IE: You can’t burn 500 gallons of gas if the end product costs 500$. That applies not just to transportation but also how much material and thus mining the raw materials you need, including refining them, the amount of chemicals you can use per panel, how much electricity you can use in production of the device including precursors etc.
One of the numbers I was looking at compared air sourced heat pump at night when it’s coldest vs this kind of resistive heat battery. Solar panels are far cheaper and better for the environment on a kWh/day basis so even if the COP is 3 (or less it gets colder at night) * 90%(losses from battery) = fewer panels you more than offset it by needing far more batteries.
Obviously solar thermal setups have advantages if you need lots of heat, but they are also wasted most of the year. If 8-10 months a year you’re only using them for hot water then annual efficiency is closer to 25% than 90%.
Anyway, the real question here costs both in equipment and labor. Solar hot water panels involve plumbing and need radiators etc they quickly pay for themselves when heating a large home but don’t scale down very well.
Running the numbers I was surprised how cost competitive the sand bucket is for something like a chicken coop. Sure the panel(s) are wasted most of the year, but you don’t exactly need an electrician to set this up either. Probably also worth considering for redundancy in some situations.