The cost for repairing a standard hot water heater varies based on the nature of the repair work, not the size of the unit, so large water heaters typically don't cost more to maintain and repair. Large commercial units, however, may command higher repair bills.
Residential water heaters generally use common parts, and the popularity of major brands means parts are typically easy to acquire. Commercial units sometimes use specialized parts, potentially driving up repair bills. Tankless water heaters, however, can sometimes command high repair costs. Traditional water heaters usually rely on only a few electrical parts, while tankless designs often use multiple stages that require electrical components. This, combined with the more complex designs, can make repairs time-consuming and expensive.