How much stress solar panels put and how much area they occupy on your roof?

All of us have space constraints and have limited shed-free roof area for one reason to another. Therefore, it is a need to know the surface area of the solar panels well in advance before installing them on the roof.

Another point to know is the load-bearing capacity of your roof.

With time, the roof strength starts deteriorating due to scorching heat, rain, wind, snowfall, and other weather conditions.

And

Your roof may not withstand the same amount of stress when it was new.

The Solar Panels do put stress on the rooftop.
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Therefore, you should know how much stress the solar panels are going to put on the rooftop and whether your roof is capable of handling that stress or not.

"The over-sized panels may not fit in your roof-top while the small-sized panels can leave your space under-utilized"

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Average Stress-bearing Capacity of the Roof

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The average minimum weight-bearing capacity of the roof is 30 pounds/ft² or 150 kg/m².

(1 pound/ft² = 4.9 kg/m²)

Depending on the type of roof, the load-bearing capacity can be more.

If the weight-bearing capacity of the roof is more than the stress put by the solar panels

then

​you can install the solar panels on the roof.

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Weight of the Solar Panel

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If I talk about the weight of the panel, it is around 2-2.5 pounds/ft² or 10-12 kg/m².
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Okay

Let us find the weight of some good solar panel manufacturers from their datasheets:
 
Waaree Solar Panels (Arka Series: 60 cells)

Data Source: Waaree

The stress (kg/m²) is calculated as follows:

Stress =Weight/surface area
=19 kg/(1.660 m x 1.003 m)
= 11.41 kg/m².

Let us check out the stress by Canadian Solar Panels:

Data Source: Canadian Solar

I have made a fair comparison on the following basis:

• Both based on Mono PERC Technology
• Both are 60 cells solar panels
• The Power range is within 315 watts to 345 watts

You can see that Waaree Solar Panels in 315 watts to 330 watts range weighs 11.41 kg/m²

and

Canadian Solar Panels in a similar power range of 320 watts to 345 watts) weigh 11.39 kg/m² (slightly lighter than the Waaree solar panels)

On average, they are going to put the stress of 11.40 kg/m² on the ground

But

You can’t install them without the mounting structure, and these structures do have weight.

The combined weight of the solar panels + mounting structure is around 4 pounds/ft² (near to 20 kg/m²)

And it is very well within the limits of the load-bearing capacity of the roof.

The weight distribution should be uniform by having right number of mounting locations so that there is less risk of cracking and leakage in the roof.

Key takeaways:

• The average minimum load-bearing capacity of the roof is 30 pounds/ft² or 150 kg/m².
• The average weight of 300 watts – 345 watts solar panels is 11.40 kg/m² or 2.33 Pounds/ft².
• The average combined weight (Solar Panel + Mounting) structure is 20 kg/m²

If your rooftop is very old and you see some cracks in it, it is your installer who needs to inspect the roof and take corrective measures before installing the solar power system on your rooftop.

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Surface area of  the Solar Panel

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Let us check out the surface area of the solar panels of the above two brands:
Waaree and Canadian Solar

​You can see that the surface area of 315 watts to 330 watts is 1.66 m² or 17.9 ft².

Let us check out for Canadian Solar:

We can find that ​Canadian solar occupies 1.69 m² that is slightly more than that of Waaree solar panels.

Key takeaways:

The average area of 320 watts to 345 watts solar panels is:
= (1.66 + 1.69)/2
= 1.675 m² or 18 ft²
Or
Area (ft²) = 5.58 feet x 3.25 feet

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Roof area required for the solar panels

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The roof area is not equal to the total surface area of the solar panels.
It is usually more than that.
 
When panels are connected in the string, we should leave some gap between 2 panels in the row so that the air can pass through that can keep the panel cool, improving its efficiency.

When you have more than one row (one behind another) then we need to keep the second row slightly away from the first row so that its shadow doesn’t fall on the second row.

(The solar panels are very sensitive to the shading, and even the small shaded portion can deteriorate their performance significantly).

In addition, how much roof area is needed also depends on the latitude of the location.

When you’re at higher latitude, you need to give more tilt to the solar panels to get the maximum sunlight.

This means that you need to put the second row (behind the first row) more away from the first row so that shadow may not fall on the second row.

In that case, you need more space to accommodate the solar panels.

If you have a tilted roof facing the sun, the area required is comparatively less when your roof is flat.

Therefore, you cannot have an exact multiplying factor to get the right area of the roof.

Take the multiplying factor between 1.25 to 1.75 to get the area required for the solar panels.

