The photon energy is proportional to its wavelength, but its not relevant.
Look up Plank's Law. Its an equation for black body radiation (like, effectively, the Sun). That'll tell you how much heat the Sun radiates at any given wavelength.
That's not the same spectrum as falls on your house, the atmosphere has absorbed or scattered some of it. But the distribution that falls on your house is well known.
A dielectric mirror is fairly narrow band, compared to this distribution. A dielectric mirror might work well for a factor of two bandwidth, but solar radiation is significant for about a factor of ten.
Also dielectric mirrors are: 1. Hard to design 2. Expensive relative to paint 3. Rather fragile compared to paint.
Point 3 is important. You can't just pressure wash your siding to clean it without stripping the dielectric mirror. But if you don't clean your siding the mirrors can't reflect the sunlight!
As to heat, heat is technically thermal energy transferred from system A to system B by a temperature difference. It has units Joules. In practice it also refers to the thermal energy a system has.
Thermal energy is the kinetic energy a material's atoms and molecules have because they're vibrating about.
Finally, these are hard questions because they start to hit upon the fundamental definitions in physics and everyday misuse of definitions by physicists themselves. Furthermore laymen have used the word "heart" far before physicists stole the term to refer to something similar but technicals in their field. They did the same thing with the words mass and weight when we needed to distinguish between the force and the mass in the 17th century (its perfectly Ok to say "weight" to refer to mass in most situations including most engineering, chemistry and physics applications)