That would be enough to be useful.
Years ago I had a desktop clock (different, ancient tech) that used a potted plant--or a piece of fruit, potato, etc.--as the battery. It worked. Actually, it would go a while on our tap water alone (impurities)!
They didn't describe it as energy from photosynthesis, but I suppose that's what it was: the chemical action of the plant in the soil only has one power source: the sun.
Now, how MUCH better is the newer tech than what we had...