As an example, if we use solar to generate the electricity to power the indoor farm, the panels will take up roughly 20-40% of the sun's energy, depending on the panel.
So to grow the crops indoors we need to use at least 2.5x as much land.
The sun is not a source of emissions, so why spend time and money replacing it?
It's more complicated than that, since the wavelength distribution matters--we can effectively transform green photons that the plant would have reflected into red or blue photons that it will absorb. (The plant still benefits from some green light, but less than in sunlight.) We can also supply each plant with its exact optimal PPFD and DLI. For example, lettuce may be grown under shade cloth, deliberately wasting much of the incident sunlight, because the extra light won't make it grow faster and will make it taste bitter. In a vertical farm, we can just set the LED current and spacing wherever we want.
I've heard that 1 m^2 of modern solar panels will support >1 m^2 of a low-light crop (like lettuce, unlike grain) under modern LEDs. I haven't done the math myself, and this obviously varies with climate. I think vertical farms (growing entirely by artificial light) are still uneconomic vs. greenhouses almost everywhere, per my other comment here. Supplemental artificial light in a greenhouse is of course highly economic in many climates, and Dutch growers have been using it for decades.
The cost to heat or cool a vertical farm should be lower than for a greenhouse with equivalent growing area, since it's got lower surface-area-to-volume ratio and doesn't need to be transparent. That may be important for stuff like high-end strawberries, where tight control of the day-night temperature swing enables higher sugar content. I again wouldn't expect a useful benefit for grains, though.
This seems like a good point: just building greenhouses, providing full control of water usage and easy mechanical access, delivers all the value without the inefficient trip through solar panels and LEDs.
On the other hand, wind power would not use up cropland, and the solar panels could be on non-crop land, or water. Stacked grow trays might be more efficient to operate on.
The new Frankenstein rice that does CD4 photosynthesis, and also makes carotenes, seems like a good choice to grow in them. Maybe it can be persuaded to make protein, in the bargain.
So to grow the crops indoors we need to use at least 2.5x as much land.
The sun is not a source of emissions, so why spend time and money replacing it?