For mitigating the negative impacts of high penetration photovoltaic power generation systems on the existing electric power system, the increase in the daytime electricity demand by the shift from the night-time or the other energy use is essential. Focusing on a heat-pump water heater (HPWH), this study proposes an autonomous scheduling method of the daytime operation of HPWH. In the proposed method, the reference stop-time is determined based on the past hot-water demand. The actual start-up time and stop-time varies with days depending on the change in the amount and the timing of the hot-water demand with days. The proposed method was tested by using the hot-water demand observed in 12 households for a year. As a result, the ratio of in-use HPWHs increases lineally with the increase in the extraterrestrial irradiance when the extraterrestrial irradiance is larger than the certain value. The sudden increase and/or decrease in the ratio of in-use HPWHs are avoided by dispersing the reference stop-time depending on the amount of hot-water demand. Consequently, the numerical simulation shows that the proposed autonomous scheduling method effectively works to schedule individual HPWH so that total number of in-use HPWHs follows the daily change in the extraterrestrial irradiance.
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