Calculating solar pressure

[IcedB]

Can anyone explain how the following formula is derived:
Force (F) = 2(P x A)/c
P=power A= Area

[Tide]

It would appear that's just twice the momentum flux of radiation reflecting off of some area:

2 n \frac {h \nu}{c} A = 2 \frac {P A}{c}

where n is the photon density. In other words, energy flux is the same as momentum flux divided by c.