Technically, this is not a bug. It is due to the behavoir of every splitter (energy splitter or the internal splitter in engines, which activates when multiple outlets are connected). An energysplitter does what it says. Its splits the available units exactly in half, regardless of actual need.
In your case: the windmill produces 20 units. The splitter consumes one of it, leaving 19. those are divided in 9.5 for your stove and 9.5 for the charging station.
Sadly, until they...