Just as an object being held above the ground has mechanical potential energy, individual charges have what is called electric potential energy. This value is measured in volts, is a scalar value (meaning it has a magnitude but no direction) and calculating the electric potential energy isn't difficult with the right information available.
Determine the values of the two charges, in Coulombs. At least one of the charges must be known as well as the distance between them. Use the equation F = (q1 x q2)/(4 x pi x E x r²) where q1 and q2 are the charge values, F is the electric force acting on the charges, r is the distance between the charges and E is the permittivity of space equal to 8.8 x 10^-12 F/m to solve for the charge values.
Convert the given distance between the charges to meters, if it is not already in these units.
Multiply the values of the charges together, then multiply by Coulomb's constant, 9 x 10^9 Newton meters squared per Coulomb squared. Divide this product by the distance between the charges and you've got the electric potential between the two charges.
Set the zero point to an infinite distance if you have a single point charge. This will turn the voltage to zero since setting the distance "r" to infinity yields zero. Electric potential energy is defined as the work required to move two charges from a theoretical separation of infinity to a finite distance, and the zero point is conventionally used to determine the amount of work done on the charge.