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This paper studied the payback period of grid-connected photovoltaic (PV) panels by the net present value method. The PV performance data were acquired by on-site measurements of two rooftop projects in subtropical Hong Kong. The sensitivity of various variables to the payback period was evaluated by the Extended Fourier Amplitude Sensitivity Test. The monetary payback periods were evaluated at different values of the most relevant variables and compared with the embodied energy and greenhouse gas payback periods. The PV panels of the two projects produced 122–143 kWh/m2 electricity per year in Hong Kong, which saved 139–163 HKD electricity tariff per square meter per year. The sensitivity analysis showed that the monetary payback period was sensitive to the initial cost and tariff increase rate uncertainties. The PV monetary payback period varied from 13.4 to 16.8 years at different tariff increase rates and investment costs, based on the current carbon trading benefit. The monetary payback period was much greater than the embodied energy and greenhouse gas payback periods, which were 10.8–12.7 years and 5.3–6.2 years, respectively. Implications of the payback period differences were discussed.