Solar panels on south direction – How scary is it?

Solar-panels-south-roof

South facing solar panels

You certainly benefit more (a lot more) with solar panels on north facing roof then on south facing roof. However, if you own a house with most of the open roof space in south orientation, how big of a disadvantage is that roof for solar panels?

Well, let me just start out saying that no one in their right state of mind will recommend you to install solar panels on south facing roof in Australia. 

But frankly, it is not all bad news. You won’t be able to take the same level of advantage as the north facing panels every month of the year but you will produce decent amount of power in the summer months. However, is that enough for good ROI?

In this article, I will present some data that shows the reality of south roof. Side by side will be a north roof, so you can compare the two. 

South-face-solar-panels

Before I get to the business, I would like to disclose my data source.

  • The solar irradiance and solar power generation data is from pvwatts.nrel.gov
  • This data can have error of +/- 10% for annual energy production total and +/- 30% for monthly totals for a similar actual system (according to pvwatts manual for modeling explanation)

For the solar panels, I have considered standard polycrystalline panels. I am also assuming the panels on both directions are free of shading.

Most residential roofs in Australia have either 15° or 22.5° pitch. And for economic reasons, most solar installations today are flush mounted on these roofs. As a common ground, I have chosen 20° pitch for all my calculations below. Location is : Melbourne, Victoria.

Solar irradiance: How much sunlight does south direction receive?

Solar irradiance is the amount of sunlight that strikes on a surface and measured in Watts/m2.

Annually, in Melbourne, a south-facing surface ( in this case, surface is a roof with 20° inclination) receives only 73% of the total solar irradiance when compared to the north surface with 20° inclination. See table below.

This table compares the total daily solar irradiance for each month of the year. Value for each month is the average of the days in that month.

Month Solar Irradiance SOUTH Roof , MELBOURNE (kWh/m2/day) Solar Irradiance NORTH Roof, MELBOURNE (kWh/m2/day) Difference
January 6.38 6.92 8%
February 5.32 6.51 18%
March 3.86 5.64 32%
April 2.32 4.32 46%
May 1.25 2.69 54%
June 0.96 2.61 63%
July 1.18 2.7 56%
August 1.82 3.52 48%
September 2.84 4.29 34%
October 4.53 5.76 21%
November 5.38 6 10%
December 5.92 6.27 6%
Average 3.48 4.77 27%

What is clear from the table above is that winter months are absolutely brutal for solar panels facing in the south direction. The only months with single digit deficits when compared to North are December and January.

High deficit in winter is due to the sun path that is shorter and towards north. Because of that, south roof is deprived of sunlight all day.

In peak summer months of December and January, the sun is directly above us. So the south roof is exposed to sunlight more than ever. 

You might be thinking that Melbourne’s bad winter weather may have some impact on those numbers in the table above. That is true.

But even if you lived in a more sunnier place than Melbourne, the comparison between North and South roof doesn’t get much better. 

Here is a comparison for Brisbane,

Month Solar Radiation SOUTH Roof BRISBANE (kWh/m2/day) Solar Radiation NORTH Roof BRISBANE (kWh/m2/day) Difference
January 6.44 6.41 -0.5%
February 5.36 5.85 8%
March 4.38 5.6 22%
April 3.25 4.93 34%
May 2.2 3.87 43%
June 1.89 3.94 52%
July 2.17 4.44 51%
August 3.01 5.26 43%
September 4.31 6.11 29%
October 5.06 5.88 14%
November 6.17 6.3 2%
December 6.59 6.48 -2%
Annual 4.24 5.42 22%

As expected, overall the numbers look slightly better than Melbourne. But the deficits compared to north are still very significant.

So, do these numbers add up to consider solar panels for south direction?

From financial standpoint, these numbers suggest that south facing solar panels won’t save you much on your utility bill most of the months in a year. Which means that the payback period will be very long as well. And, except for few summer months, it is likely that you’ll be relying heavily on grid power for most of the year – which defeats the purpose of having solar in the first place.

If your roof orientation is southeast or southwest, installing solar panels on either orientation will produce more solar power than a strictly south facing roof. But the annual deficits can still be very large.

Instead of polycrystalline panels which I used for this analysis, a more efficient monocrystalline panels can be used to increase the production. However, these panels are 10-20% more expensive as well and the difference in the production may not be significant.

In summary,

Solar panels installed on a south facing roof can not really compete with systems on North, East or West roof orientations, and the margin is quite significant. Which is why the topic is only appropriate to talk about during Halloween season.

Ultimately, the goal of having solar is to reduce your monthly electricity cost as well as save the environment. Numbers show you won’t achieve much of either with south facing solar panel system.

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