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How Many Solar Attic Fans Do I Need?

I’ve been reading about how a solar attic fan system can save me money and keep my house cool. Perhaps you’re also interested in installing an attic cooling system?

I know that attic fans offer energy savings and other benefits both in summer and winter, and I wanted to find out what kind of system I need for my home. There’s lots of information out there, but it’s hard to get a concrete answer. But I’ve gathered together all the best information for you, so here’s my comprehensive answer to the question, how many solar attic fans do I need?

What Is  A Solar Attic Fan

A solar attic fan is an electronic device you install on your roof that vents out air from inside your attic. When the solar fan vents out air, it is replaced by cooler air drawn in through attic vents. Solar fans are powered by a solar panel placed on the outside of your roof. Ideally, you should position solar panels on a south-facing slope angled so that they can track the sun’s path.

Solar panels are a collection of photovoltaic cells, also called solar cells. These react with light energy from the sun to produce electricity through a chemical reaction known as the photovoltaic effect. So when the sun is shining, or there’s enough indirect sunlight getting through the clouds, the solar panel drives the fan and expels air from your attic.

what size attic fan do i need

Why Install Solar Attic Fans?

Solar attic fans offer two main benefits, one that applies when the weather is hot and the other in cooler and wetter weather.

1) What do solar attic fans do in hot months?

In the hot summer months, the air in your attic can heat to 120°F and above. This is a problem because this hot air transfers heat through your ceiling into your house, making it hotter and harder to cool. A hot attic increases your aircon costs and makes your home less comfortable to live in.

A well-installed solar attic fan system replaces hot attic air with cool air, thereby cooling your home.

how to size a solar attic fan

2) What do solar attic fans do in cold months

During cold, wet winters, your attic space is at risk of mildew, dampness, and other water damage. Also, ice can collect in the eaves of your roof and damage it through a process known as ice damming.

A solar attic fan deals with the damp by expelling humid air and drawing in dry air, reducing attic humidity.

For Further Reading: Are Solar Attic Fans Worth It?

Important measurements

We measure the air venting capacity of a solar fan in cubic feet per minute or CFM.

CFM tells you how much air per minute a fan can displace under optimum operating conditions. Of course, you rarely get these conditions in the real world, but it’s a helpful way to compare fans.

We measure the electricity-producing capability of a fan’s solar pane in watts.

If we know the size of our attic space, then we know how many fans of what wattage we’ll need for an effective attic solar fan system. For the tables below, I’ve assumed we are using a standard 20 Watt solar fan.

Attic Size

We measure the attic size in square feet.

Your attic area is simple to calculate. All you need to do is to measure the length and width of your home and multiply them together. This will give you your attic square footage.

The next thing is to consider how steep your roof is.

how to calculate attic space

Attic Slope or Pitch

The steepness of your attic roof is measured as x/12, where x represents the slope of your roof.

If you have a reasonably flat roof, the pitch may be 2/12, meaning that in the space of 12 feet, the roof rises only 2 feet. A steeper roof pitch is 12/12, where your roof height increases at a 45° angle. The slope of your roof is significant because it governs how much air space there is in your attic. An attic with a low pitch doesn’t hold much air, so you won’t need many solar fans to vent it out and draw in cooler air. With a steep-sloping roof, the opposite is the case; it will contain a large volume of hot air, so you’ll probably need several fans to do the job

The tables below are good guidelines for how many solar fans you need.

