Answer
5 nW.
Work Step by Step
The power is the work done per unit time. The work done to move a charge Q against a potential difference V is QV.
In this problem, we are given the charge density, so multiply by the effective surface area of the cell (length times circumference) to find the charge that is moved.
$$P=\frac{W}{t}=\frac{QV}{t}=\frac{Q}{t}V$$
$$=3\times10^{-7}\frac{mol}{m^2\cdot s}(6.02\times10^{23}\frac{ions}{mol})(1.6\times10^{-19}\frac{C}{ion})(0.10m)\pi(20\times10^{-6}m)(0.030V)$$
$$=5\times10^{-9}W$$