We derive a simple, robust scaling law, which explains naturally how the well-known anticorrelation between final solar wind speed and freezing-in temperature results from the loss of radiated energy in the corona. Furthermore, if the Sun injects roughly fixed electromagnetic energy per particle, this law provides a unified theory for the source of solar wind: fast tenuous solar wind from dark coronal holes; slow dense wind from hotter, brighter regions; and bound but unstable plasma in extremely hot regions, which may be related to solar transients. The scaling law is not an extension or a new variant of previous solar wind models, but rather a requirement on all solar wind models, which should apply quite generally to magnetically driven winds. Thus, this paper reveals an underlying connection between the solar wind and its coronal source.