How many calories does a plant photosynthesize per day?

[chingel]

Hello,

Related to a thought of why animals don't use photosynthesis (typical answer is that photosynthesis would provide only marginal energy compared to the needs of an animal), I am interested in how much energy does a plant produce using photosynthesis?

Any example or estimates is good to know, for example mediterranean climate and sun angle, clear day, calories per average leave per hour or per hour for m2 of leaves; or whatever else example anyone might have data for.

 

[phyzguy]

I got interested in this a few years ago, and compiled the attached spreadsheet on the energy gathered by various food crops, and how efficient they are at converting sunlight into food. I'd be interested in your comments.

 

[chingel]

That is a pretty low efficiency, but that is only how much of the energy the plant uses to grow the crops.

I have a question about the units, is the calories/kg row meant to be kilocalories? Why do you multiply it through by 10^6 to get megacalories, or is that row not meant to be kilocalories?

 

[phyzguy]

Both the Food value (cal/kg) and the Mcal/acre/year columns are in food calories (i.e. kilocalories). I just converted it to megacalories to make the numbers of order 1. When I converted to Watts, you see that I multiplied by 4.2*10^9 to convert mega-food-calories into Joules, so I think I've done it right.

You're right, this efficiency is just how efficient the crop is at producing human consumable food, but I think this is what you were asking about in your original post. If you look at all of the organic matter produced by the plant, I think the efficiency goes up to a few %, but I don't have those numbers.