Solution
= Solution
After three <half-lives>, $P(T)=P_0/8$. Therefore the minimum initial thermal <power> is
$$
\boxed{P_0=8(250\ \mathrm W)=2000\ \mathrm W.}
$$
Equality gives the stated terminal <power> exactly in the idealized <exponential decay> model. The associated duration is $415.2$ days. No thermal-to-electrical efficiency is needed because both specified powers are thermal.