  The solar-wind energy flux measured near the ecliptic is known to be
independent of the solar-wind speed. Using plasma data from Helios, Ulysses,
and Wind covering a large range of latitudes and time, we show that the
solar-wind energy flux is independent of the solar-wind speed and latitude
within 10%, and that this quantity varies weakly over the solar cycle. In other
words the energy flux appears as a global solar constant. We also show that the
very high speed solar-wind (VSW > 700 km/s) has the same mean energy flux as
the slower wind (VSW < 700 km/s), but with a different histogram. We use this
result to deduce a relation between the solar-wind speed and density, which
formalizes the anti-correlation between these quantities.
