Stems back to matching the power of the two stroke glow fuel engine to the airplane. The numbers "15" and "40" equate to the engine displacement in cubic inches i.e. a 0.15 cuin engine and a 0.40 cuin engine.

When the four stroke engines first came out, because they only "hit" on every other stroke there was a general opinion that for a conversion from a two stroke to a four stroke you doubled the size of the displacement. For a Kadet LT 40 which flies well on any 0.40 cuin two stroke, you needed a 0.80 four stroke to get the same performance. THIS IS NO LONGER TRUE!!!

Now if you want to switch to electrics, the easiest is just look at the eflite range of motors they name them to the equivalent two stroke power.

MTC YMMV