In the fullness of time, the Earth will be devoid of exhaustible fuel sources. By then, an entirely new power grid will need to be in place. It will presumably resemble the Largest (aka most productive) Sustainable Power Grid (LSPG) consistent with the laws of physics and chemistry. Although the absolute need for such a grid is still far in the future, it is none too soon to start planning the whole and constructing parts of it. Some of that is happening already, with wind and solar farms springing up in favorable locations. Yet three obvious questions remain: (#1) How productive can the LSPG be expected to be, (#2) how soon should it be built, and (#3) what sort of penalty will have to be paid if it remains incomplete when the need finally arises. We address these and related questions with the aid of a simple dynamic model.