Abstract: A maximum power level sensor and method for reliably providing instantaneous estimates of maximum power level for a photovoltaic system including one or more photovoltaic cells under prevailing conditions of irradiance, array temperature and spectrum of sunlight. The maximum power level sensor produces a reference voltage signal which is proportional to the maximum power level of the photovoltaic system. The maximum power level sensor comprises a reference circuit including at least one reference photovoltaic cell and a thermistor network in parallel with the reference cell. The thermistor network comprises at least one negative temperature coefficient thermistor, at least one resistor in parallel with the thermistor, and at least one resistor in series with the thermistor and the parallel resistor. The thermistor is positioned in or on the photovoltaic array. The reference cell is preferably constructed of materials which are similar to those of the photovoltaic cells in the photovoltaic system.