7 ms·

Hot water tank heated by electricity and powering on at noon is flattening curve. You can say hot water tanks are cheapest, simplest and fastest deployed energy

Hot water tank heated by electricity and powering on at noon is flattening curve. You can say hot water tanks are cheapest, simplest and fastest deployed energy storage device. Solar + hot water tank can provide any house in US with 100% solar hot water (from PV!) for 80% of time, remaining 20 % of time you can have 10-99% solar heated water. So we should focus on saying to people that if they buy solar and add electric heating element to hot water tank, then PV system will pay itself much sooner and their batteries will last longer. Becasue it is known and predictable load, you need hot water every day. And hot water is order of magnitude more energy then TV, lighting... By lowering household usage like this we can make energy transition faster, cheaper. Also proper construction - house heated only 10 days in a year - https://www.youtube.com/watch?v=5KHScgjTJtE https://www.youtube.com/watch?v=5KHScgjTJtE

opwieurposiu 2y ago
I installed PV solar hot water at my house, works great. Makes about $2 a day worth of power.
- Calwestjobs 2y ago
  Congrats, using as much energy directly on site is crucial for fast and cheap energy transition of economy.
epistasis 2y ago
Converting a gas water heater to electric and/or solar is one of the best bang for the buck on decarbonization too. Something that should be done before buying an electric car or swapping out your gas furnace for a heat pump. Though I'm terrible at following my own advice, I still have a gas water heater, just because I needed to replace my car and furnace before I needed to replace my water heater. That said, the sunk cost fallacy applies to carbon emissions just as hard as it does to dollars so I have little excuse for not replacing it except laziness (and space on the breaker panel...)
- ipdashc 2y ago
  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
  - bryanlarsen 2y ago
    That only applies if you're only going to get less than 1 year of use out of a heat pump. A heat pump has an "embodied CO2" of 1.7t, which is about the same as the annual CO2 emissions of a gas furnace.
  - epistasis 2y ago
    Can you explain how it's not a fallacy? Compare your lifetime emissions of replacing a gas car with an EV now versus at the "end of life" of the gas car. Emissions will always be lower if you replace now rather than later. Imagine if everybody switched to EVs right now, en masse. Emissions over the next decade, and every subsequent decade, would be massively lower. Waiting for every gas car to reach end of life before switching is always going to be higher emissions, always. Similarly, the "waste" already happened when the gas heater was manufactured. There's no additional waste when it's decommissioned. It's a sunk cost, there's no getting that back. The only question is if you switch to lower emissions now, or you switch to lower emissions later. Now, if you bring money into it, sure, there could be a financial motivation to keep emitting higher amounts of emissions. But if you take monetary considerations out of it, it's always better to stop emitting sooner rather than later. I'd love to have some serious push back against this. The best I've ever got is "that doesn't sound right..." without any engagement with the quantitation or the ideas. Which is exactly what I would expect if it was a fallacy.
    - deleted 2y ago
      [deleted]
    - relaxing 2y ago
      I think you’re assuming when you switch to EV the ICE car disappears. But more likely it got traded to someone else who has less money, eventually making it possible for someone to own a car who otherwise wouldn’t have. So the emissions stayed the same and you added the carbon embedded in the new EV.
MostlyStable 2y ago
Slightly less convenient/has more impact on how we percieve our environment, but HVAC (the number 1 power use, hot water is #2), can also be a decently good battery, if your house is well insulated. Where I live, power is incredibly cheap over night, so I over-heat or over-cool my house (depending on season) overnight, and then let it gradually equilibrate during the day. I realize that some people won't be willing to have a very warm/very cold house that gradually shifts to the more ideal comfortable range, but for people who are willing to deal with that (it personally doesn't bother me), it's a pretty easy way to shift a lot of power use and, if you have Solar or Time of Use billing, save a lot of money.
- Calwestjobs 2y ago
  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.
  - pfdietz 2y ago
    > Because hot water energy usage can not go lower in current society Couldn't heat energy in waste water be recovered? Or is that already maxed out?
andbberger 2y ago
has PV finally overtaken solar hot water?
- Calwestjobs 2y ago
  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).
  - Calwestjobs 2y ago
    by multistage heat pump i meant https://en.wikipedia.org/wiki/Absorption_refrigerator https://en.wikipedia.org/wiki/Absorption_refrigerator i.e. rv propane refrigerator.
  - kragen 2y ago
    How much does the piping cost?
    - Calwestjobs 2y ago
      even with tariffs in place, whole PV system will provide you more kWh per year per $ invested then solar thermal system. rated output is not what you get 100% of time. price per performance is crucial. price per imaginary watts is nonsense.
      - kragen 2y ago
        You're probably right, but do you have some idea how much the piping costs for a solar thermal hot water system? Because that's what I was asking about.
        deleted 2y ago
        [deleted]
megaman821 2y ago
Using a hot water tank as a battery is an incredibly simple idea. I wonder how much electric hot water heaters on a timer could flatten California's duck curve.
- ok_dad 2y ago
  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.
- applied_heat 2y ago
