Power generator

@LownirPL
what @Dereric said

@SamTheFox
Your solution was also my first idea, but in my opinion the problem is that you have to deactivate it manual. Something like:

on start
if (fuel [Battery]) less than 0.5 then
Set AG8 to true
if (fuel Battery) more than 0.95 then
set AG8 to false

(AG8 is only an example, but I suggest you to bind your solar panels or power generators in an AG because it makes things a little bit easier.

@SamTheFox ok thanks

this requires you to simply deploy solar panels using vizzy

on start
if (fuel [Battery]) less than 0.5 then
part (insert the partID of a solar panel) [Activated] to (true)

if you want to magically add battery to your craft, vizzy can't do that