Let us understand more clearly in 6 simple steps with the help of an exampe.

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Step 1: Calculate your energy requirements

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Let me tell you the simple way of estimating the your average daily energy consumption in terms of units of electricity.
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1. Take out your previous month electricity bill and find the number of units consumed in that month. Your energy consumption will be different in different months of the year. Therefore, take out the utility bills of the last 12 months and add all the units consumed in that year.
2. Then divide the results by 12 days to get average monthly units consumption.​
3. On further dividing it by 30, we get the average daily units consumed.

Let us understand with the help of an example:

Energy Consumption chart for a year

Determine the feasibility of your solar roof CLICK HERE!

In the above table, you can see that the average daily consumption is 12 units or 12 kW-hr of energy.

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Step 2: Know the Peak Sun Hours in your region

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The solar insolation plays a very important role in determining the surface area of the panels.

A region receives good sunlight will require less panel area, to produce same number of electricity units, than that of region with less sunlight.

For example, a residential solar power system installed in Delhi/NCR (India) with 5.5 sun peak hours per day will require less panel area to produce same number of units than that a system installed in New York (USA) with average daily sun peak hours of 4.33.

Well, there are many places in USA which receives more sunlight than the places in India.
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You can check the solar radiations value from: 

https://pvwatts.nrel.gov/pvwatts.php (National Renewable Energy Laboratory of the U.S. department of energy)

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Step 3: Calculating the Ideal power of the panels

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After getting the average daily solar insolation or the daily peak sun hours of your region, divide your average daily consumption by it.

Read: How to size the panels of your solar power system?

I am taking the example of Delhi/NCR, in this particular case the ideal power of the solar panels is 12 kW-hr/5.5 hours = 2.2 kW or 2200 watts.

This is the ideal power (when there are no losses) of the panels which is required to meet your energy needs.

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Step 4: Calculating the actual power required from the panels

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We are living in the real world and we cannot ignore the losses in the system.

The 2.2 kW solar panels will not produce 12 kw-hr of energy because some of their power will get lost during energy conversion,  transmission and heat loss to the surroundings.

Read: What are the significant losses in the solar power system?

In order to meet the energy demand, we need to slightly increase the power of the panels to compensate for the said losses.

I am simply multiplying the ideal power of the panels by factor 1.5 to compensate all the losses in the solar power system.

In the example, the real power of the panels become 2200 watts x 1.5 = 3300 watts.

Now, the 3300 watts panels will meet the daily energy need of 12 kW-hr.

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Step 5: Get the standard dimensions of the solar panels

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At this point, it is important to know the standard dimensions of the solar panels available in the market.

The dimensions of one panel will help us in calculating the total surface area of the panels required to meet the energy demand.

You can find above that the average area of 330 watt solar panel is 18 ft²​.

(The thickness of the panel is of least concern while determining its surface area)

We need 3300 watts from solar panels. Therefore, number of 330 watts solar panels that will add to 3300 watts are 3300/330 = 10 numbers.

The even numbers of panels are preferred while designing the solar power system as it is easy to make series and parallel combination.

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Step 6: Find the surface area and the weight of the solar panels

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4 Now, the total surface area of the panels is the surface area of 1 panel multiplied by the numbers of panels.

In our case, it is:

= 18 ft² x 10
= 180 ft² or 16.72 m²
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Now, use the multiplying factor range 1.25 to 1.75 to get the idea of the space occupied by the solar panels.

And

It is 225 ft² to 315 ft² of shed-free space is required to accommodate 10 x 330 watts of solar panels on the roof.

And

The total weight of the 10 x 330 watts solar panels along with the mounting structure is:

Total Surface Area of the solar panels x average weight in Pounds/ft²:

=180 ft² x 4 Pounds/ft² {The average weight of one solar panel of around 330 watts (60 cells) + mounting structure}.
= 720 Pounds or 326.6 Kg

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A final note

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1. Measure the shed-free area of your roof-top in advance. 
2. If the shed-free area is less than the surface of the panels then it is not feasible to install them on the roof-top.
3. You should cover your roof-top in such a way that even after installing the panels, you are still left with sufficient free space to do cleaning and maintenance of the system.
4. Look for cracks or other points that give you feeling of roof weakness, consult with your solar installer before having a solar power system.
5. Remember to leave sufficient space between two rows, otherwise the shadow of the front row may fall on the second one and the output power will be affected.
6. Leave sufficient gap between panel and the ground and also between 2 panel connected in a string so that air can pass through, keeping panels cool.

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