  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
  - Taniwha 2y ago
    We also control (or used to control) our street lights in AoNZ using ripple control (it was implemented with a tuned reed relay in some towns). Back in the 70s friends built a ripple transmitter and sent morse across Ōtepoti by turning on/off their suburb's street lights (at 2am)
dzhiurgis 2y ago
My hot water heats up in less than 2 hours and if I don’t fire it up at night I won’t have hot water in morning. At this point getting some batteries would likely be cheaper than new boiler + plumber to install it.
- PaulDavisThe1st 2y ago
  It loses heat overnight, or you use all the hot water contents overnight?
  - dzhiurgis 2y ago
    Sun stops producing useful amount after 6pm which coincides with dinner and whole family taking a shower.
    - NullPrefix 2y ago
      Increasing water heater capacity might be cheaper than increasing battery capacity.
      - dzhiurgis 2y ago
        New cylinder - 1.5k NZD + half day of labour (so another $500). My current one stores about 6-8 kWh. 15kWh battery - 5.5k NZD + and hour of DIY. So technically battery is more expensive but more useful. Also easiest with water heater would be cranking up the temperature, but I really hate dealing with scolding water coming from taps (especially with small kids around). Another thing with battery I can charge with whatever solar excess I have, but with hot water my only option is 16A. Either way I do not care ATM - I export using spot price which has been 2x of what I actually pay for power - https://www.emi.ea.govt.nz/Wholesale/Reports/W_P_C?DateFrom=20250301&DateTo=20250307&RegionType=POC_UNI&TimeScale=TP&_rsdr=W1&_si=v%7C3 https://www.emi.ea.govt.nz/Wholesale/Reports/W_P_C?DateFrom=...
        kragen 2y ago
robomartin 2y ago
Photovoltaic water heating is the worst possible idea (and use) for solar panels. Frankly, I have no clue why they are pushing this concept. Add to that electric stoves, ovens and cars and you have an expensive disaster in your hands. Most of the homes around me have somewhere around 3.5 to 6.0 kW of installed solar. This is barely enough to support these homes. With changing rates and TOU billing, everyone is paying hundreds of dollars per month for electricity (between billed power and leasing costs). Wasting --because it would be wasting-- the energy they produce to heat water would cause every single one of these homes to go back to bills they were getting in the pre-solar era. Electric water heaters run somewhere between 3KW and 5KW...which is crazy. In a place like SoCal, in the summer, your air conditioning system is going to consume that much power. The monumental increase in energy usage cannot be understated. I have THERMAL hot water heating, similar to this: https://www.stiebel-eltron-usa.com/products/solar-thermal-hot-water-systems-individual-components https://www.stiebel-eltron-usa.com/products/solar-thermal-ho... Just two to four panels are enough for most homes. Instead of burning gas or electricity to heat water, you run a little circulation pump and get water hotter than you can handle, by far. This is supplemented with gas to keep the desired temperature when the sun isn't up. I've been using these systems for well over 30 years, they work well and they are the smart way to make hot water from the sun. My 13 kW solar array isn't being used to inefficiently turn photons into electrons to then burn the energy making water hot.
- kragen 2y ago
  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/stiebel-eltron-renewables-msrp-rev.2018.1.pdf#page=4 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-report-q2-2024 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-preisindex/ 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-termotanque-solar-300lts-acero-kit-elec-tanque-pre-sg300-_JM 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 https://news.ycombinator.com/item?id=43468177 for a recent comment where I did a brief sketch of an LCoE estimate.
kavalg 2y ago
Why are people even considering an electric heating element, when you can get at least 2-3 times the efficiency of a DHW heat pump that would probably cost you ~ $4000. In my experience, I have found that for PV panels it is often the roof area / orientation that limits the energy capacity that you can install. Installing a heat pump instead of resistive heater can effectively reduce this 2-3 times. Yes, heating DHW with a heat pump is not that trivial. There could be problems when the tap water is hard (limescale problems in heat exchangers), you often need 2-3 times larger tank in order to cover the daily cycle, but still looks more efficient than a big battery and an electric heater. PS: I've accumulated lots of knowledge on the topic. DM me if you are interested in exchanging on this.
- kragen 2y ago
  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-preisindex/ 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 Even in countries like the US with aggressive anti-renewable-energy regulation, it's hard to see how the heat pump comes out cheaper.
  - kavalg 2y ago
    USD 4000 will buy you something like 12 - 16Kw R32 heat pump. If you have enough roof area to install 50kW PVs then you must live in a mansion :). Another thing is that heat pump installations are usually dual purpose (space heating/cooling and DHW).
    - kragen 2y ago
      Thanks! That's about US$0.20-US$0.40 per watt, which is probably cheap enough to compete with overprovisioning solar panels. What's the usual duty cycle for such a heat pump? Can you buy smaller ones? 12kW seems like it could provide 8000 liters of hot water per day, which seems like a lot even for a mansion. Maybe a public bathhouse. It is certainly true that energy-intensive buildings cannot be self-sufficient on solar, but perhaps you can put the solar panels near your house instead of on it.

opwieurposiu 2y ago

I installed PV solar hot water at my house, works great. Makes about $2 a day worth of power.

