PV cell output calculation, when a charging limiter is involved
Irradiance is the solar power available per unit area (eg. W/m2) at any given instant. Insolation is the total energy (power * time) received over a specified period. For charging a battery the insolation is generally more important, but peak irradiance may also be a factor.

In your case the regulator limits useful irradiance to what it needs to produce 12V at 1A. Any excess power will simply end up wasted or not used, so the maximum power collected flattens off at a bit over 12W. How this relates to irradiance depends on how much your panels can actually collect. If they can't produce more than 12W anyway then the normal formulas apply. If they can then the irradiance required to produce 12W becomes the effective upper limit. This obviously also limits the useful insolation, but by how much? The answer isn't simple because it depends on the shape of the irradiance curve (useful insolation is the area under the irradiance curve after its top has been chopped off).                          

You may think that having a panel which can produce a lot more than you can use is a waste, however it may still have advantages. One is that the power collected is more constant for longer, so the battery is more likely to get fully charged even on cloudy days. Another is that if a load is also connected then excess power can be diverted to it, leaving more available to charge the battery. Also the battery can be charged at a more gentle rate than in a setup which must charge rapidly to collect as much energy as possible during the short time of peak irradiance.