Abstract: With the promotion of the “self-generation” mode in photovoltaic power generation and the introduction of peak and valley tariff policies, the issue of selecting the economically efficient installed capacity for air source heat pump coupled with electric boiler heating system has become more prominent. In light of this, we propose a method for calculating the installed capacity that takes into account the impacts of PV power generation and peak and off-peak tariffs. We use the optimal unguaranteed days（OGD） of the air source heat pump as the reference value for selection. A new module has been developed by Trnsys to create a system model with automatic optimization capabilities. After verifying the accuracy of the model using test data, we calculate the recommended OGD values for five representative cities based on the model. The results show that the OGD value is influenced by both the building load size and the installed PV capacity. When the average OGD value is used as the recommended value, the maximum error in the annual cost value of a single building corresponding to the average value of OGD is 8.17%. This ensures the accuracy of the calculation. On average, the OGD value decreases by 46.83% with warming in each climate subzone. The maximum OGD value in Beijing is only 5. This suggests that the addition of photovoltaic power generation to such systems in cold B zone cities can be considered without auxiliary heat sources.