A method of controlling a heating system that operates using power supplied from a power supply source, the heating system including: a heat pump unit that generates heat using the power supplied from the power supply source; and a radiator unit that radiates the heat generated by the heat pump unit, the method of controlling the heating system comprising: obtaining, from the power supply source, information specifying an output modulation period during which power consumption by the heat pump unit is to be reduced; and controlling, based on the information obtained in the obtaining, an amount of the heat generated by the heat pump unit, wherein in the controlling, the heat pump unit is caused to: generate a first amount of heat per unit time in a period other than the output modulation period; and generate a second amount of heat per unit time during the output modulation period, the second amount of heat being less than the first amount of heat, wherein the heat pump unit heats water flowing therein with the generated heat, and outputs the heated water, and in the controlling, the heat pump unit is caused to: output heated water of a first temperature in a period other than the output modulation period; and output heated water of a second temperature during the output modulation period, the second temperature being lower than the first temperature, wherein further in the obtaining, an inlet water temperature and an outlet heated water temperature are detected on a per unit time basis, the inlet water temperature being a temperature of the water flowing into the heat pump unit and the outlet heated water temperature being a temperature of the heated water exiting the heat pump unit, and in the controlling, the temperature of the heated water exiting the heat pump unit is reduced from the first temperature to the second temperature by causing the temperature of the heated water exiting the heat pump unit to reduce by a given amount of variation each time a difference between the outlet heated water temperature and the inlet water temperature exceeds a threshold. a heat pump unit that generates heat using the power supplied from the power supply source; and a radiator unit that radiates the heat generated by the heat pump unit, the method of controlling the heating system comprising: obtaining, from the power supply source, information specifying an output modulation period during which power consumption by the heat pump unit is to be reduced; and controlling, based on the information obtained in the obtaining, an amount of the heat generated by the heat pump unit, obtaining, from the power supply source, information specifying an output modulation period during which power consumption by the heat pump unit is to be reduced; and controlling, based on the information obtained in the obtaining, an amount of the heat generated by the heat pump unit, wherein in the controlling, the heat pump unit is caused to: generate a first amount of heat per unit time in a period other than the output modulation period; and generate a second amount of heat per unit time during the output modulation period, the second amount of heat being less than the first amount of heat, generate a first amount of heat per unit time in a period other than the output modulation period; and generate a second amount of heat per unit time during the output modulation period, the second amount of heat being less than the first amount of heat, wherein the heat pump unit heats water flowing therein with the generated heat, and outputs the heated water, and in the controlling, the heat pump unit is caused to: output heated water of a first temperature in a period other than the output modulation period; and output heated water of a second temperature during the output modulation period, the second temperature being lower than the first temperature, output heated water of a first temperature in a period other than the output modulation period; and output heated water of a second temperature during the output modulation period, the second temperature being lower than the first temperature, wherein further in the obtaining, an inlet water temperature and an outlet heated water temperature are detected on a per unit time basis, the inlet water temperature being a temperature of the water flowing into the heat pump unit and the outlet heated water temperature being a temperature of the heated water exiting the heat pump unit, and in the controlling, the temperature of the heated water exiting the heat pump unit is reduced from the first temperature to the second temperature by causing the temperature of the heated water exiting the heat pump unit to reduce by a given amount of variation each time a difference between the outlet heated water temperature and the inlet water temperature exceeds a threshold.