West roof orientation- is it a blessing in disguise?
If you are hesitating to put solar panels on your west-facing roof, this article is for you. Let me just say that installing solar panels on west roof orientation may not be as bad as you are thinking right now. But before you draw any conclusions, it is important that you understand its potential, and how it compares to the North roof. Why North? Because that’s the best roof orientation you can have for solar.
I’ve done a little data analysis for west roof and the results are discussed below.
If you consume more energy in the late afternoon – evening hours, because that’s when the kids are home after school, west roof orientation can be a real blessing for solar installations. To explain why, in one of the sections below, I will show you hourly generation of solar electricity from west roof and a comparison against 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.
What if your roof is northwest or southwest orientation? How does roof pitch affect how much solar power is produced? I will go over that as well.
First, let me disclose my data source:
- My primary data source is pvwatts.nrel.gov. It is a popular destination for meteorological data for solar power systems, maintained by U.S. government.
- 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. Although this won’t matter when I make a comparison, because it will cancel each other out.
The specifics of the two roofs for comparison are as follows:
- North roof: Azimuth angle = 0°, 20° inclination, Melbourne, no shading.
- West roof: Azimuth angle = 270°, 20° inclination, Melbourne, no shading.
Regarding solar panels: standard polycrystalline panels that is flush mounted on the roof.
Now, let’s get started, shall we?
Comparing solar irradiance: West roof vs. North
If you recall, solar irradiance is the amount of sunlight per unit area of a surface. It is measured in watt-hour or Kilowatt-hour (kilowatt=1000 watt). Usually, solar irradiance is measured for the entire day.
Below is a comparison of “daily mean” of solar irradiance for each month of the year.
|Month||Solar Irradiance WEST (kWh/m2/day)||Solar Irradiance NORTH (kWh/m2/day)||Difference|
Data in the table above shows two things:
- Solar irradiance in winter months is too low compared to summer for both roof orientations. This is true because winter is just really bad here in Melbourne.
- There is very little difference between west and north roof in the summer; the difference swells as the winter arrives. This is because of the sun path being higher in the summer, and low and towards north in the winter.
If we look at the entire year, west surface receives 15% less sunlight compared to north – in Melbourne. Not so bad, right?
Now, let’s compare solar production between the two roofs with 5 kilowatts of solar panels on each.
Monthly solar power generation: West vs. North roof orientation
Solar irradiance determines how much solar power is generated by a system. In this section and the next that follows, I will compare solar generation between these two roofs; both with 5 kilowatts of system on each.
First, let’s compare total output for each month..
If you are new to this stuff, let me make a connection between solar irradiance and the solar output for a 5kW system: So
In January, total solar irradiance measured for a day is 6.62 kWh (for west). Remember that this value is always derived by measuring irradiance on a surface area of one square meter.
Luckily, one square meter surface area translates to 1 kilowatts of solar panels. So, what this means is 6.62 kWh from one square meter of space is same as saying 6.62 kWh energy from one kilowatts of solar panel.
So, if one kilowatts of solar panels produces 6.62 kWh of energy in a day, a five kilowatts system would produce: 6.62 kWh x 5 Kilowatts x 31 days = 1026 kWh, in January.
But we don’t get all of 1026 kWh out of the system. In fact, the graph shows only about 850 kWh for West roof in January. The reason are the losses in power due to factors such as temperature (Jan is hot), cable loss, inverter efficiency, panels soiling and more. These losses can vary between 10-20%. 850 out of 1026 is 82%. That means 18% loss (which I assumed for my calculation).
The difference in total output between West and North roof orientation in the summer months is insignificant. So, that’s something to be a little happy about. However, the difference in winter months are very significant.
On average, for the year, west roof cranks out only 15% less energy compared to north. But, does 15% less energy translate to 15% less savings on electricity bill. If north roof saved $1000 per year, will west roof save $850 (15% less)?
I really can’t answer that question for you, only because I don’t know how much electricity you consume each hour of the day, and more importantly whether you are home in the late afternoon – evening hours to take advantage of that solar electricity while its being generated at the highest rate. But you know that more than anybody.
