A plant's ability to make sure of sunlight saturates at about half the brightness of full daylight and it only absorbs light in half the spectrum of sunlight so if you had magical 100% efficient solar panels and LEDs you could roughly quadruple the effective area you have under cultivation if you put solar panels on top then four layers of growing plants. But of course solar panels and LEDs are very far from perfectly efficient so yeah, you're better off with the greenhouse until we get cheap fusion power or something.