Guide for roofs with a low slope

Attic SlopeAttic SizeAttic Fan Size in Watts
1/12 – 4/12800 ft²1 Solar Attic Fan
1/12 – 4/121,200 ft²1 Solar Attic Fans
1/12 – 4/121,600 ft²2 Solar Attic Fans
1/12 – 4/122,000 ft²2 Solar Attic Fans
1/12 – 4/122,400 ft²3 Solar Attic Fans

how many attic fans do i need

Guide for roofs with medium slope

Attic SlopeAttic SizeAttic Fan Size in Watts
5/12 – 8/12800 ft²1 Solar Attic Fan
5/12 – 8/121,200 ft²2 Solar Attic Fans
5/12 – 8/121,600 ft²2 Solar Attic Fans
5/12 – 8/122,000 ft²3 Solar Attic Fans
5/12 – 8/122,400 ft²4 Solar Attic Fans

Guide for roofs with a high slope

Attic SlopeAttic SizeAttic Fan Size in Watts
9/12 – 12/12800 ft²2 Solar Attic Fans
9/12 – 12/121,200 ft²2 Solar Attic Fans
9/12 – 12/121,600 ft²3 Solar Attic Fans
9/12 – 12/122,000 ft²4 Solar Attic Fans
9/12 – 12/122,400 ft²5 Solar Attic Fans

Note that these numbers are guidelines based on 20W fans in regular attic conditions.

attic fan sizing calculator

Remember to take other factors into account

This guide is great, but if your home is in the middle of the Nevada desert, then, guess what, your attic will get hotter than if you live up in Washington State.

So you need to apply some common sense to this model. The number of fans required is just a recommendation, and if you know that you have an unusually hot or cool attic space, you can adjust the number of fans accordingly.

Intake Air Vents

Now, just wait a minute before you rush out and start buying lots of solar attic fans because we need to consider something else.

A solar attic fan is a way of driving hot air out from your attic, but we must replace this hot air with cool air from somewhere.

For an effective solar attic fan system, you also need air intake vents

Intake Air Vents-how many solar attic fans do i need

Fortunately, most homes have soffit vents. These are air ducts placed under the eaves of your house at the edge of your roof. Before you plan your solar attic fan system, it’s essential to check how much soffit ventilation your home has.

If your soffit vents are open spaces, then a good rule of thumb is to make sure you have 4 ft² of soffit ventilation per solar fan.

More commonly, your soffit vents will be covered by a grill. If so, you should double that figure to 8 ft² of soffit ventilation per solar fan.

If you don’t have enough ventilation, installing soffit vents or air vents in the gable end of your house is straightforward. Just remember that these air vents are vital for an effective solar attic fan system.

Peak solar hours

The effectiveness of your solar attic fan depends on the amount of direct sunlight you get.  You can calculate this depending on where you live. It’s measured in peak solar hours. The hottest states of the southern US receive more direct sunlight than the cooler states to the north.

These states will have hot attics since there’s more direct sunlight heating your roof, but also more solar energy to power your fans. It’s good that your solar attic fans get an extra push in the parts of the country where they are needed the most.

us Peak Sun Hours Map

Hybrid Fans

You can reduce the number of attic fans you’ll need by installing hybrid fans. These are simply solar-powered attic fans that also connect to your mains electricity supply. When the sun is shining, the solar panel powers the fan, but if the fan is needed at other times, it is powered from your mains supply.

There is an advantage here in that you don’t need to buy and install as many fans in the first place. The drawback is that it costs money to power these fans when they’re not running on solar. However, it’s cheaper to power a fan than an air conditioner so a hybrid solar fan will still save you money on your electricity bills.

Summary

We’ve looked at the benefits that solar fans offer both in hot and cold weather, seeing how they can help cool your home and prevent water damage.

We’ve established that the number of solar attic fans you need varies depending upon the area of your attic space and how steep the slope is.

I’ve given you some tables to use as guidelines for how many solar attic fans you’ll need. But I’ve also added some important points. You should always make sure you have enough soffit ventilation. Also, place your solar panels on a south-facing slope and angle them carefully to maximize power generation.

There are many different solar attic fans out there; you should check our product reviews for more information.
Use the tables as a guideline for the number of solar attic fans you need, and you can reduce the requirement by buying more powerful models or hybrid fans.

If you have any comments or questions on how many solar fans do I need, please share them with us.

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