Work has to be done to move charge carriers through a power source so energy is lost when current flows through the source and the source gets hot.In a solar cell the materials resist the flow of charge. In a chemical cell the chemicals react together.This means that not all the energy is transferred to the circuit and the potential difference measured at the terminals is less than the actual e.m.f. measured when current is not flowing.From Kirchoff's second law:The e.m.f. = the sum of potential differences + lost volts
ε = IR + Ir