When an auxiliary electric heater (AEH) is incorporated in a thermosyphon solar water system, the system performance becomes complex and difficult to simulate and analyse. Using the fixed-node, equally-convecting-mixing model, we have performed detailed simulations and analyses of the effects of the power of the auxiliary electric heater (PAEH) in the storage tank on the performance of a TSAEH and have also derived a formula for estimating the optimal value of the power of the auxiliary electric heater.