Solution:
This post has an explanation of the physics by Sir Alan and an example by Heureka.
Here is a reproduction of Heureka’s math, using the data from this question.
I=P4⋅π⋅r2P=23 wattr=19 500 000 mI=234⋅π⋅195000002I=4.8⋅10−15 Wm2
Closest answer from list: 5 x 10−15 W/m2
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It’s usually not necessary to reinvent the wheel, but you must be careful not to inflate the tire on the wheel with hot air from a blarney bag.