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!).