One of the new solar power devices that has come out in recent years is the solar powered attic fan that you install in your roof. Like just about all solar powered devices, they come at a premium price compared to their utility powered counterparts. Usually, the buyer hopes that the money saved by not paying for energy eventually pays for the difference in price. This payback is usually measured in years. A couple of years ago, I needed an attic fan for my garage. It was a particularly hot summer so I needed some extra venting so that I could comfortably work in the garage. I bought a low volume fan to do the job. Since then, the same company came out with a solar powered attic fan essentially rated to handle the same space as the utility powered fan that I bought. What I would like to do here is to figure out what the payback would be if I had bought
the solar powered fan. Here’s some data: Utility powered fan handles 1170 cubic feet of air per minute (CFM). The manufacturer claims it’s good for a 1650 square foot area. 120 volts AC, 3.4 amps. Big Box Hardware Store price is $74.00. Solar powered fan handles 800 CFM and is good for 1200 square feet. 24 volts DC motor. Amps unknown but not needed for our discussion. Big Box Hardware Store price is $217.00. Keep in mind that both fans are made by the same company. This particular solar powered fan uses an external solar panel rather than having it integral to the fan’s dome. The difference in cost is $143.00. I would like to find out how long it would take to get that $143 back through savings in energy costs. Calculations: Watts = volts x amps. To find the wattage of the utility powered fan we plug 120 volts and 3.4 amps into the equation and get 120 x 3.4 = 408 watts. A kilowatt (KW) is a different way of saying 1000 watts. So, 408 watts = 0.408 kilowatts. We pay for energy by the Kilowatt-Hour. For every 1000 watts of electricity we use in an hour, we pay the utility’s electric rate. On my last utility bill, I was charged 13.7 cents per KW-Hr. Since my fan only uses 0.408 KWs per hour, I pay 0.408 x $0.137 = $0.056 per hour of usage. That’s 5.6 cents I get charged for every hour that my fan runs. Now I’m going to find out how many hours my fan has to run in order to use $143 of energy. $143 / $0.056 per hour = 2,554 hours. After using my solar power attic fan for 2,554 hours, I will have made up the difference in purchase price through energy cost savings. How many years is that? That’s tough to answer because we never know how long the fan is going to run during any given year. I my case, living in the midwest, the fan doesn’t need to run that often. Just as an example, let’s say my fan runs for 6 hours a day for 60 days during the summer months. That’s 6 x 60 = 360 hours per year. At that rate, it would take 2554/360 = 7.1 years to payback the price difference. Any usage of the solar fan after that would mean money in my pocket. Of course your payback period will be less if your utility rates are higher or if you need to use the fan more often. If your rates are lower or you use the fan less often, the payback period will be longer. Discussion: Although I am in favor of using solar power wherever possible, it still has to make financial sense to use it. If you can afford to use a solar powered device even when there is no practical payback vs a utility powered device, more power to you. You are definately helping the environment. For the rest of us, a solar powered attic fan may not make
sense to use. Chances are that the utility powered fan doesn’t draw the full 3.4 amps during normal operation but something less. This would make the payback period even longer. The one feature that may make it practical is that you don’t need to run a 120 vac circuit to the fan. Right there is an additional savings in wiring and accessories. Not having to hire an electrician to do the job may make using the solar powered fan a financial bargain. This would mainly apply to installing one fan. If you need 3 or 4 fans, using the untility powered fan would probably be the better choice even if you had to
hire an electrician. Once you have one circuit installed, it’s usually not that much more costly to extend the circuit to other fans. When the cost of electricity goes up and the price of the solar powered attic fan comes down, the decision will be much easier to make. Go solar. Something else I just thought of. The utility powered fan has a thermostat. It usually only comes on when the temperature reaches between 90 and 110 degrees Fahrenheit. The solar powered fan comes on whenever the sun is shining; no thermostat. In the summer, this is no big deal. But in a northern winter, having the sun heat your attic by say 20 degrees helps to keep your house warm. Having a solar fan in this case may actually cause your furnace to work harder on a sunny day than without the fan. Something to think about.