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 $\mathrm{mA}$ for 820 $\mathrm{h}$ and 120 $\mathrm{h}$ , respectively, at 1.5 $\mathrm{V}$ . Compare to normal 120 $\mathrm{-V}$ ac house current at $\$ 0.10 / \mathrm{kWh}$ .