Battery-powered electricity is very expensive compared with that available from a wall receptacle. Estimate the cost per kWh of
(a) an alkaline D-cell (cost $1.70) and
(b) an alkaline AA-cell (cost $1.25). These batteries can provide a continuous current of 25 mA for 820 h and 120 h, respectively, at 1.5 V.
(c) Compare to the cost of a normal 120-V ac house source at $0.10/kWh.