Understanding Solar Energy

If you want to DIY a solar PV water heater I made a whole website about it with instructions and a simulator to estimate what your payback period could be.

https://www.pvh2o.com/

Problem being that electric water heating is a lot more expensive in e.g. Germany where gas prices are lower than electricity prices per kWh delivered. (~12 vs. 39 eurocent per kWh)

So blindly converting a gas water heater to electric will roughly quadruple your water heating cost.

Is it a fallacy though? It doesn't make sense to buy a new EV if you still have a gas car that's working fine. In the same vein, I wouldn't want to throw out my gas furnace or water heater to replace with electric, creating waste and requiring the manufacturing of a new unit

Yeah that too, but that has limits, for example european union regulates building industry in such way that every new build, rebuild has to be done in a way that your heating energy requirement is already lower than your hot water energy requirement. Because hot water energy usage can not go lower in current society, but buildings can be improved a lot. So yes as you said if building is modeled in software tools like OpenStudio ( Revit, archicad uses this sw developed in collaboration by NREL, ANL, LBNL, ORNL, and PNNL ) before build, to make building not waste energy and capture as much sun in cold period as possible then even such strategies can be used. You can not preheat/ precool 1870s handhewn cabin, all energy will be lost very fast. It sounds obvious to you and me but most people do not really understand this deeply enough to "click" in their heads.

time of use billing - tool to incentivie you to use "off-peak" power, but i guess it will be deprecated in favor of "realtime" billing in future, because there will be so much solar (almost zero $ per kWh on market) that your energy provider will incentivize you to draw energy during peak solar "activity" AND off-peak hours. it will be simpler for them to give you market price every 15 minutes window than 4hour window at same time every day.

Congrats, using as much energy directly on site is crucial for fast and cheap energy transition of economy.

What size of DHW heat pump costs you ≈$4000? Which currency are we talking about here?

If you aren't limited by roof and other outdoor area for PV panels, US$4000 buys you about 50000 watts of "low cost" solar panels at current wholesale prices: https://www.solarserver.de/photovoltaik-preis-pv-modul-preis...

At a nominal capacity factor of 15%, that works out to about 5000 liters per day of domestic hot water:

    ~ $ units -t '50000W 15%/(30K 1kcal/kg/K)' kg/day
    5162.5239

How about lifetime costs? A resistive water heater is going to last longer and you can get non-metallic water heaters for extremely long life.

If you don't have net metering (or just a terrible power purchase rate), why not just sink that extra solar energy into a water heater?

Heat pumps come with a lot of restrictions. What about constant noise? Plus stating that they are cheap ain't correct. Our housing unit in Switzerland recently needed to replace older oil heater and one of the options was heat pump... which was by far the worst choice based on various criteria and we at the end voted for the oil again.

They have been controlling hot water tanks in New Zealand for decades… probably since the 70s. They use what they call a ripple signal by adding 400 hz on top of the 60 and then relays on your hot water tank detect the 400hz and switch it off

There’s a company doing that in Hawaii, I think it’s called “shift energy”. I interviewed with them, it seemed like a great operation, but a bit hobbled by being a startup in Hawaii. I respect it though, I’d do the same.

It loses heat overnight, or you use all the hot water contents overnight?

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.

well just piping for hot water system is more expensive then PV panels.

But biggest expense is instalation costs(humans) so it depends how you calculate. But PV system can be used for hot water, tv, car, charging kids bikes, lawnmower etc. Solar thermal can be used only for hot water (or cooling if you use multistage heat pump but that is viable only in office buildings or hockey stadiums and such).

This is by design in the regulatory infrastructure, from local permitting offices all the way up to CPUC and rate structures.

We pay about $3/W for solar installation in the US, but Australia pays about $1/W.

For batteries, there's still a supply crunch and the only people getting really good prices are those people who buy in huge bulk or are willing to take a risk on a lesser known manufacturer. If you want well-proven brands the prices can still be very high for small purchases, and a solar installer is not going to want to take a risk with a new supplier.

