Attic Fan: Solar vs. Electric
Last month we wrote about installing an attic fan to save some cash on your energy bills. For those of you who missed that, the concept behind an attic fan is simple: in the Summer, attics will heat up to well above the outside temperature (up to 150 degrees). When this happens, the attic transfers heat to the top floor of your house. The job of an attic fan: exchange the extremely hot attic air with outside air, thus reducing the attic temperature. This, in turn, reduces the heating effect on the top floor of your house, allowing you to use less energy cooling the home. For more, click that link and read the original article.
Solar or Electric Attic Fan?
In the original article, I wrote that solar attic fans were more costly up front ($300-500), while electric versions were much less ($150-200). Solar versions tend to move less air because they run at low power. To sum up what we said, here’s a quote from the other article:
There are several varieties of attic fans including solar and electric varities. As a general rule, solar fans operate at a lower wattage and thus can’t cool as much area. Check the CFM and appropriate square footage rating on the fan. If you already have electric in the attic, we recommend installing an electric fan. It will run much faster, and its cheaper. If you decide to go with a solar fan, make sure you will get sufficient air flow. It may be a waste of your investment if you don’t. Consider running electric if you don’t have it to avoid the solar option.
Peter (a commenter) argues that the solar attic fan is actually cheaper. His analysis:
Hi Fred,
Here’s a rough payback calculation:
A top of the line 1300 cfm solar attic fan will run you around $600, installation (no electrician required) another $300, so total installed cost is $900 minus your $270 Federal tax credit = $630 out of pocket cost.
An electric attic fan of similar capacity will cost around $125, installation $400, so the total installed cost = $525 out of pocket.
First cost differential: Electric fan is $105 less expensive.
Solar fan is free to run.
Electric fan uses 0.5 kw per hour of operation. If electricity is $0.12/kw-h, cost to run the fan is $.06 per hour.
So the electric fan will run for 1,750 hours on $105 worth of electricity. Say the fan runs only when the solar attic fan would run, 6 hours a day. The breakeven point or payback period is 292 days!
While I think Peter is good natured (and perhaps making some bucks off of solar attic fans), this analysis strikes me as misleading.
Energy Use: Solar vs. Electric Comparison
Most premium solar attic fans run 10 or 20 watt motors. We’ll say 20 watts to get the 1300 cfm Peter claims in his example. Peter argues that an electric fan will be require .5 KW (or 500 watts). It simply doesn’t make sense that a 20 watt solar-powered fan moves anywhere close to as much air as a 500 watt fan. A more fair comparison would be to assume the 20 watts produced by the solar panel had to instead be drawn from the grid… a 20 watt vs. 20 watt comparison.
At 20 watts, assuming 14 hours of use each day, that’s .02 KW * 14 hours = .28 KWh per day. At $0.12/KWh, that’s about $.04 / day. Assuming the break even point happens at $105 as in Peter’s example (we think that part of his analysis is correct), that gives us 2,625 days. Assuming the attic fan would run only 5 months (155 days) out of the year–generous, actually, because it would probably only run for 3.5 months–the payback period is an astounding 17 years… much longer than Peter’s claimed 292 days (which would be about 3 years since the fan only runs in the Summer).
Now, most electric fans will run a higher wattage that 20, but they will also move more air, and thus do a better job of cooling. Variable speed fans may be adjustable down to a 20 watt consumption level which would make our analysis valid.
Which is Greener?
Of course, one could argue that by using solar energy to save electric energy is better for the environment. This may be true, but its also hard to know whether you could actually save more electricity by installing an electric fan… In our estimation, there’s too many variables to make a good prediction on which is better for the environment.
The Bottom Line on Solar Attic Fans
In our view, solar attic fans just don’t make sense. Are we missing something?
(photo credit: muffet)
Comments & Conversation on this Article...
7 Responses to Attic Fan: Solar vs. Electric
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August 14th, 2009 11:08 am
Hey there, Peter here, thanks for the response to my comment!
The alternating current powered electric fan I used for my comparison is the Broan model 355, which is rated at 1,200 CFM, 4.3 amps, 120 volts ac. Watts = Volts x Amps = 120 Volts x 4.3 Amps = 516 watts. Here’s a link to the manufacturer’s spec sheet: http://www.broan.com/ImageLibrary/broan/pdf/Specifications/99041108.pdf
Is there an alternating current powered attic fan available that will move 1,200 cfm using only 20 watts of power? If so, please post the specs. All the ones I have seen are around 500 watts.
