East/West Solar vs South-Facing

[TheTalkingMule]

The installer I love keeps insisting on east/west for my flat roof urban install. My rowhome is oriented directly east/west so I can easily set my rows up efficiently facing directly south. How could that possibly not be the most logical setup?

I have net metering.
I fully intend to get a Powerwall in a year.

In my mind total annual production is simply stronger facing due south, so that's how I want to go. Especially since I'm paying per W installed. Thoughts?

 

[Solarguy]

The installer I love keeps insisting on east/west for my flat roof urban install. My rowhome is oriented directly east/west so I can easily set my rows up efficiently facing directly south. How could that possibly not be the most logical setup?

I have net metering.
I fully intend to get a Powerwall in a year.

In my mind total annual production is simply stronger facing due south, so that's how I want to go. Especially since I'm paying per W installed. Thoughts?

One possibility is that the structural attachment points to tie in the PV mounting structure might be better oriented for the east/west layout. Just a thought.

 

[Big Earl]

East/west will give you a longer, more consistent period of solar production over the course of the day, where a south-facing configuration will give you a fat lump in the peak of the day with lower generation at the start and end of the day.

Here's an example:

You'll want to crunch the numbers for your particular installation and see what makes the most sense for you.

 

[Dave EV]

Can write more later, but yeah, if you have a limited number of panels and room to space rows apart, south facing panels will beat east/west panels.

You'll also get more energy in the winter, too.

But if you're looking for the highest total annual production and have limited roof space, east/west panels can let you get more panels on the roof and more production overall annually.

East/west installs are common on commercial flat roofs.

The devil is in the details, though, and a lot depends on where you are and your specific install.

 

[Sharkbait]

But if you're looking for the highest total annual production and have limited roof space, east/west panels can let you get more panels on the roof and more production overall annually.

I agree that fixed, south facing is the best (with a minimum number of panels) most cost-efficient solution. My ground-mounted system faces due south, 19º tilt. Perfect bell curve peaking at noon (plus or minus depending on season). Now, if money is no issue for building your system, you could have many more panels divided equally between mounting panels at 90º and 270º azimuth and collect more insolation as @DaveEV indicated. I can easily see why a salesperson would be pitching that solution ($$$$). If you have even more money to burn, you might as well build a system that tracks the sun from dawn to dusk. You'll likely have some nice costs for maintenance as well.

 

[Dave EV]

A great tool for modeling production is PVWatts Calculator

I quickly modeled two 4 kW systems in Philadelphia - one mounted 20* south which is estimated to generate 5362 kWh/yr with 283 kWh in Dec and 548 kWh in June.

And another with 2 kW facing east and west at 15*. That is estimated to generate 4600 kWh/yr with 176 kWh in Dec and 547 kWh in June.

So summer time production is basically the same, but you incur a big hit in the winter as expected losing about 14% less overall production. If you increase the size of the E-W system 14% to get the same overall production, you still end up with significantly less production in winter than summer - 206 kWh in Dec, but 638 kWh in June.

As said earlier - this is only part of the picture - detailed rate plans, shading, cost to install, cost of panels, etc all need to be taken into account to determine the best solution.

 

[Big Earl]

If you have extra roof real estate to burn, a configuration that splits the difference, with half of the array facing SE and the other facing SW, you could end up with the best of both. Thinking out loud for future home building.

 

[nwdiver]

I've faced a similar conundrum recently. I'm working on a 23.1kW array that is limited by its service drop to ~15kW so unless I want to upgrade the service I'm looking at an oversize ratio of ~1.5. I thought that an E/W array would be a clever solution to this to maximize production while limiting peak exports. While it certainly produces more in the morning and evening than a 180S array I was very surprised that the annual production is still higher with 180S.

If you have net metering and no other constraints 180S is still your best bet. You'll lose ~15% with E/W. If you're oversized by ~50% that drops to ~5% but you're still better off with 180S. IF net metering goes away the math changes and you may be better off gaining some self-consumption but losing some net exports.

 

[wws]

Another factor might be Time-of-Use rates. With solar becoming so cheap, the utilities are sliding peak rates more into the evening hours. When combined with net metering, it may be more cost effective - dollar-wise - to orient the array more westward than just pure south to better capture afternoon and early evening sun.

For example, my PG&E EV-A weekday peak time is 2:00 PM to 9:00 PM (3:00 to 7:00 PM on weekends). The weekday mid-peak rate is from 7:00 AM to 2:00 PM. Dollarwise, the peak rate is about $0.45/kWh and mid-peak about $0.25/kWh. So it makes sense to try and harvest as much as possible from the 'after 2:00 PM' sun - even at the expense of some morning sun.