These systems are not super complex, most technical people could figure them out fairly easily, and in fact off-grid disconnected systems are really easy to do. It's the grid tie that will kill you or first responders to your house, we have made the process of setting the whole thing up very expensive because nobody on the regulatory side has an incentive to make it straightforward and cheap. And since NEM3 killed solar in California, all the installers are barely scraping by and need to rely on very high margins on few projects.

PV panels have dropped in cost in nominal euros by 21% over the last year, which is roughly the long-term trend since solar became profitable without subsidies around 02014: https://www.solarserver.de/photovoltaik-preis-pv-modul-preis...

Possibly you are only looking at prices inside the US, where anti-renewable-energy regulations drive the cost of solar energy through the roof.

I'm working in the IIoT domain too. Your workflow is interesting towards the end. Any particular reason, why you don't write it to some db like Timescale or Influx at the end without any prometheus conversion?

As someone working in the IoT space - why pay for kepware for something that can be done in a few weeks by a developer? Telegraf or even a bit of programming will save a lot of pain

Does this include thermals on batteries? And how much power is spent on keeping them at the optimals? What about SoC/SoD figures?

Because without that the 20 year promise is bullshit.

I can sort of name ballpark figures for the above, the thing I can't get is how this can even approach profitable w/o hype and subsidies.

Jenny Chase perhaps:

> On Friday my colleagues suggested I get a tattoo reading "COWARDS", to save me time saying it in solar forecast calibration meetings.

iCal them „simple“ and „complex“ power. For someone who isn’t truly informed, a „Simple Energy“ solution seems much simpler than one based on renewable energy. With „simple“ power, solving climate change appears straightforward: just build more nuclear plants, which conveniently replace coal and gas on a 1:1 basis since they are baseload power generators.

Renewable energy, on the other hand, is (for now, the transition time) complex. It requires a better, smarter, and much larger interconnected grid, as well as intelligent management of supply, demand, and storage. It means considering and understanding multiple aspects at once. This complexity often leads people who are convinced that more simple power is the answer to dismiss the idea of renewables too quickly—because nuclear seems so much simpler.

I understand the appeal of simple energy. The sad part is that many people likely believe this is the scientifically correct position. And they are often so convinced that, even when presented with current studies and reasonable arguments against new nuclear plants, they quickly assume that the other person is just an irrational, biased anti-nuclear activist. After all, the simplest solution must also be the right one, right?

Being informed in this context doesn’t just mean knowing the pros and cons of nuclear, wind, or solar power. It requires a deep understanding of what is technically and financially feasible today—including energy forms, grid transformation, storage solutions (not just lithium-ion batteries), follow-up costs, sustainability (mining, waste disposal), as well as political, economic, military, and social implications. And how all of these factors interact.

But none of that is necessary if you just want to build more simple power plants.

The transition to 100% renewable energy is as complex as the development of the internet. If we were still relying on letters, telephones, fax machines, newspapers, radio, and TV, the idea of transitioning to a globally available, instant multimedia internet would have seemed just as utopian and impossible.

It was also underestimation of China. Outright chauvinism there.

There are only three energy storage forms that are relevant for the next decade. All the others looked promising, but the learning curve on batteries has rendered them irrelevant. Your link is from 2020, it is out of date.

The best energy storage form is "final form". Some energy products can be stored. For example if you are using the energy to create heat, you can store heat for use in the future. Heat storage sucks as a way to store energy destined for electricity, but is a great way to store energy destined for use as heat.

The utility of batteries for daily storage is obvious and well proven.

Thirdly, the best annual storage is pumped hydro. It's the cheapest and it can be used pretty much everywhere -- all you need is water at one end of an elevation change and a way to build storage at the other end.

All the other forms that you'd think would fit in between the two are being quickly subsumed by the rapid price drops in battery pricing. The cutover points are rapidly shifting -- batteries are now cheapest for biweekly-ish.

And the primary sources are getting so cheap that overbuilding is an alternative to storage. Rather than storing for the reduced amount of daylight in the winter, just overbuild. More overbuilding and a few days of storage will let you handle a stretch of cloudy, windless days in January. No annual storage required.