Seems like something may be up with the airflow ratings – if a solar attic fan can move 1,200 cfm using power from a 25 watt solar panel, why does it take an ac electric fan 500+ watts of power to do the same thing?
Or are the ratings correct and is there a large difference between the efficiency of the solar attic fan’s dc motor and the conventional attic fan’s ac motor? I am not sure but maybe someone else can shed some light on this.
Thanks for bringing this up, it will be good to get some firm answers and accurate info out there.
August 14th, 2009 11:51 am
Is a gable fan a more economical alternative to an attic fan?
It would be much easier/cheaper to install as you don’t have to cut into the roof and it comes in solar or powered models.
August 14th, 2009 12:35 pm
Peter,
You raise an excellent point about A/C vs D/C motor efficiency. I’ve got a good contact who can explain the relative losses in AC vs. DC. conversion… It should definitely be taken into account in the analysis. It would be hard to believe the loss is more than 50%, though….
Tom,
A gable fan is a type of attic fan…. In terms of whether it’s more economical/efficient than a roof mount, it would depend on a number of factors, including where the air would be driven to (soffit vents vs. roof vents vs. another gable vent on the other side)… Gable installation might be a little easier/cheaper than rooftop installation too, with less chance of creating leaks…
August 17th, 2009 11:19 am
I think you guys need a brief intro fan dynamics. Not all fan blades are alike, and thus the power required to turn the blades will differ significantly. The fan blades used by solar attic fan manufacturers tend to be high efficiency, low head producing blades. This means that the fan can put out a large volume of air for relatively little power consumption. However, the fan cannot put out any significant amount of head pressure at those rates, so back pressure restrictions must be kept to a minimum. By using high efficiency (80+%) variable speed DC motors and combination with proprietary fan blade designs, a solar attic fan can in fact deliver the rated amount airflow under maximum solar power output. I have tested it myself with an anemometer. Unfortunately, all of this energy efficiency comes at a price, which is the reason solar attic fans cost quite a bit more than their AC powered counterparts.
AC powered fans by contrast have unlimited power available for the fan blade and thus are not resigned to using energy efficient parts to manufacture the fan. Whereas manufacturing a solar attic fan is all about reducing the energy consumption of the fan, manufacturing an AC powered fan is all about reducing the cost of the fan. AC powered fans can absolutely be made to use much less power. The reality is however, these manufacturers will use whatever parts are most cost effective. Since the typical AC powered fan manufacturer produces many more types of fans than just attic fans (many which required high head pressure output), often economy of scale takes over in dictating which motor and fan parts will be used for a given project.
Hope this helps.
August 17th, 2009 11:59 am
What about wind-powered turbine ventilators? Properly installed they do a fine job of lowering the attic temperature, and they are quite common here in Arizona. They are inexpensive to purchase and install, and require zero electricity. A very slight breeze will start them up. You need gable or soffit vents of sufficient size for intake air.
August 17th, 2009 12:29 pm
Turbine vents actively move air through the attic anytime the wind blows, which is great. These vents also have the advantage of not having any electrical parts and require no energy consumption. However turbines vents have a major disadvantage in only being able to operate when there is wind, which is not so great as this does not always correspond to when the attic is hot. Turbine vents may be a good choice in some cases as a back up ventilation system, but unfortunately they are not reliable as a primary active ventilation solution due to unreliability of wind power during peak hours of maximum solar radiation (insolation).
This is really where the solar attic fans stand out as an ideal solution for attic ventilation; the solar fan will runs at its fastest speed and will move the most amount of air when the roof/attic is receiving the highest amount of solar radiation. This is a major advantage over other active ventilation methods as the system becomes self-regulating, directly tying the volume of attic ventilation airflow with the amount of heat being received by the attic.
Appropriately sizing the correct amount of ventilation that is needed is very important in achieving desired performance from a solar attic fan, and can sometimes be difficult for the average consumer. The majority of complaints toward solar attic fans is that they just don’t move enough air to effectively ventilate an attic as described in the above article. However this is a common misconception by consumers that unfortunately did not purchase the correct size/number/type of fans for their specific application. While some manufacturers do offer low-powered solar attic fans, there are others that manufacture solar fans which have the same airflow performance as electric fans. The technology works well, but as with anything you must use the right tool for the job in order to get the results you expect.
January 11th, 2010 3:03 am
my question is there a way to utilize solar panels to run outside ceiling fans on my houses porch?