orbit - How much light does Jupiter project onto the surface of Ganymede?

How much? Well how accurately do you need it? How do you want it quantifying? And in what wavelength range?

Jupiter scatters a fraction of its incident sunlight. It also has its own luminosity (predominantly in the infrared).

A quick calculation:

The solar constant (flux at 1 au) is about 1370 W/m$^{2}$. Jupiter is situated about 5.2 au from the Sun (it varies by about +/- 5%) and has a radius of 70,000 km. The albedo is about 0.34.

Thus it receives about $7.8times 10^{17}$ W from the Sun and radiates about $2.6times 10^{17}$ W back into space. Assuming this is done more-or-less isotropically into a hemmisphere, then Ganymede, at a distance of 1,070,000 km from Jupiter, receives a flux of only 0.1 W/m$^2$ multiplied by the fraction of the sunlit hemisphere that can be seen.

This compares with the $sim 50$ W/m$^2$ it receives from the Sun!

This surprising result (to me) puts into perspective all the simulations you see of things in orbit around Jupiter. The planet is still pretty faint compared to the Sun.

I'd be grateful if someone could double-check the sums!

[The intrinsic infrared luminosity of Jupiter is less than a fifth of what it receives from the Sun, so this would increase, but not double the received power.]