I think this is going to turn out like the exotic panel chemistries: batteries are simple and have powerful continual improvement in performance and price, while the others turn out to be more complicated. In particular solid state wins over mechanical anything almost every time.

I can also make some methane depending on what's on the lunch menu, but that doesn't mean cheap renewable natural gas is a solved problem.

> why didn't progress in solar and batteries happen sooner?

The rate of progress in cost reduction has been astonishing. It's unlike anything except Moore's Law. This catches people out.

As well as the usual suspects: cheap fossil fuels, failure to take global warming seriously, belief that nuclear power would see similar exponential cost reduction rather than opposite, and of course anti green politics.

But if 95% cost reduction is the result of not taking it seriously, would taking it seriously earlier have been even better? Hard to say.

Battery progress was in some ways slowed but also accelerated by oil companies who kept buying up patents on solar and battery stuff that looked promising, and then sat on the patents, refusing to license it.

One oil company bought Cobasys, which owned all the NiMH patents. Thereafter, Cobasys refused to license NiMH batteries to anyone making a vehicle, except large ones like transit busses. Several early EVs used NiMH batteries until Cobasys was acquired and set up the restrictions.

This really lit a fire under researchers and battery industry to try and improve lithium ion, which had hit the market in the early 90's. Once the price of Lithium Ion started falling, the market very quickly forgot about NiMH batteries. In about ten years prices have fallen to one fifth of what they were. That fall has slowed, but it's still dropping.

It's a false assumption that technological progress happens automatically or even that it's based upon the passage of time.

Progress happens as a result of many choices made by individuals to invest time and energy solving problems. Why is solar rapidly improving now? Because way more people are invested in making it better.

Nascent technologies almost always face an uphill battle because they compete against extremely optimized legacy technologies while themselves having no optimization at first. We only get to the current rapid period of growth because enough people pushed us through the early part of the S curve.

Solar and batteries got cheaper when we scaled up and built a lot. You have to pay current prices to get the next price drop, because it's all learning by doing.

If we had pushed harder in the 80s, 90s, and 2000s, solar might have gotten cheaper sooner. Solar fit in at the edges of the market as it grew: remote locations for power, or small scale settings where running a wire is inconvenient or impractical. The really big push that put solar over the edge was Germany's energiwende public policy that encouraged deploying a ton of solar in a country with exceptionally poor solar resources; but even with that promise of a market, massive scale up was guaranteed.

It's in many ways a collective action problem. Even in this thread, in 2025 you will see people wondering when we will have effective battery technology, because they have been misinformed for so long that batteries are ineffective that they don't see the evidence even in the linked article.

Also, most people do not understand technology learning curves, and how exponential growth changes things. Even in Silicon Valley, where the religion of the singularity is prevalent and where everyone is familiar with Moore's law, the propaganda against solar and batteries has been so strong that many do not realize the tech curves that solar and batteries enjoy.

A lot of this comes down to who has the money to spend on public influence too, which is largely the fossil fuel industry, who spends massive amounts on both politicians and in setting up a favorable information environment in the media. Solar and batteries are finally getting significant revenues, but they have been focused more on execution than on buying politics and buying media. They have benefited from environmental advocates that want to decarbonize, without a doubt, but that doesn't have the same effect as a very targeted media propaganda campaign that results in zealots that, whenever they see an article about climate change, call up their local paper and chew out the management with screaming. Much of the media is very afraid of right wing nuts on the matter and it puts a huge tilt on the coverage in the mass media in favor of fossil fuels and against climate science.

No, but I don't see a good reason why you can't recycle the cells especially given they contain a thin layer of silver. Google already finds local recycling firms, since it's required by WEEE.

(The 500 years question has issues for all the other sources of energy as well!)

The resources all start off chemically bound in rocks that aren't famous for spontaneously generating or storing electricity.

I don't see it being meaningfully more expensive to process smashed up old PV or batteries than starting from the natural state, and my expectation is that it would be easier.

The exception would be if some of the chemical pathways turn into low-concentration atmospheric gasses that then diffuse all over the world, which is how we got the problem with CO2 (and unrelated problems with CFCs).

