at equator sun rays drop at a 90* angle - full power
at tropiques they drop at (90-23.5)*angle - ~70% power
at polar circles they drop at 23.5* angle - ~30% power
at poles it's just 0* that means they're tangent to ground - 0% power
(that's valid only at equinox)
the general idea is that the energy is proportional to cosy where y is the angle that sun rays make with the ground
the higher the latitude, the lower the angle => less energy