Two point charges q1   and  q2 are placed in an external uniform electric field as shown in figure. The potential at the location of q1   and  q2 are V1   and  V2, respectively. The effect of q1   and  q2 is not included in V1   and  V2, i.e., V1   and  V2 are potentials at location of q1   and  q2 due to external unspecified charges only. Then electrical potential energy for this configuration of two charged particle is

The potential energy of the system can be found by computing the work done by external agent in assembling the system without changing the kinetic energy of the system. Let us first bring q, from infinity to the desired location, then in doing so we have to do work against external electric field which is equal to W1=q1V1  (potential at infinity is considered as zero). Now, we will bring q2 from infinity to the desired location, to do so, we have to do work against external electric field and against electric force of q1 so work done is W2=q2V2+q1q24πε0r So total work done isU=q1V1+q2V2+q1q24πε0r