Sun radiation is about 1.4KW/m2 at the Earth orbit and the pressure is 0.00001N/m2 - basically the same numbers as GP mentioned for the photonic drive - can be

Sun radiation is about 1.4KW/m2 at the Earth orbit and the pressure is 0.00001N/m2 - basically the same numbers as GP mentioned for the photonic drive - can be calculated as 1.5KJ (for roundness of numbers) of photons divided by c which gives 0.000005kg-m/s of momentum per second, double that for bounce and thus 0.00001N. As photons are usually the things being thrown for the thrust in the mass-less thrust schemas, then those photonic thrust numbers seems to be a theoretical limit .