I also wondered if geothermal could be run off solar....what I see now, I doubt it. Our biggest complaint is we notice the power draw...the lights get dim (I haven't checked with the electric company as far as what their thoughts might be). I know it's real complicated with the settings, stages etc. on the thermostat and on the geothermal units but we just think the heat strips should not be coming on as often (so we feel it's in the settings). We're waiting on the installer's pro to get back but in the interim we are puzzled. We put the strips in as 'backup' but this geothermal stuff is pretty complicated.
It probably depends on region you are in. I was thinking about solar source heat pump also but the cost and expected technical difficulties pretty much killed the project before it started. At the end we decided on classic GT instead unproven, exotic and unnecessarily system.
We are in south central Iowa and have Waterfurnace geothermal in operation fourth year. Our "fluffed coil" heat exchanger contains about 2500 ft of pipe and is in our pond about 300 ft from the house. We have 5 ton unit (5 kW) and heat about 4000 sq ft by floor heating. Our last month heating bill was 97 dollars for 2150 kWh.
We didn't have enough water in the pond first year so about half of the heat exchanger was above ice. Inlet temperature dropped to about 24F and return to 21 F but the house was still reliably heated. I think that floor heating is primary reason for low heating cost. We have set temperature entering the floor to 100F so the required temperature differential is much less than for forced air.
