It is very simple.

1. Electric engines are far more efficient than fossil fuel engines.

2. The generation of electricity from a large powerplant to charge an Electric Aircraft's batteries whether a fossil fuel powerplant or a sustainable energy one is going to be an order of magnitude more efficient than converting fossil fuels into energy in a minipowerplant on the airplane. This is a simple consequence of the efficiency of scale.

3. Electric Aircraft can maximally reduce the cost of energy by charging when the grid is oversupplied with electricity, this provides a capability to trade waiting time for reduced energy footprint/grid impact and in many situations the energy stored in an Electric Aircraft's batteries that can be used to then accomplish real work might have simply gone to waste if there hadn't been another battery hooked up to the grid.

4. The production of aviation fuel in particular is costly and energy intensive requiring discrete production infrastructure whereas the charging of a battery for an electric aircraft just takes normal old electrons like any old battery.

If the cost of using an Electric Aircraft is minimal, than comparing it to fossil fuel powered Aircraft isn't really relevant since the fossil fuel powered Aircraft is going to use far more energy and resources to accomplish the same amount of work.

That doesn't mean there aren't drawbacks to Electric Aircraft, but one of their undeniable advantages is that they are far less wasteful with energy use than normal Aircraft in a way that is difficult to compare directly with simple numbers.

How do you value alternative electricity production that is going to waste?

That is a question with a manifold of implications and it makes your question far more difficult to parse than you realize.