Just like many people these days, I'm looking to save money on my electricity bills. If I can "Go Green" at the same time, that is just an added benefit. I viewed many websites and businesses that say you can save "x" amount on your electricity bill if you have them install any dizzing array of products, but there really wasn't any real life data to back up the claim.
Time to collect some data. I teamed up with Ward Vestal and Kangaroo Building Services of McKinney, Texas and decided to install their 90% blocking Phifer Sun Screens on the southern facing windows of my house located in Melissa, Texas. The graphs below are real-time temperatures collected from the windows updated every 5 minutes.
The solar shades were installed on April 5th, 2010 on twelve windows with a total area of 271.5 square feet. The rule of thumb for windows is to calculate 60 BTU/hr influx for solar radiation on a window with closed blinds. Mutliplying the square footage of my windows and assuming 90% blockage will give me the amount of BTU's that my air conditioners should not need to cool which is 14,664BTU/hr. As seen on the graphs, the windows are in direct line of the sun between 12pm and 4:30pm. My main AC unit will cool 48,000BTU/hr. This means it should work roughly 30% less than before during the part of the day the sun is hitting the windows thereby saving me money. I will compare this with my month electrical utility bill with a correction for cooling-degree days to compensate for climate variations to verify the actual cost savings.The table below is data comparing this year to last with my current electricity bill on our 3400 square foot home.
The graphs below show the current temperatures of the outside blind, inside the blind, inside the window, and inside the room of my house. You can see an immediate difference in the temperatures inside and outside the solar shades. The difference between the blue line and the red line should be the cost savings I see on my monthly bills since that is the radiant heat being blocked. There is no way for me to currently calculate how much radiative heat is being blocked, which is why the proof should be in my power bills.
Update 6/11/10. Looks like I'm saving about $21/month with the solar shades so far. Since we had uncommonly hot weather (CDD is up almost 25% for the month of May), my overall power bill went up, but I can see the duty cycle on my AC is less than it was before which means the Solar Shades are doing their job. Also, since my windows only see direct sun for 4.5hours each day, my benefit is less compared to if the sun was hitting the widows all day. I suspect that cost savings would be in the $60/day range if I had a rectangular house and sun hitting the windows all day from sunrise to sunset. This is a 14% cost savings per month on my electricity bill!
Update 8/14/10. We now have four months of data which gives me enough data to calculate out a linear regression fit. The BTU calculation shows $81.42 worth of cost savings, but the linear regression which helps factor in our recent hot streak shows $166.68 in cost savings. The truth is somewhere in between there and most likely towards the upper end because we have seen a huge drop in our electricity bill even though it is getting much hotter outside. We are very happy with our investment!
Update 9/17/10. This was the hottest month ever since I've been keeping track of my electricity bills. 657.5 cooling degree days for 7/30/10 to 8/28/10. Wow did we ever have a hot streak! But we're keeping nice and cool with our Solar Shades! My electricity bill was $272.32 but that is a far cry from the high of $358.78 that it was during the very hot month of July last year and at 6% less cooling degree days. Can I be any happier with our purchase from Kangaroo Building Systems?! They have put a smile on my face and money in my pocket!
Update 11/09/10. It's starting to cool down and it's almost time to take the solar shades off so we can get all that daylight in through our windows to passively heat up the house. At $317 worth of cost savings for a summer, out investment will pay off in just a little over two years.
Update 7/31/11. North Texas is in a hot streak. We have seen triple digit weather now for over 30 days in a row. We also started cooling off our upstairs to a constant 78 degrees, and my electricity bill still hasn't hit the all time high bill we've seen before. The Solar Shades still continue to pay back huge dividends. The return on the investment will be under two years now! Best investment ever!
Blue line is with no solar shades. Purple line is with solar shades. Difference between the two lines is about 11% cost savings. Also note that the purple line increases spacing from the blue line when it gets hotter outside. That means as the temperature goes up outside, we use less power to keep cool thanks to the solar shades!
