Electrical potential difference between planets
I dunno how or why a planet would have a significant net charge, but supposing it did, here's some initial calculations. 

If the planets have a different net charge, you can treat them as capacitor plates.
Capacitance in its simplest form is  

$C = \frac{\epsilon *A}{d}$
The permittivity $\epsilon$ of vacuum space is 8.85 x 10-12 farad per meter .
Let's choose a nearby (nonexistent) planet at 5 LY = 5 * 9.461e+15 m .  

Looks like the capacitance is pretty close to zero.  So the energy available,

$E = \frac{CV^2}{2}$  is going to be nonexistent unless there's a seriously large voltage.