For that to be the case, you have to be assuming that within that cell there really is an ideal 1.5 V voltage source.My understanding is the internal resistance of a cell ( at a given current) is just that .... An alkaline cell with 1 Ohm int resistance , with a 1 Ohm load connected will have a total circuit resistance of 2 Ohms , hence current of 1.5/2 = 0.75 A ... to find heat generated in cell I x I X R = 0.75 x 0.75 x 1 = 0.562W
You also have to be assuming that ALL of the heat being produced is coming ONLY from resistive joule heating -- that there isn't any heat being produced due to chemical reactions. You also have to be assuming that there aren't any other mechanisms at play, such as the production of hydrogen gas.