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I saw a post on CleanTechnica last week that highlighted a report prepared for the International Window Film Association that said "Window films are a more cost effective retrofit opportunity for homes than other measures, however, especially in homes with single pane glass."

Regarding the figure at the top of this post, which was taken from the report:

...the orange bars represent the effect of film on single pane existing glass, and the red bars represent the effect of film on existing double pane glass. The blue bars as before are the other common energy efficiency measures used during retrofits. Typically, there are fewer energy efficient features considered for retrofitting a home compared to the energy features considered for new construction.

Where TDV stands for Time Dependent Valuation:

 TDV energy emphasizes the impact of energy features that reduce peak load (primarily air conditioning load). This is beneficial for window film savings, since the energy saved is typically space cooling energy which occurs during the peak period.

I can't say that I really understand TDV, and I definitely have to spend more time delving into this, but CleanTechnica read the report and determined "The paypack period for window film was identified to be less than two years, with up to 70% return on investment (ROI)."

In concluding the portion on existing residential applications:

In the warmer climate zones of Riverside and the Central Valley, installing window films can save between 1 and 2 TDV per $100 spent when applied to double pane glass, and over 2 TDV per $100 spent when applied to single pane glass. In fact, in homes with single pane glass, there is little that one could do to improve energy performance more cost effectively.

We have touched on solar film and solar screens before and there is some nice info in the comment section on that post, but I had trouble finding performance information.  So what about the 2 year payback?

If you look at the figure below from my electricity consumption from 2011, you can see that my average electric consumption before it starts getting warm, which is between April and September, is around 400 kWh or $55 per month.

electricity consumption of townhome in Atlanta

If we assume that the amount over $55 between the months of April through October is from the hot air outside which forces our electric AC unit to run more we get a total of....

• April bill = $71, minus $55 => $16
• May bill = $81, minus $55 => $26
• June bill = $135, minus $55 => $80
• Jully bill = $159, minus $55 => $104
• August bill = $177, minus $55 => $122
• September bill = $120, minus $55 => $65
• October bill = $74, minus $55 => $19

.....an extra $432 dollars that I spend cooling my house in the warmer months.

My home does face due south, which means I get the full force of the summer sun, but window film is only going to prevent a portion of the heat from entering my house.  If it's hotter outside that it is in my home, then that heat will still find a way in, so let's assume that window film lowers the amount of energy I spend cooling my home by 30%.  For instance, I still have to run our AC at night during the real hot months and window film is not going to help me much when the sun goes down. So if we take 30% of $432 we would save about $130 in savings over the warmer months from solar film.  Unfortunately, I doubt I can get solar film installed for under $260.  While there was an energy efficiency tax credit for windows in 2011, I'm not sure it applies to window film, and it doesn't look like there are any energy efficiency tax credits for windows in 2012 (only for geothermal heat pumps, small wind systems, and solar energy systems).

So can someone help me out here?  Do you have solar film?  Do you think it would save me more than 30%?