How much energy do plants produce?
Consider a magic system that follows the Eragon model; "doing it by magic takes just as much (biological) energy as doing it the mundane way".
In Eragon, magical energy can be drawn from other living things besides the caster. Thanks to questions such as How much can a magician lift if constrained by her own body's energy?, Magic and physics with human power output and How can wizards do such powerful things running on pure human metabolism?, we have some general idea of how much energy animals (including humans) can contribute for this purpose.
What about plants? Eragon is stated to also draw magic from grass and trees and such, but is this realistic? If I had a "magic converter" that could be hooked up to some plants and losslessly convert their "energy" into electricity (or whatever, really; I'm using electricity because it's a form for which we're used to thinking about measurable energy), how many watts could I reasonably expect to produce, continuously? (If any?)
Since the above is probably too non-specific, let's talk numbers for some specific producers:
- One square meter of a typical lawn.
- One "average" 10m tall tree.
- One bush/hedge that is about 1 meter tall/wide.
Note 1: it's okay if this process (while active) stunts the plants' growth, but it shouldn't kill them.
Note 2: I did find this article which, if I did the math right, appears to claim that "a one-square-meter garden" can generate about 3 watts; is this plausible? (It does go on to say that "15 square meters [...] would be enough to charge a cell phone", which is hardly impressive.)
Note 3: For comparison purposes, a human is probably good for around 50-75W. I'm guessing other animals are at least in that ballpark (adjusted for mass, obviously!).
1 answer
According to this answer to a different question:
On Earth, a large tree on average collects maybe 200 calories of energy in a day.
(Thanks, Dan! This also gives the same answer, but could be where Dan got the number.)
That's about 2.3 watt-hours, or slightly better than a AAA battery (but only a little better than half a AA battery); maybe enough to run an under-powered cell phone under modest load. Continuously, that works out to a whopping tenth of a watt. Hardly impressive.
An adult human, on the other hand, should be able to supply around 50 watts. (Again, this is continuous, and trying to avoid radically dietary alterations; the numbers for short bursts are much, much higher.) As a magic power source, one adult human is roughly equal to one forest, or several hundred trees.
If the magic user doesn't want to totally drain (and thus, kill) plants, it's probably not worth even bothering with them.
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