>>ivank+(OP)
I live at 52.5 degrees North. Cosine to the power 4 being what it is, my neighbours and I could be experiencing significantly lower levels of daylight intensity in all seasons than people who live at lower latitudes. In the summer, we will see similar or slightly more daylight integrated over the duration of the day because of the longer days.
The OA does not specify the latitude at which he(? assuming the OA is the site owner's work) is living.
>>keithp+ta1
Cosine to the fourth power is the wrong formula, it is for camera lenses and takes into account the apparent size of the pupil from both sides, as well as distance attenuation for objects on a fixed plane. Solar illuminance is proportional just to the cosine, no power.