Answer
See answers.
Work Step by Step
a. We are told that the D-cell battery provides 25 mA at 1.5 V for 820 h, and costs \$1.70.
$$Energy = Pt=IVt=(0.025A)(1.5V)(820h)\frac{1kW}{1000W}=0.03075kWh$$
$$cost/kWh=\frac{\$1.70}{0.03075kWh }\approx \$55/kWh$$
b. We are told that the AA-cell battery provides 25 mA at 1.5 V for 120 h, and costs \$1.25.
$$Energy = Pt=IVt=(0.025A)(1.5V)(120h)\frac{1kW}{1000W}=0.0045kWh$$
$$cost/kWh=\frac{\$1.25}{0.0045kWh }\approx \$280/kWh$$
c. Compare the energy costs per kWh with those from house current.
For a D-cell, \$55.28/\$0.10 = 550x as much.
For a AA-cell, \$277.78/\$0.10 = 2800x as much.