Yes, batteries are getting better at such a rate that you can recycle old batteries at end of life, lose 10% of the material in that process and build a new battery with new tech and less material that is better than the original.

The resource extraction issue is more than these are so useful we're going to build an ever growing amount of them.

Luckily they're made from widely available materials, with even more widely available substitutions possible e.g sodium batteries.

Aside from silver for contact wires, PV panels are made (or could be made) with mundane materials available in essentially unlimited amounts. The total mass flowing through new PV panels each year if the US were fully solar powered would be much less than the volume of mundane material flowing through the system already. For example, the EPA estimated that in 2018 the US generated 600 million tons of construction and demolition waste (of which 143 million tons went to landfills.)

https://www.epa.gov/facts-and-figures-about-materials-waste-...

You can't repair, but you can recycle (although doing so likely isn't very profitable until the exponential price decrease stops)

We had an insulation upgrade recently when we ripped out our gas furnace and put in a heat pump. The biggest improvement was from spraying foam into the space below the first floor, where it rests on the outer basement walls. There had been too much air leakage there. There was also attic insulation upgrading. No walls had to be penetrated.

The house (built just a decade ago) feels much better insulated now.

> Air seal and upgrading insulation: correct me if I'm wrong, but that implies either tearing open all of the exterior walls or ripping off all of the siding, no?

If you do the exterior walls yes, but most heat loss is through the attic and roof. Air sealing and super- insulating the attic floor is pretty cost effective. Likewise sealing cracks around windows and doors.

> the annual cost to run a heat pump sits well between a natural gas furnace and resistive heating.

Why is that? Maybe I'm missing something fundamentally, but this should be a strict function of COP, $/kWh (elect.) and $/kWh (gas) right? The insolation thing is kind of red herring, because that saves kWh-needed and that goes into both, right?

And yes COP will probably be bad/worst on some days of the year. But on most days even Chicago should get a pretty decent COP from a low-temperature heat pump. Is natural gas just so cheap in Chicago?

> You say "likely never" but eventually we'll have covered the earth's surface in solar panels. Unless we are transitioning to space based solar and transmission we'll want fusion to increase energy generation beyond surface irradience of earth.

> In the meantime, solar panels for massive generation also incur transmission costs to centralize that energy for any major energy usages. We might want to keep having high power generators next to super-high energy consumers. For instance our (theoretical) hyperspace communication and computation array. Right now those usages are things like Arc Furnaces, Aluminum smelters, data-centers, ...

Well before we cover the entire globe in PV, the mere fact that the panels absorb a lot of light means they will change the planet's albedo, heating things up.

But any source of power on that scale will also increase the planet's equilibrium temperature (regardless of if it's PV, fusion, or even if we figure out how to harness dark energy/zero point shenanigans) so we want space-based power before then — and the industrial capacity to use that power in space, because simply beaming it down to Earth is still going to heat up the planet just like any other power source.

Before we even get to that point (in fact, already today) humanity is manufacturing enough metal to make a global power grid with only 1 Ω of resistance the long way around. The limiting factor is geopolitical, not technical, because it's literally just China making enough of the relevant metals.

> covered the earth's surface

In any discussion of far-future considerations like this we need to remember that thermodynamics requires all energy used for work to become heat. Covering the Earth's surface in solar panels is one thing, but by the time we're covering every inch in fusion power plants, we've turned the Earth's surface to lava from all the waste heat. There's a physical upper bound on energy usage within the Earth's biosphere that prevents useful energy production from scaling to infinity, and as a result we'll find ourselves optimizing for economics rather than being limited by energy production per unit of area.

As for industry, yes, it seems likely that industry will still represent a large and relatively centralized consumer of power which may benefit from a large dedicated installation. But I'm not convinced that fusion will ever be more economical than even traditional nuclear fission energy (just because your fuel is relatively cheap doesn't mean a thing if the plant itself is essentially disposable because of the energies involved).

As for space, maybe, though considerations of space exploration aren't driven by economics, so whether or not anybody ever figures out a viable fusion reactor design for a spaceship could have as little relevance to civilian power generation as your classic RTG did.