The letter suggests utilizing a recursive-least-squares identification algorithm for real-time estimation of supercapacitor equivalent capacitance and resistance. Estimation is required since both parameters are subject to age, temperature, and terminal-voltage-based variations in addition to typical 20% tolerance of manufacturer provided values. The proposed approach allows calculating the device instantaneous state of energy used as a fuel gauge instead of the commonly adopted state of charge. Experimental results are given to verify the feasibility of the proposed method.