So assuming a 100% efficiency in the conversion of the products of the photosynthesis to light, a 12h+12h light/dark cicle, 100% canopy coverage, ignoring the branches, ignoring that plants need energy for other activities and ignoring that the plant will release 1/2 of the light to the sky instead of to the floor, we get 2W/m^2 for illumination at night.

For a 3mx3m room (10ftx10ft), I'm happy with a 18W LED lamp, and streets are usually darker. (I can't find the official recommendation.)

I'm ignoring a lot of inefficiency sources, so assuming a 100% efficiency after photosynthesis is too optimistic. Organic reactions are usually far from 100% efficiency. I think that a 10% is still optimistic, but it's difficult to give an accurate estimation.

So I think it's not ruled out by the laws of physics, but I don't expect it to be viable as a street illumination method. (On the scale from 1 to Nessie, I'd rate it as Yeti.)

Most organisms are exceedingly energy efficient. [Build me an humanoid-like robot with all its attendant capabilities that can be powered on 2000 calories of snickers bars and apples...]

In order for these things to put of the kind of wattage we're familiar with even in a candle, you'd have to be feeding those plants some really crazy food (not what they usually do), or be charging them up with some super crazy laser (kinda defeats the point of a bio-light).

However, it's still super cool. And you could probably find all sorts of ways to actually charge up a plant in the long run, and as technology matures. Maybe have a whole 'forest' providing energy to a single 'lamp', or be able to provide some direct reservoir of maple syrup the plants can tap into for fuel, or store up energy over a few days for a few hours of light. The energy can come from somewhere else, but it does have to come form somewhere.

But yep - there are some basic laws of physics around energy expenditures that need to be taken into account here.

https://www.engineeringtoolbox.com/light-level-rooms-d_708.h...