Solution
= Solution
The stored energy is $0.75(7)=5.25\,\mathrm{Wh}$, and the load draws $P=IV=(20\times10^{-6})(2.5)=50\,\mu\mathrm W$. Hence
$$
\boxed{t=\frac{5.25}{50\times10^{-6}}=105000\,\mathrm h\simeq12.0\text{ years}.}
$$
This is an ideal energy-budget estimate using 365.25 days per year. It assumes the stated energy is usable at this load, constant current and <voltage>, and negligible self-discharge, ageing and reserve capacity. No biological operating model is required.