The physics
The potential V at a point is work per coulomb. It is the work to bring a + charge there from far away.
V = kq / r (k = 9 × 10⁹ N m²/C²)
A hand carries a charge q slowly. It does work q times the change in V. Only the two end points matter.
W = q(VB − VA) from far away: W = qV
Potentials add as plain numbers, with their signs. Fields add as arrows.
The energy of a group of charges is kq₁q₂/r for every pair, counted once.
U = Σ kqiqj / rij