# To grow one $\rm m^2$ of crop plants, how many $\rm m^2$ of solar panels do you need?

In vertical farms you need to use LEDs to make plants grow, which in turn have to be powered e.g. by solar panels. If solar panels had the same efficiency as plants, then you would need the same amount of solar panels as you had plants before, if your LEDs had perfect efficiency. So the claim that vertical farms would use less surface area seems strange at first.

But it appears that solar panels are more efficient than plants. Although the accepted (at the time of writing) answer focused on creating electricity (burning biofuels), which is advantageous to solar panels as there are no further conversion steps.

So the question is, does it make sense to go the (Sunlight -> solarpanel -> electricity -> light -> plant) detour?

As you probably lose efficiency in every step this seems wrong intuitively, but reasons this might make sense are:

• plants might be able to convert a smaller percentage of the light spectrum into energy, if solar panels can capture the entire spectrum and LEDs produce only that spectrum, you gain some efficiency
• plants might only be able to use energy up to a certain point and "throw the rest away". If solar panels can capture all the energy you could split it up to multiple plants with dimmer lights

So the question is: To grow one square meter of crop plants, how many square meters of solar panels do you need?

(This might depend on the latitude)

• Intriguing question. But do most vertical plant farms actually use solar power, or do they use other electricity sources, e.g. nuclear? If they are powered by electricity from the grid, they'll get a mix with only a small portion of solar energy. Feb 16 at 10:52
• @Polygnome sure, but this is more of an abstract - "is this a sensible thing to strive for in the future?" type of question, and ideally we want to get rid of any non renewable plants including nuclear in the relative near future. Fusion might change things again, but until then this seems quite relevant Feb 16 at 11:02
• @PM2Ring Would focusing on plants for agriculture help? I guess there is often a differentiation between leafy greens, vegetables and energy rich food (grain/rice/potatos) in vertical farming. So that would still leave multiple categories. Feb 16 at 11:13
• Are you also considering that vertical farms are not single floors, but often several layers of plants? I think one attractive aspect of vertical farms is the smaller total area footprint in countries with sparse/expensive land (e.g. Japan, Netherlands), because you can build several farms on top of each other, so to speak. Feb 16 at 11:14
• @JohnW. But you still need to light every floor there. So the question is how many square metres of solar panels you need to light one square metre of plants. If you only need $0.25m^2$ of solar panels per $m^2$ of plants, then you could do 4 floors with solar panels on top as a self contained system. If you need more solar panels, then you would have to place them somewhere else than on the roof and then you wouldn't really save land would you? I mean that is exactly the point of the question. Do you actually save land area? Or just seem to save it before considering electricity generation Feb 16 at 11:19

The most efficient solar panel boasts with an efficiency of 22.8%. While the best case for plants have 28.2% of sunlight absorbed by chlorophyll. Even if there are further energy losses in its path to become sugar the vertical farm doesn't circumvent those steps and is therefore irrelevant. So assuming the most generous case (100% electricity -> absorbed by chlorophyll) you would still end up using about $$1/3$$ more land area to provide for your vertical farm.

Traditional farming requires fertile soil, abundant water and sunlight. While the vertical farm setup only requires 1 of the 3. The solar panels could be located in a sunny location not suitable for crops while the vertical farm could be located next to the food processing plant. This way it doesn't compete with traditional farming for space and also reduce carbon emissions associated with transport. Farming in a closed environment reduces the need for pesticides to $$0$$ and also reduces water losses due to evaporation and drainage to effectively $$0$$.

So the answer to your question would be: It depends. If you have abundant fertile soils and water then the good old fashioned way is hard to beat. Scarce water, wasteland unsuitable for anything else? Then yea vertical farming could make perfect sense.

EDIT: There is also the alternative of using greenhouses on said wasteland. In that case I don't really see a case for using vertical farms powered by solar panels with technology available today.

• I already argued that you still go through the same steps in a sense with a detour so naively this should be worse, but I also mentioned reasons, why this naive "more steps is worse" argument might be wrong and you are not really addressing them. Besides, your average efficiency numbers appear to be wrong: en.wikipedia.org/wiki/Solar_cell_efficiency "This is above the standard rating of 37.0% for polycrystalline photovoltaic or thin-film solar cells." And you could also build a greenhouse on a wasteland instead of a vertical farm - then you would use the sunlight directly... Feb 16 at 11:40
• Just had a look at the source for that sentence in wikipedia, and the source did not really corroborate the sentence. So you might be right on that one Feb 16 at 11:47
• Here is where I sourced the efficiency for solar panels: solarreviews.com/blog/what-are-the-most-efficient-solar-panels Granted it applies to commercially available solar arrays right now and might be subject to change in the near future. Good catch on the alternative of using greenhouses, I didn't think of that and will update my answer. Feb 16 at 12:06
• @FelixB. In my answer I'm trying to make the best possible case for vertical farms. I think it does address "why this naive "more steps is worse" argument might be wrong" if it still doesn't make sense assuming $0$ losses in each step. Feb 16 at 12:15
• en.wikipedia.org/wiki/Photosynthetic_efficiency typical crop plants seem to be in the 1-2% range. Let us say this is due to the fact that they only use 1-2% of the light spectrum and lets say that we can create LEDs that only emit that particular light. Then the plants would be able to use ~100% of the light of the LEDs and if the LEDs are 50% efficient and the solar panels 20% efficient, this would still result in 10% efficiency overall. I.e. you could power 5 floors of plants. Where did you get the 28% from? Feb 16 at 12:26

A few consideratiosn to take into account:

• The amount of energy absorbed by plants on a $$1m^2$$ depends on the type of the plants, their density, how efficiently they cover the area with their leaves, etc. In other words, the main gain may come from the fact that a solar panel covers $$1m^2$$ continuously, using the surface more efficiently.
• If plants absorb fraction $$\alpha$$ of light, whereas the solar panels absorb fraction $$\beta$$, then plants will end up absorbing fraction $$\alpha\beta$$. One however could play on the solar panels absorbing wider spectrum and then illuminating plants with a more efficient light.
• Plants, solar panels and any other devices or organisms do use only part of the available energy and throw the rest away - throwing away a part of the available energy is required by the second law of thermodynamics (often neglected in the green energy debates).
• Burning a battery actually produces more energy than using it to power an electric circuit... it is just that harnessing this energy is challenging.