Inspired by this [answer](https://worldbuilding.stackexchange.com/a/51459/4995), If that Lunar-plant is used as a weapon (pointed toward specific cities/forest/seas/lakes), how much damage it could really do? Is that able to burn cities, or anything?

I assume for fun Full Moon.

To recap here's how the weapon works:

* Fill the moon with mirrors, and direct them to reflect light on a specific point of the Earth
* Due to mechanical orientation errors, and small imperfections the light is not concentrated into a dot, but into a small circle of about 3 kilometers diameter and with a gaussian distribution of sunlight.
* Like in solar plants, mirrors are slightly curved (parabolic shape, with focus point on Earth) in order to try to focus sun of a single mirror to a single spot.

The working principle is the same of Solar plants, just on much bigger scale:
[![enter image description here](https://i.stack.imgur.com/BvYOv.jpg)](https://i.stack.imgur.com/BvYOv.jpg)
(picture from [wikipedia](https://en.wikipedia.org/wiki/Solar_power_plants_in_the_Mojave_Desert))