I agree, efficiency (and performance if limited to 500W) is seriously limited at cold ambient air. Thanks for the details and thoughtful analysis.
Florida might be a great place for these, for those that can learn to live with the slow recovery.
I live off grid at 5600 ft in central eastern AZ. Moderate winters with some snow, plenty of single digit F nights. I went with a homebuilt 4x32ft horizontal riser aluminum over copper flat plate solar water heater, which drains back to an homebuilt 800 gallon insulated tank. With copper pipe coils for domestic HW and in floor heat, it solves my hot water issues as well as space heating via in floor heat. Propane backup Dec-Feb. It has served me well, for my small 1100SF, super insulated (Canadian style double wall) home.
My interest in this thread is because I like to think about the future of off grid home design- without propane. Heat pumps with ground source seem very appealing for that. After my experience this last summer experimenting with a 12000 BTU water chiller, I'm now newly skeptical about efficiency claims on refrigeration/heat pumps (air exchange). They hide the real performance in the new seasonal average performance data.
Well bores filled with 1" PEX (closed) loop are popular for ground source, and require little space, but are damned expensive. I've got sandstone at about 5 food deep so dug trenches aren't viable. 65F all year round at about 10-12 foot deep (confirmed), but not cheap to get to. It would be a great temperature for direct winter greenhouse heating (in foor for large surface area), heat pump, water chiller.