 

[ai4px]

Solar edge makes a modeling tool you can download. Then you can tinker with the projected output for each configuration. Designer | SolarEdge | A World Leader in Smart Energy . Dead south with a tilt at the same angle as your latitude will make the most annual power, ideal if you are on grid and back feeding. SE and SW are better if you plan to go offgrid and want to stretch the production time throughout the day. Our house has hip roof and we’ve got 35 panels on SW face and 25 on SE face. One thing I didn’t correctly assume was the production in the morning. I thought about foggy mornings, and wanted to make more power in the afternoon, so put more panels on the SW face. Truth is painful.... in the summer it rains about every day at 2-3pm. I saw the mistake looking at my hourly production graphs.

Hope this helps.

 

[TheTalkingMule]

Thanks for all the valuable input! I'm going back to these guys and pushing for dead South. Fairly sure they're pushing east/west for reasons other than profits, but you never know.

My proposal has 20 panel east/west and I think 20 panrls fit facing south as well, so I want my 15% production boost. Will post if they give me a specific rationale.

I'm trying to develop a standardized flat roof rowhome product for Philadelphia with this installer and this process for just my house is taking forever.

Thanks again.

 

[SageBrush]

I'm trying to develop a standardized flat roof rowhome product for Philadelphia with this installer and this process for just my house is taking forever.

Awesome !

All the best

 

[mspohr]

Thanks for all the valuable input! I'm going back to these guys and pushing for dead South. Fairly sure they're pushing east/west for reasons other than profits, but you never know.

My proposal has 20 panel east/west and I think 20 panrls fit facing south as well, so I want my 15% production boost. Will post if they give me a specific rationale.

I'm trying to develop a standardized flat roof rowhome product for Philadelphia with this installer and this process for just my house is taking forever.

Thanks again.

Perhaps they can show you some numbers on why E-W is better for you. I'm sure they have modeled both configurations.

 

[SageBrush]

Our local PV raising group puts up about 20% more panels on an E-W roof compared to one facing south for equivalent production. Since the panels end up being about 1/4 -- 2/3 of the total cost the sub-optimal configuration (all else being equal) adds about 5% - 10% to the total cost.

 

[mspohr]

Our local PV raising group puts up about 20% more panels on an E-W roof compared to one facing south for equivalent production. Since the panels end up being about 1/4 -- 2/3 of the total cost the sub-optimal configuration (all else being equal) adds about 5% - 10% to the total cost.

Solar is so cheap now that you can "waste" some panels on less than perfect exposure and still come out ahead. I have a difficult site with lots of very tall trees (100 ft +) that shade different panels in different seasons and times of the day however, the whole installation still has about a 5 year payback so I'm happy. We ended up installing some panels facing South, some East and some on poles where I adjust them to East in the Winter and flat in the Summer... it works well. 30 kWh/day during the dead of Winter and 65 kWh/day in summer (on sunny days).
Here's the past few months. I should end up at net zero for the year.

 

[MorrisonHiker]

We put in as large of a system as possible this year and ended up having panels facing southeast, east and west. We're actually thinking of adding some panels facing northwest next. The return on investment would take longer but there's more to life than return on investment. Unfortunately, we have to wait another nine months before the utility will allow us to install new panels.

 

[mspohr]

We put in as large of a system as possible this year and ended up having panels facing southeast, east and west. We're actually thinking of adding some panels facing northwest next. The return on investment would take longer but there's more to life than return on investment. Unfortunately, we have to wait another nine months before the utility will allow us to install new panels.

Do you really ask for permission from the utility to add panels?

 

[MorrisonHiker]

Do you really ask for permission from the utility to add panels?

We didn't. However, when we asked our solar installer about adding more panels, they told us that they can't get approval from the utility until one year after our first system received PTO.

We did mention the purchase of another electric car and that should allow us about 3000 kWh per year. Unfortunately, it sounds like they want to see how much our first solar system produces before we can add more.

 

[Sharkbait]

Thanks for all the valuable input! I'm going back to these guys and pushing for dead South. Fairly sure they're pushing east/west for reasons other than profits, but you never know.

If you have space, a ground-mounted system is the only way to go IMO. I have a large parcel, so I'm on the ground. Easy maintenance if you need it (I haven't) and much cheaper and/or easier to clean. I pay a local company $70 once a year for cleaning with de-ionized water. You could also grow dense hedges around the system and trim at the inclined panel height, so the system isn't obtrusive when enjoying the backyard. Homeowners insurance also covers the system as an on property structure, so no additional or separate coverage required. I highly recommend a short talk with your insurance company to make sure you're covered for weather, and especially fire if you build on your roof. My system was built to local code and withstand winds up to 140 mph.

 

[TheTalkingMule]

I would absolutely go with ground mounted if I could, but I'm in the city on a 16ft wide plot of land and the driveway has a 37ft house on the south side of it. Maybe when they build a matching house on the north side of the driveway I'll suspend another array between to two. That would be a conversation starter!