cheap, clean, and reliable? If yes, panels plus a heat pump probably beat a dedicated collector. If no, solar-thermal likely wins. This single question decides most cases.
Is it a simple thermosyphon or a complex pumped system? Simple systems in warm climates are close to bulletproof; every pump and controller you add is another thing to fail.
What is it displacing? Replacing electric-resistance heating or diesel is a large win; replacing gas, a smaller but real one; replacing an efficient heat pump, probably not worth it.
Is there a competent installer and proper freeze protection? Most disappointment traces to bad installation, not bad technology.
Is it being sold as growth, or as fit? Be wary of a pitch that never mentions heat pumps. Trust the one that can tell you exactly why this roof, this town, this grid.
Where this matters most
Who is working on this
We are researching which organizations in our directory of 8,493 actively work on this solution, and we only list an organization once we have verified it. That research is ongoing. In the meantime, search the directory yourself:
Questions
Is solar water heating obsolete?
No, but it is being overtaken in the wealthy, electrified world by rooftop solar panels plus heat pumps, which do the same job more flexibly. In the sunbelt Global South, with weak grids and expensive fuel, it is still often the best option there is.
Why is a proven, sensible technology actually shrinking?
Global capacity fell in 2024 for the first time in a generation, mostly because China's market has declined since 2014. As cheap photovoltaics and efficient heat pumps arrived, they became the better path to low-carbon hot water in grid-connected homes.
Should I get solar panels or a solar water heater?
For a grid-connected home, photovoltaic panels paired with a heat-pump water heater are usually the more flexible choice. A dedicated solar thermal collector makes its strongest case off-grid, in warm climates, or where the electricity is dirty or unreliable.
How much energy does it actually save?
Typically seventy to eighty percent of water-heating energy compared with an electric-resistance heater. Because water heating is often a quarter or more of a home's total energy, the absolute saving is large.
What is the simplest kind?
A thermosyphon system: a collector mounted below an insulated tank, with hot water rising by natural convection, so there is no pump and no electricity. Cheap and nearly maintenance-free, it dominates across the Mediterranean, Latin America, and sub-Saharan Africa.
Sources
- Project Drawdown - Solar Hot Water Framework and classification. Cited, not reproduced.
- REN21 - Renewables 2025 Global Status Report, Solar Thermal Global capacity and 2024 decline figures.
- IEA Solar Heating and Cooling Programme - Solar Heat Worldwide
- Solar water heating - history and technology overview
The solution taxonomy follows the framework popularised by Project Drawdown. The analysis above is our own; for their carbon modeling and rankings, visit them directly.