> I assume location must play a large part in this? Solar must be more cost-effective in, say, the Mojave desert, than it is in Alaska.
Yes, each peak kilowatt of utility-scale solar produces about 240 watts average in Arizona, 140 in Maine, and 100 in Germany ("capacity factors" of 24%, 14%, and 10%). I assume the number for Alaska would be even lower.
> Every solar panel you put on the ground is going to take up solar energy that could otherwise be absorbed by a plant, which in turn means that plant can't absorb carbon from the atmosphere.
Yes, and also it will reflect less heat back into space than the plant or bare dirt would, locally raising the temperature. These will start to be important problems when the quantity of power produced by solar panels is about 100 times larger than current world marketed energy consumption. I expect that this will happen in about 30 years. However, merely switching all our energy to solar will have an effect that's about 100 times too small to matter.