That's an interesting point! Indeed without the Sun, the planet would be devoid of life. So as a corollary one might conclude that energy from the Sun is necessary for life and hence for value creation. Coupling a currency to the Sun's output, however, implies an underlying assumption of growth (through the Sun's energy). This is a problem because yearly economic growth is a lot less steady than the yearly amount of energy we get from the Sun. In turn, this might lead to periods of significant in- and deflation. Fixing this does not seem obvious to me!