A method of controlling a heating system that operates using power supplied from a power supply source, the heating system including: a heat pump 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, the heat pump operating in either a heating mode for generating the heat to be radiated by the radiating unit or a defrost mode for removing frost formed on the heat pump, the method comprising: obtaining, from the power supply source, an output modulation instruction specifying an output modulation period during which power consumption by the heat pump is modulated; determining a first defrost condition to be used in a period outside the output modulation period and a second defrost condition to be used in the output modulation period, the first defrost condition and the second defrost condition each including a defrost initiation condition for causing the heat pump to start operating in the defrost mode and a defrost termination condition for causing the heat pump to stop operating in the defrost mode; and controlling operation of the heat pump to cause the heat pump to start operating in the defrost mode based on the defrost initiation condition determined in the determining being met and cause the heat pump to stop operating in the defrost mode based on the defrost termination condition determined in the determining being met, wherein in the determining, a continuous operation time of the heat pump in the defrost mode in the output modulation period under the second defrost condition is made to be shorter than in a period outside of the output modulation period under the first defrost condition by making at least one of the defrost initiation condition or the defrost termination condition included in the second defrost condition different from that of the first defrost condition. a heat pump 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, the heat pump operating in either a heating mode for generating the heat to be radiated by the radiating unit or a defrost mode for removing frost formed on the heat pump, the method comprising: obtaining, from the power supply source, an output modulation instruction specifying an output modulation period during which power consumption by the heat pump is modulated; determining a first defrost condition to be used in a period outside the output modulation period and a second defrost condition to be used in the output modulation period, the first defrost condition and the second defrost condition each including a defrost initiation condition for causing the heat pump to start operating in the defrost mode and a defrost termination condition for causing the heat pump to stop operating in the defrost mode; and controlling operation of the heat pump to cause the heat pump to start operating in the defrost mode based on the defrost initiation condition determined in the determining being met and cause the heat pump to stop operating in the defrost mode based on the defrost termination condition determined in the determining being met, wherein in the determining, a continuous operation time of the heat pump in the defrost mode in the output modulation period under the second defrost condition is made to be shorter than in a period outside of the output modulation period under the first defrost condition by making at least one of the defrost initiation condition or the defrost termination condition included in the second defrost condition different from that of the first defrost condition. obtaining, from the power supply source, an output modulation instruction specifying an output modulation period during which power consumption by the heat pump is modulated; determining a first defrost condition to be used in a period outside the output modulation period and a second defrost condition to be used in the output modulation period, the first defrost condition and the second defrost condition each including a defrost initiation condition for causing the heat pump to start operating in the defrost mode and a defrost termination condition for causing the heat pump to stop operating in the defrost mode; and controlling operation of the heat pump to cause the heat pump to start operating in the defrost mode based on the defrost initiation condition determined in the determining being met and cause the heat pump to stop operating in the defrost mode based on the defrost termination condition determined in the determining being met, wherein in the determining, a continuous operation time of the heat pump in the defrost mode in the output modulation period under the second defrost condition is made to be shorter than in a period outside of the output modulation period under the first defrost condition by making at least one of the defrost initiation condition or the defrost termination condition included in the second defrost condition different from that of the first defrost condition.