An electricity storage device for a solar energy harvesting device comprising at least a first electricity storage unit, at least a second electricity storage unit, a battery monitoring unit and a power converter. The second electricity storage unit is for coupled to an exterior power supply system which charges/discharges the second electricity storage unit in a predetermined voltage. The battery monitoring unit controls the power converter according to the output voltage of the first electricity storage unit for converting the electricity of the first electricity storage unit to the second electricity unit. When the output voltage of the first electricity storage unit is larger than a start voltage, the electricity of the first electricity storage unit is converted to the second electricity storage unit. When the output voltage of the first electricity storage unit is less than a stop voltage, the battery monitoring unit disables the power converter.