Hot-water supply system, and control method for water heater

A hot-water supply system (1) is equipped with a power conditioner (31) and a water heater. The power conditioner (31) is equipped with: an instruction acquisition unit (303) that acquires an instruction for suppressing the supply of power generated by a generating means to a commercial power system; and a power output unit (305) that, when the instruction acquisition unit (303) receives an instruction, outputs part of the power generated by the generating means, which is less than the generated power, for a period specified by the instruction. The water heater is equipped with: a surplus power information acquisition unit (605) that acquires surplus power information showing surplus power based on the difference between the power generated by the generating means and the power output by the power output unit (305) during the period specified by the instruction; and a boiling unit (604) that, when the surplus power information acquisition unit (605) acquires the surplus power information, boils water with a capacity corresponding to the surplus power.