Solar Panels on EAST facing roof

East-roof-solar-potential

Can you put solar on east facing roof?

It’s a fact that a north-facing roof anywhere in Australia is ideal to install solar panels. However, if you don’t have a roof space facing north and only have a open roof space facing east, how will the solar output from that roof compare against a north roof? 

Note: In case if you have usable roof space on both sides of the roof, east as well as west, and considering splitting solar panels between the two sides, you can read my East-West roof analysis here.

One thing is certain and that is; east roof will produce more solar power in the morning than any other roof orientation. If you are a family that uses quite a bit of power in the morning hours, east-facing solar panels can be of a significant advantage to your family. However, these panels need to work very hard to produce electricity rest of the day, especially in the winter time.

What if you have a roof that is slightly northeast or southeast facing? How does your roof’s pitch/inclination play a role? After reading this article, you’ll have a good understanding of these issues and know a true potential of your east roof.

As always, here is a disclosure of data source I am going to use for my analysis below.

  • Primary data source is pvwatts.nrel.gov. Keep in mind the data is not real “measured” data for solar irradiance. Instead pvwatts provides computer simulation based modeled irradiance data. This data is used in various engineering applications. For this analysis, it is the best high-resolution data there is available out there.
  • According to pvwatts manual for modeling explanation, the modeled data can have error of +/- 10% for annual energy production total and +/- 30% for monthly totals for a similar actual system.  
  • Pvwatts also did a study where, when comparing modeled data against measured data of the actual system, the modeling under-predicted the actual performance by 1.8% – which  was remarkable.

The point is: Please use the data cautiously. Although, for the comparison, because I’ll be using the same data source, the margin of error will essentially cancel each other out. 

Regarding the panels, I will assume standard polycrystalline panels for the solar power system.

We will make a comparison between two roofs. The specifics of these roofs are:

  • North roof : Azimuth angle = 0°, 20° tilt, Melbourne, no shading.
  • East roof : Azimuth angle = 90°, 20° tilt, Melbourne, no shading.

Let’s start the comparison:

Solar irradiance: East roof vs. north roof

If you recall, solar irradiance is the amount of sunlight received per unit area, or one square meter. 

The table below compares total solar irradiance measured for the entire day – for each month of the year. Values representing each month is the average for that month.

Month Solar Irradiance EAST (kWh/m2/day) Solar Irradiance NORTH (kWh/m2/day) Difference
January 6.74 6.92 3%
February 5.94 6.51 9%
March 4.97 5.64 12%
April 3.41 4.32 21%
May 2.06 2.69 23%
June 1.89 2.61 27%
July 2.01 2.70 26%
August 2.85 3.52 19%
September 3.69 4.29 14%
October 5.30 5.76 8%
November 5.76 6.00 4%
December 6.14 6.27 2%
Average 4.23 4.77 11%

First of all, as you can see, total daily solar irradiance is much higher in the summer than in the winter season. Then, between the two roofs, east and north differ by small amount in the summer months but the difference grows as we move towards winter season.

On annual average, east surface receives 11% less sunlight compared to North – in Melbourne.

Large deficit in winter months is due to the fact that sun path is lower and more towards the north in that time of the year. This favors the north roof solar panels; the eastern roof doesn’t see much daylight except for few hours in the morning (see intraday comparison graph below).

While in summer, the sun path is higher and thus, the east roof continues to receive decent amount of sunlight even in the late afternoon hours. Here is an illustration of sun path between summer and winter months.

sun-path 

Now, let’s put 5 Kilowatts of solar panels on each surface and compare the generation based on the solar irradiance we just discussed. 

Monthly solar power generation: East vs. North roof

Let’s take a look at total solar production for each month of the year between the two roofs. 

Note: Monthly total output is for 5 Kilowatts system in Melbourne, Victoria.

east-north-solar-generation

Solar power generation depends on the amount of solar irradiance i.e. the amount of sunlight that strikes the solar panels.

East roof produces the least amount of solar energy in winter – 29% less in June compared to North, and in summer months of November – January, the east and north roofs look almost the same.

What about the production within a day? Does east roof produce any electricity after 12pm? Let’s look at that as well..

Intraday solar generation: East vs. North roof solar system

intraday-summer-solar-generation

intraday-winter-solar-generation

In case you were wondering, first bar graph represents an average summer day of January and the second bar graph represents an average winter day of July. The graphs illustrate the daily production profile for two extremes seasons for solar production.

In summer, east facing roof starts to crank out more solar electricity than north right from the sunrise until about noon. After which, north roof starts to produce at higher rate rest of the day.

In winter, as the sun path is tilted towards north, it favors the north roof in the morning. Solar generation between the roofs is almost same in the morning hours. For second half of the day, east roof is far worst than north in this time of the year.

Few observations from the SUMMER graph..

  • Peak generation hour (hour with the highest generation) for east roof occurs between 10am to 11am; for north roof it occurs between noon to 1 pm.
  • A 5 kilowatts system on east roof produces 3.5 kWh of electricity during the peak hour (10am-11am) and 27.1 kWh for the entire day. For comparison, north roof produces 27.8 kWh for the entire day, which is 2.5% more than east roof.
  • Ideally, a 5 KW system should produce 5kWh of energy during solar peak hour. But we don’t live in ideal world and that’s certainly not the case above. There are two reasons for that: (i) energy loss in the system such as cables, inverter, soiling etc. (ii) panel efficiency loss due to high temperature.

If you plan on installing solar panels on your East roof, it is important that you understand the seasonal variation and intraday profile of solar production. The latter one will help you plan on how to self-consume the solar power while it’s being generated. Today, the more you self consume, the more savings you’ll see on your electricity bill. That’s because exporting electricity is worth very little.

What about Northeast and Southeast roof orientation?

If you recall the sun path diagram from earlier, any roof facing even slightly towards north generally receives more sunlight through out the year. And it’s a huge advantage during winter months when the sun path gets shorter.

So, if you have a northeast roof, it’s generation deficit can be as low as 5% compared to North. For strictly east roof, this deficit was 11%.

In contrast, southeast roof will not perform as good as the east or northeast roofs. The deficit, compared to north can be as high as 25% in Melbourne.

Below is a summary for different roof orientations. 

Solar-comparison-roof-orientation

If you’d like to learn more about south roof, here is my blog post dedicated for south roof.

How do different roof pitch affect solar electricity production?

For this post, I’d considered 20° roof pitch. The question is, what happens if you have smaller or larger roof pitch than 20°?

The answer is simple: If you have a flatter roof (less inclination/pitch), that roof is exposed to more sunlight than a higher pitch roof on any given day.

For most Australian homes where 15° and 22.5° are the most common roof pitch, it means solar panels that are flush mounted on 15° roof pitch would produce slightly more power than a 20° roof. And solar panels on 22.5° pitch would produce slightly less power than on 20°.

In summary,

Few years ago, when electricity companies were paying high rates for exported electricity, decision about putting solar panels on east roof was easy.  Why? Because all you cared about was the production.

Today, though, the electricity companies pay very little or don’t pay at all for the exported electricity.  So, the decision about putting solar panels on east roof is about the production AND the consumption. More solar power consumption means less import from the grid, which means smaller utility bills.

So, if you are seriously considering solar, one of the first things I recommend is understanding your solar power need. Based on your consumption pattern, how many solar panels or what size solar system would you need for your home? Initial assessment of your own need will educate you about solar and prepare you for the shopping.

Check out my other post about Calculating solar size need

I hope you learned something from this post. If you have any questions, please contact me or leave comments below.

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