Exactly what winnie said.
If you do not have knowledge of calculus, then a way to see how power in the load varies as load resistance varies with constant source voltage and constant source internal resistance is to perform calcultations for incremental load resistance changes and calculate the load power. You will find that this maximum power transfer occurs at Rload = Rinternal.
Edison understood that a practical power distribution system could not be based on maximum power transfer as many others thought in 1879. Rather Edison understood that a low source impedance and moderately constant voltage distribution system was required.