Summary

Electric potential energy is work required from external force to assemble charges from infinity. Infinity set is the zero reference.
Elastic potential energy: U_E=\frac{1}{4\pi \epsilon_0}\frac{q_1q_2}{r}=k\frac{q_1q_2}{r}
Positive U_E means repulsive interaction from like charges.
Negative U_E means attractive interaction from unlike charges.
For multiple charges, add potential energy of each pair.
U_E is a scalar.
Energy is conserved, as charges move potential is traded for kinetic.