Calwestjobs 2y ago
Congrats, using as much energy directly on site is crucial for fast and cheap energy transition of economy.

epistasis 2y ago

Converting a gas water heater to electric and/or solar is one of the best bang for the buck on decarbonization too. Something that should be done before buying an electric car or swapping out your gas furnace for a heat pump. Though I'm terrible at following my own advice, I still have a gas water heater, just because I needed to replace my car and furnace before I needed to replace my water heater. That said, the sunk cost fallacy applies to carbon emissions just as hard as it does to dollars so I have little excuse for not replacing it except laziness (and space on the breaker panel...)

ipdashc 2y ago
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
- bryanlarsen 2y ago
  That only applies if you're only going to get less than 1 year of use out of a heat pump. A heat pump has an "embodied CO2" of 1.7t, which is about the same as the annual CO2 emissions of a gas furnace.
- epistasis 2y ago
  Can you explain how it's not a fallacy? Compare your lifetime emissions of replacing a gas car with an EV now versus at the "end of life" of the gas car. Emissions will always be lower if you replace now rather than later. Imagine if everybody switched to EVs right now, en masse. Emissions over the next decade, and every subsequent decade, would be massively lower. Waiting for every gas car to reach end of life before switching is always going to be higher emissions, always. Similarly, the "waste" already happened when the gas heater was manufactured. There's no additional waste when it's decommissioned. It's a sunk cost, there's no getting that back. The only question is if you switch to lower emissions now, or you switch to lower emissions later. Now, if you bring money into it, sure, there could be a financial motivation to keep emitting higher amounts of emissions. But if you take monetary considerations out of it, it's always better to stop emitting sooner rather than later. I'd love to have some serious push back against this. The best I've ever got is "that doesn't sound right..." without any engagement with the quantitation or the ideas. Which is exactly what I would expect if it was a fallacy.
  - deleted 2y ago
    [deleted]
  - relaxing 2y ago
    I think you’re assuming when you switch to EV the ICE car disappears. But more likely it got traded to someone else who has less money, eventually making it possible for someone to own a car who otherwise wouldn’t have. So the emissions stayed the same and you added the carbon embedded in the new EV.

MostlyStable 2y ago

Slightly less convenient/has more impact on how we percieve our environment, but HVAC (the number 1 power use, hot water is #2), can also be a decently good battery, if your house is well insulated. Where I live, power is incredibly cheap over night, so I over-heat or over-cool my house (depending on season) overnight, and then let it gradually equilibrate during the day. I realize that some people won't be willing to have a very warm/very cold house that gradually shifts to the more ideal comfortable range, but for people who are willing to deal with that (it personally doesn't bother me), it's a pretty easy way to shift a lot of power use and, if you have Solar or Time of Use billing, save a lot of money.

Calwestjobs 2y ago
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.
- pfdietz 2y ago
  > Because hot water energy usage can not go lower in current society Couldn't heat energy in waste water be recovered? Or is that already maxed out?

andbberger 2y ago

has PV finally overtaken solar hot water?

Calwestjobs 2y ago
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).
- Calwestjobs 2y ago
  by multistage heat pump i meant https://en.wikipedia.org/wiki/Absorption_refrigerator https://en.wikipedia.org/wiki/Absorption_refrigerator i.e. rv propane refrigerator.
- kragen 2y ago
  How much does the piping cost?
  - Calwestjobs 2y ago
    even with tariffs in place, whole PV system will provide you more kWh per year per $ invested then solar thermal system. rated output is not what you get 100% of time. price per performance is crucial. price per imaginary watts is nonsense.
    - kragen 2y ago
      You're probably right, but do you have some idea how much the piping costs for a solar thermal hot water system? Because that's what I was asking about.
      - deleted 2y ago
        [deleted]

megaman821 2y ago

Using a hot water tank as a battery is an incredibly simple idea. I wonder how much electric hot water heaters on a timer could flatten California's duck curve.

ok_dad 2y ago
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.
applied_heat 2y ago
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
- Taniwha 2y ago
  We also control (or used to control) our street lights in AoNZ using ripple control (it was implemented with a tuned reed relay in some towns). Back in the 70s friends built a ripple transmitter and sent morse across Ōtepoti by turning on/off their suburb's street lights (at 2am)

dzhiurgis 2y ago

My hot water heats up in less than 2 hours and if I don’t fire it up at night I won’t have hot water in morning. At this point getting some batteries would likely be cheaper than new boiler + plumber to install it.

