Ok then, it easy.
Simple and simple-minded calculation.
I'll crib from what we know about solar energy intensity here on Earth, 1470W/m2.
The sun and moon subtend the same solid angle size in the sky so the geometry is easy.
The output energy of a hot object scales as T^4. Sun surface temp ~6000K Moon surface temp, let's say 100K.
(100K)^4
-------------- = 7.7*10^-8 (0.000000077)
(6000)^4
So an extravagantly generous upper limit is ~ 100 micro Watts / m2