So, what I am about to show you next instead, is the expected solar production for each hour of the day from a 5 KW system on west roof. Does your energy usage habit match this generation pattern? Let’s find out..
Intraday solar generation: West vs. North roof
We previously saw that solar production varies wildly between summer and winter months. With that in mind, in this section, I am going to compare the two roofs for each season separately.
Things to note from the graphs:
- West roof produces more power later in the day, as expected.
- Production in west roof peaks between 2-3pm in the afternoon.
- There is decent amount of power being generated even after 5pm (thanks to longer days).
- Sun is higher in the sky in summer; west roof starts producing generously by ten o’clock in the morning.
As you can tell, all great things happen in the summer for west facing roof. Solar panels gleam with plenty of sunlight, lots of power produced, and production spread across many hours of the day (like the north). So, if you “naturally” consume less power in the morning and more in the evening, you know what that means.
In winter though, things get dreary, for both roofs. Shorter days limit the production to only few hours. West roof struggles for power generation in the morning hours, comes alive only before noon, then drops quickly in the evening hours.
So, after seeing these graphs, do you think west roof is a real blessing for you and your family? Just remember that the more solar electricity you consume, the less you will import from the grid – means more savings on your power bill. If you don’t consume any, you will see your utility bill drop by a very small amount, if any. Also, lately, the financial incentives for “selling” electricity to the grid is little to none. So, that’s not an option anymore.
Payback period for West facing solar panels.
If you are making an investment, you surely care about the returns as well. In case of solar panels, the financial return on your investment is: when do I recover the money I invested in solar panels?
Well, that is going to depend on several factors. But, most importantly: what is your existing electricity rate and how much of the electricity that is produced is self-consumed.
For this exercise, I will make two realistic assumptions: let’s say you currently pay 25 cents per kWh flat rate (this information is in your electricity bill) and you are likely to consume 75% of the electricity that is produced by your 5 kilowatts solar panels. 25% is exported to the grid at 5 cents per kWh.
Based on these assumptions and the production data I already showed above:
West solar panels (@5kW) = 6308 kWh, self consumption @75% = 4731 kWh
North solar panels (@5kW) = 7419 kWh, self consumption @75% = 5564 kWh
If you replaced 25 cents per kWh electricity with the self-generated solar power;
Savings in power bill,
West roof = $1183 (self-consumed) + $79 (export to grid, FiT) = $1262
North roof = $1391 (self-consumed) + $93 (export) = $1484
Today, a good quality solar panel systems can cost between $7k-10k (after rebates). If you paid $8000 for yours, the payback period, based on the savings above, would be 6.3 years for West and 5.4 years for North.
Solar panel system on west roof takes a year longer to payoff but considering how long the system is going to last, which is 25+ years, it is not a bad investment choice. In reality, it may be longer than 6.3 years: will you consume 75% of total output that comes mostly from the late afternoon – evening hours? Check out the intraday production graphs above.
West, Northwest and Southwest: which roof is better for solar?
North-west orientation of roof is better for solar panels than strictly west – both in terms of total output and spread of production throughout the day.
Southwest roof will produce the least amount of solar electricity. I have written about south roof separately. If you have a southwest facing roof, please read my analysis of south roof as well: South roof – How bad is it?
Here is a “compass-comparison” of production from different roof orientations.
And finally, a chart that shows how surface inclination, or roof’s pitch in this case, affects solar generation for a west-facing roof.
Unless you are talking about roof’s pitch that is 50° or more (that’s where the production starts to fall sharply), the pitch angle is something you shouldn’t worry about. Most Australian homes have roof pitch between 15° and 25°.
If you are just starting out looking for solar power system, my recommendation would be to first have a good understanding of your power usage habit. What time of the day do you consume more power – is it in the morning, afternoon, evening, or consistent throughout the day? As long as the electricity generation matches your usage habit, solar panels, even on west roof, can make a good financial case.
I hope that I have answered some of your burning questions on the west roof. If something sounded confusing or you need further explanation, please don’t hesitate to contact me or leave the comment below.