PaulDavisThe1st 2y ago
It loses heat overnight, or you use all the hot water contents overnight?
- dzhiurgis 2y ago
  Sun stops producing useful amount after 6pm which coincides with dinner and whole family taking a shower.
  - NullPrefix 2y ago
    Increasing water heater capacity might be cheaper than increasing battery capacity.
    - dzhiurgis 2y ago
      New cylinder - 1.5k NZD + half day of labour (so another $500). My current one stores about 6-8 kWh. 15kWh battery - 5.5k NZD + and hour of DIY. So technically battery is more expensive but more useful. Also easiest with water heater would be cranking up the temperature, but I really hate dealing with scolding water coming from taps (especially with small kids around). Another thing with battery I can charge with whatever solar excess I have, but with hot water my only option is 16A. Either way I do not care ATM - I export using spot price which has been 2x of what I actually pay for power - https://www.emi.ea.govt.nz/Wholesale/Reports/W_P_C?DateFrom=20250301&DateTo=20250307&RegionType=POC_UNI&TimeScale=TP&_rsdr=W1&_si=v%7C3 https://www.emi.ea.govt.nz/Wholesale/Reports/W_P_C?DateFrom=...
      - kragen 2y ago

robomartin 2y ago

Photovoltaic water heating is the worst possible idea (and use) for solar panels. Frankly, I have no clue why they are pushing this concept. Add to that electric stoves, ovens and cars and you have an expensive disaster in your hands. Most of the homes around me have somewhere around 3.5 to 6.0 kW of installed solar. This is barely enough to support these homes. With changing rates and TOU billing, everyone is paying hundreds of dollars per month for electricity (between billed power and leasing costs). Wasting --because it would be wasting-- the energy they produce to heat water would cause every single one of these homes to go back to bills they were getting in the pre-solar era. Electric water heaters run somewhere between 3KW and 5KW...which is crazy. In a place like SoCal, in the summer, your air conditioning system is going to consume that much power. The monumental increase in energy usage cannot be understated. I have THERMAL hot water heating, similar to this: https://www.stiebel-eltron-usa.com/products/solar-thermal-hot-water-systems-individual-components https://www.stiebel-eltron-usa.com/products/solar-thermal-ho... Just two to four panels are enough for most homes. Instead of burning gas or electricity to heat water, you run a little circulation pump and get water hotter than you can handle, by far. This is supplemented with gas to keep the desired temperature when the sun isn't up. I've been using these systems for well over 30 years, they work well and they are the smart way to make hot water from the sun. My 13 kW solar array isn't being used to inefficiently turn photons into electrons to then burn the energy making water hot.

kragen 2y ago
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/stiebel-eltron-renewables-msrp-rev.2018.1.pdf#page=4 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-report-q2-2024 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-preisindex/ 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-termotanque-solar-300lts-acero-kit-elec-tanque-pre-sg300-_JM 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 https://news.ycombinator.com/item?id=43468177 for a recent comment where I did a brief sketch of an LCoE estimate.

kavalg 2y ago

Why are people even considering an electric heating element, when you can get at least 2-3 times the efficiency of a DHW heat pump that would probably cost you ~ $4000. In my experience, I have found that for PV panels it is often the roof area / orientation that limits the energy capacity that you can install. Installing a heat pump instead of resistive heater can effectively reduce this 2-3 times. Yes, heating DHW with a heat pump is not that trivial. There could be problems when the tap water is hard (limescale problems in heat exchangers), you often need 2-3 times larger tank in order to cover the daily cycle, but still looks more efficient than a big battery and an electric heater. PS: I've accumulated lots of knowledge on the topic. DM me if you are interested in exchanging on this.

kragen 2y ago
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-preisindex/ 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 Even in countries like the US with aggressive anti-renewable-energy regulation, it's hard to see how the heat pump comes out cheaper.
- kavalg 2y ago
  USD 4000 will buy you something like 12 - 16Kw R32 heat pump. If you have enough roof area to install 50kW PVs then you must live in a mansion :). Another thing is that heat pump installations are usually dual purpose (space heating/cooling and DHW).
  - kragen 2y ago
    Thanks! That's about US$0.20-US$0.40 per watt, which is probably cheap enough to compete with overprovisioning solar panels. What's the usual duty cycle for such a heat pump? Can you buy smaller ones? 12kW seems like it could provide 8000 liters of hot water per day, which seems like a lot even for a mansion. Maybe a public bathhouse. It is certainly true that energy-intensive buildings cannot be self-sufficient on solar, but perhaps you can put the solar panels near your house instead of on it.