In terms of efficiently using sun's energy. Let's not forget passive solar.
With no mechanical parts to break, a sufficiently heavy or phase change thermal mass can go a long way when the house is properly insulated. 4 m2 of windows (40 sf) is close toe 4 kw. (circa 12000BTU )
There's an interesting summary of the situation in the UK here.
In short, the most widespread option is likely to be heat pumps, despite some significant disadvantages, such as cost, and that fact that they don't always work well with existing heating infrastructure.
It may be that hydrogen, generated by renewable power, will also play a significant role - ...
There are a few factors here:
Temperature range. Domestic heating water ranges from about 90 F (35 C) to 140 F (60C) The latter is used in radiators and fan coil heat exchangers, the former used in in-floor heating.
Swimming pools are heated to only to mid 80 F. Depending on how much water you move, you need water only a few degrees to maybe 15 C warmer.
I reached out to a manufacturer of swimming pool heat pumps, and it turns out that they go up to 40C or so.
So it seems that the difference is the maximum temperature they can reach, and presumably this influences the cost of the materials used.
They might also forego cooling, eliminating a component.
This is reflected in the weight: one generic poolside ...