Solar panels on flat roof; best angle

[ghoti]

We just had a solar panel install in the United Kingdom (latitude of 57 degrees). The panels are installed on a flat roof at an angle of 10 degrees (i.e. nearly flat). I was expecting an angle of 35 degrees for maximum efficiency, but the contractor claims that a 10 degree angle is standard for a flat roof install.

I know that the panels will harvest less energy because of the smaller angle. My questions are:

• Is the contractor's information accurate (or did he just buy the wrong fittings ...)?
• Is the difference in capacity substantive?

Thanks.

 

[SeaGal]

Surprised that wasn't specified before the install. But yes less angle is common on roofs for wind-protection.. Renusol tubs are 15 degrees. Won't make huge difference and when cloudy may well be better.

 

[ghoti]

Many thanks for the reply SeaGal. This is clear.

The angle was specified in the quote before the install. What is installed does not correspond to that quote. This really bugs me but that is separate from whether it's worth asking for changes.

I take your sentence "Won't make huge difference and when cloudy may well be better" to mean that I'm actually OK as I am. Is this fair?

 

[SeaGal]

You can use the tool below for your location to compare expected production vs. angles and direction etc.

JRC Photovoltaic Geographical Information System (PVGIS) - European Commission

Photovoltaic Geographical Information System (PVGIS)

re.jrc.ec.europa.eu

 

[TomC4306]

Won't make huge difference and when cloudy may well be better.

Excuse me?

It will make a material difference in what you can harvest.
They were installed 47⁰ off axis . (Assuming equinox aiming)
You paid for 22⁰ off axis.
I'd be pissed.

 

[chrisski]

Without running the calculators.I’d say you may lose out on 50% of production.

At the shortest day of the year, winter solstice, with the sun low in the sky, I’m finding my 1550 watts of flat roof panels max out at at 600 watts, but my 900 watts of portable panels angles at about 30 degrees max out at 840 watts, and that is the 30 amp limit of the SCC I use.

 

[TomC4306]

Without running the calculators.I’d say you may lose out on 50% of production.

At the shortest day of the year, winter solstice, with the sun low in the sky, I’m finding my 1550 watts of flat roof panels max out at at 600 watts, but my 900 watts of portable panels angles at about 30 degrees max out at 840 watts, and that is the 30 amp limit of the SCC I use.

At what latitude?. OP is pretty far north

 

[svetz]

The best tilt angle depends on your needs and if you have any sort of net metering agreement. With an agreement you want to optimize for maximum year round production. If you use most of your power in winter, you want a steep angle, if it's in summer you'll want a shallow angle.
A program like SAM can help you with various scenarios.

 

[chrisski]

At what latitude?. OP is pretty far north

Good point. I’m at around 32 north, and even further north in percentage of production, I’d expect a more drastic loss this time of year, and perhaps even a noticeable difference for the longest days of the year when the sun is highest.

A solar calculator like in my signature block will show good data and not just heresy.

 

[ncsolarelectric]

Keep in mind, at that latitude, anything more than a 10-deg tilt will require a much wider spacing between rows. Therefore, it maximizes the number of solar panels you can fit on the roof by going with 10-deg tilt, which will generate more energy than fewer rows at 35-deg tilt. The contractor made the right decission.

 

[TomC4306]

Good point. I’m at around 32 north, and even further north in percentage of production, I’d expect a more drastic loss this time of year, and perhaps even a noticeable difference for the longest days of the year when the sun is highest.

A solar calculator like in my signature block will show good data and not just heresy.

Hearsay

According to PVWatts:
Aberdeen/Inverness UK ~57⁰ N
4kw array at 10⁰ tilt, 180⁰ aim yields 2950 kwh annually.
4kw array at 35⁰ tilt, 180⁰ aim yields 3080 kwh annually.
So, actually marginal difference.

 

[chrisski]

I am shocked how little difference the slope makes per month. The only real difference is in the summer months of the panels are too high. This shows 1000 watts of panels.
10 degree slant


35 degree slant

And straight up at 90 degrees