This is what I thought originally, too, because it's just common sense, and it was true 30 years ago. But that common sense turns out to be wrong now, as you'll see if you do the math.

The manufacturer-suggested retail price of the Stiebel Eltron SOLkit 2 you link is US$7870, according to https://www.stiebel-eltron-usa.com/sites/default/files/pdf/s.... This includes two SOL 27 Premium flat-plate solar thermal collector panels, about which that page says, "The net absorber surface of over 25 square feet results in a maximum output of 31,300 btu/day per panel (SRCC clear day rating)."

In modern units, that's 2.3m² (per panel) and 382 watts (per panel), so you're paying US$7870 for 764 watts (on a clear day). That's US$10.30 per average watt. We don't really care about peak power, since the system comes with thermal energy storage built in, but if we assume a capacity factor of 20% (which would be about right if it were fixed photovoltaic) then it's about US$2 per peak watt. (Incidentally, that's a very-well-designed panel, because, assuming the same 20% capacity factor, it's about 80% efficient!)

That's a very high price. The SEIA calculated a cost breakdown for US residential solar PV installations in early 02024 (https://www.seia.org/research-resources/solar-market-insight...), of which 20¢ per peak watt (Wp) was the PV module. The rest of the US$3.25/Wp price (even higher than Stiebel Eltron's!) was things like batteries, inverters, installation labor, etc. You don't need those for water heating; you only need low-voltage wiring, an electric heating element (a resistor) in the water tank, and some kind of safety thermal cutoff. So if all you're interested in is getting a solar water heater, you can almost certainly get it more cheaply by running wires to your roof, or to panels in your yard, instead of pipes.

20¢/Wp at a 20% capacity factor would give you a nice round US$1 per round-the-clock watt, so 764 round-the-clock watts would cost you US$764 of panels, as compared to Stiebel Eltron's US$7870 MSRP. (Which one were you saying was "the smart way to make hot water from the sun" again?)

But the situation actually favors solar PV much more strongly than that, for two reasons. First, PV modules are cheaper than that; the "mainstream" price on https://www.solarserver.de/photovoltaik-preis-pv-modul-preis... is now 0.115€/Wp (US$0.125/Wp) wholesale, though the US's anti-renewable-energy policies presumably make them a little less cheap than that where you live. Second, in most cases, even if a home PV system provides less power than you need on average, it still provides more power than you need at times. If you're choosing between turning some of the panels off in the daytime and heating up the hot-water heater, the latter sounds like a better choice. If you're just burning up surplus energy, instead of buying extra panels just for your hot water, it won't even cost you US$764.

But wait, you might ask, why not batteries? And batteries are certainly more flexible than a hot-water heater. If you use your excess solar power production to charge batteries, you can use the energy later to run your computer, cool your house, run a circular saw, heat your house, or take a hot shower. If you use it to heat up a hot-water heater, you can only use it to heat your house or take a hot shower.

But batteries have a countervailing disadvantage: they're expensive. If your hot-water tank is at 65° when incoming water is at 20°, and the tank is 300 liters like the one in the Stiebel Eltron system you linked, it's storing 56MJ of energy, or, in cursed folk units, 16kWh. Lead–acid or lithium-ion batteries generally cost in the range of US$50–150/kWh (US$15–40/MJ) so the same amount of energy storage in batteries would cost US$700–2400. The hot-water tank only costs about US$500, if you don't have it already.

There are cheaper solar hot-water systems than Stiebel Eltron's. Looking locally, this no-brand locally-made 300-liter one sold by "Energía al Sol" only costs about $2.1 million: https://articulo.mercadolibre.com.ar/MLA-1683162470-termotan...

That's about US$1600 at today's exchange rate, one fifth of the much more complex system you're talking about. Still more expensive than the PV option.

The solar-thermal option might still beat PV if you're limited on space, because, at around 80% efficiency, it requires about a fourth of the area as the equivalent mainstream 23%-efficient photovoltaic panels. Even garden-variety solar thermal collectors can often exceed 50%. They're just much more expensive than photovoltaic. I know that's crazy, but what can I say? We live in a crazy world.

As for your unfortunate neighbors, I suspect that the reason they are paying hundreds of dollars a month for under 6kW is that they installed their solar systems when prices were much higher than they are today, and that all their energy storage is in electrical batteries. Some thermal energy storage—whether in primitive "sensible heat storage" systems like an insulated tank full of hot water, or more advanced phase-change and TCES systems—would probably have gone a long way toward bringing those costs down. See https://news.ycombinator.com/item?id=43468177 for a recent comment where I did a brief sketch of an LCoE estimate.