Greenhouses work all the way to the arctic circle. At which point your not going to be getting much energy from solar power. (One trick is to use reflectors to collect more sunlight then concentrate it onto a smaller area, same result as having solar power at a tiny fraction of the cost.) Other option is to use a lot of insulation then tilt the structure. ex: http://waldenlabs.com/5-northern-greenhouse-examples/

Further, solar power takes the same area as you can use to grow crops anyway.