Modern solar panels and high efficiency GHP, are getting upto a point where it's comparable to the heat you get from direct sunlight. So I don't think it's far

Modern solar panels and high efficiency GHP, are getting upto a point where it's comparable to the heat you get from direct sunlight. So I don't think it's far off that we see green houses heated purely by solar energy, with the added benefit of being energy sink during low demand (night) and energy source during demand (day).