https://cdn-ilcjnih.nitrocdn.com/BVTDJPZTUnfCKRkDQJDEvQcUwtA...
https://reneweconomy.com.au/battery-storage-is-dramatically-...
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
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.
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
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?
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.
Another issue was that they were not available for a long time (around 6 months delivery time with no guarantee), something not relevant here but it also affected decision of owners.
With respect to the duty cycle, obviously if you have solar power, you would prefer to use it predominantly and only add up some extra power from the grid when needed. This is the essence of the sizing problem, because that leads you to 2-3x power overprovisioning and the need for heat/cold storage. Heat storage can be two types - DHW and space heating. Space heating is the easiest to estimate. You need to know your house's heat loss (either by specification or just figure it out empirically if you have already lived in it). DHW storage is more difficult to estimate, because it depends on the usage (e.g. how many showers per day). Cold storage is the most problematic, because the fluid needs to be at least 16C or lower to do useful cooling work, however you cannot go much lower than 7C unless you are using propylene glycol (expensive) and even then your indoor units may start to freeze (I am not even mentioning indoor humidity management and dew points).
Lately, the industry has been exploring PCMs (phase change materials). The idea is to store heat/cold not as sensible heat, but as latent heat of the phase change. In practice the substances used are either salts (efficient, but corrosive to the storage tank) or paraffins (more expensive, less efficient, but still viable). These come rated at a specific temperature, but usually have some hysteresis/drift and other issues. I guess you are now feeling a bit frustrated from the engineering complexity :). If batteries were cheap, long lasting and environmentally friendly, this complexity would not be needed. However, I really doubt it that in the foreseeable future batteries will beat heat storage. Given that most of our domestic energy use is space heating/cooling and DHW, I think that PCMs may actually have some moat. There are already offerings on the market, but IMHO they are still not very compelling. What I see lacking is some integrated offering, that would take into account the PV schedule and also grid prices. One a side not, batteries still have an advantage if you can sell back to the grid at a high premium or if you need to e.g. charge your car in the night. So these technologies may be complementary, rather than competitive.
A very big factor is climate. Just to give you an example, I live in the mountain with a colder climate. Cold water from the faucet is around 10C. I rarely need cooling if at all, but I need space heating around 8-9 months during the year. Just 300 km south and by the sea (Greece), cold water from the faucet is around 20-25C, you need 4-5 months of cooling and only ~4 months of heating. Some countries, such as UK have very moderate climate without extremes and things are more predictable. Where I live, we get -15C in the winter and 38C in the summer.
It sounds like you might be interested in my notes and calculations on thermal energy storage in phase change materials, some of which are listed at https://dercuano.github.io/topics/phase-change-materials.htm.... But I think TCES systems are likely to be more significant because of their technical advantages, among other things for managing indoor humidity and possibly even for seasonal thermal stores; some of my notes on the topic are at https://derctuo.github.io/notes/desiccant-climate-control.ht... and https://dernocua.github.io/notes/shower-heating-tces.html. Various kinds of thermal energy storage do seem to beat batteries on cost by around three orders of magnitude; some of my relevant notes are listed at https://derctuo.github.io/topics/thermal-storage.html. I agree with you that there is no real prospect of batteries catching up with thermal energy storage in the foreseeable future.
With respect to the particular problem you mention with needing expensive propylene glycol in your heat transfer fluid to keep it from freezing, ice rinks commonly use brine systems instead, despite the corrosion problems you mention. Brines are very cheap, some like dipotassium phosphate are minimally corrosive, and the commonly used ones are pretty nontoxic.