Most affordable/quick method to block light through windows

As Tester mentions, new windows are the most cost and energy efficient in the long haul. However, the upfront cost is substantial, especially if you want to do a lot of windows.

The concept is the same with all window coverings, add an additional layer of air between the primary window and the covering to slow down the transfer of heat. A single pane window is generally .85 r-value. Verifiable r-value numbers are hard to come by for most of these products, but if you do find some, that will give you a basis for comparison.

Other options:

• Storm windows. This is basically installing a secondary window on the outside. These can be quite expensive, or cheaply made using plexiglass. They are cheaper than new windows, reusable, and transparent. The drawback is that there is still a sizable upfront cost and you have to store them somewhere in the summer.

• Secondary glazing. Same idea, except installed inside. Similar advantages and disadvantages to storm windows. They tend to be much thinner and don't change the look of your house.

• Cellular shade. There are some interesting claims with cellular (honeycomb) shades, some claiming to add well over 2 r-value to a window. These will darken the room some and are still quite expensive.

• Window film. This is the cheap option, unless you have curtains laying around. The main drawback is that it's generally opaque and it's more difficult to reuse year-to-year.

• Heavy curtains. Tried and true method used for centuries. Obviously you won't be looking out these windows.

First, try to prevent the sun from hitting the building in the first place. Shade trees, large overhangs, louvers can all do this. There may be micro louver screens available that appear to be heavy duty insect screens but are actually very small louvers. You can easily see through the louvers, but they prevent the sun from hitting the glass.

Second, reduce the impact of what sun does hit the building by reflecting it away. Transparent reflective film on the window and light colored wall finish will work for this measure.

Third, reduce conducted heat. Insulate the wall so the hot outer surface cannot easily heat the inner surface. Use dual glazing the same way, the hot outer pane cannot easily heat the inner pane.

Fourth, prevent the hot inner surfaces from heating the room's air. Typically drapery or similar treatments. Not just the window, the wall too. Even sheer drapes you can see through are better than nothing. They trap air between the glass and the fabric, creating additional insulating value to keep the heat out of the room, and reduce the transmitted light that would have further heated the room interior.

I've covered how to reduce heat gain through the various modes of how it gets into the room, but I've only scratched the surface of ways to do this. Be creative. Figure out ways to achieve what I've outlined with materials that are readily available to you.