https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html#api_5.1

I'd be interested what others find whilst using that web app, @gary. It's a great resource but it does also highlight how local variations in conditions can affect the theoretical output.

For me, it overestimates by about a quarter which on the face of it is quite a significant margin. However, in practice there are some extras to factor in:

• Shading isn't a big issue but we're not completely immune.
• Although our kWp is calculated on the number and capacity of all the panels, there are four which are not on the main South-facing elevation. This is really important in capturing the early sun and therefore extending the period when we don't have to draw from the battery but it is at the expense of absolute amount of electricity generated.
• Our panels are now four years old so there will be some degradation in their generating capabilities now.

All that is entirely reasonable and if I factor those points in then the web app is doing a pretty reasonable predictive job. However, it's worth remembering these points since if I was making a buying decision based on expecting to generate 6MW/year and ended up getting 4.5-5 then I'd be a bit miffed.

Posted by: @majordennisbloodnok

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I'd be interested what others find whilst using that web app, @gary. It's a great resource but it does also highlight how local variations in conditions can affect the theoretical output.

We've only had our solar since last June, but so far for me it's been very accurate and within the published standard deviation each month.

Posted by: @majordennisbloodnok

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For me, it overestimates by about a quarter which on the face of it is quite a significant margin. However, in practice there are some extras to factor in:

• Shading isn't a big issue but we're not completely immune.
• Although our kWp is calculated on the number and capacity of all the panels, there are four which are not on the main South-facing elevation. This is really important in capturing the early sun and therefore extending the period when we don't have to draw from the battery but it is at the expense of absolute amount of electricity generated.
• Our panels are now four years old so there will be some degradation in their generating capabilities now.

 

All that is entirely reasonable and if I factor those points in then the web app is doing a pretty reasonable predictive job. However, it's worth remembering these points since if I was making a buying decision based on expecting to generate 6MW/year and ended up getting 4.5-5 then I'd be a bit miffed.

Yes, I think you'd need to create separate arrays for each roof aspect, and then combine them to get a representative total. Mine is straight forward as it's a single aspect that is unshaded, hence the results seem remarkably accurate.

@majordennisbloodnok what degradation per year is typical with aging?

This guy did a good analysis 

https://protonsforbreakfast.wordpress.com/2022/03/16/analysis-16-years-of-solar-pv-data/

in summary go down a bit more at first then steady slighter decline. The “spec” is 20% down in 25 years.

@majordennisbloodnok 

your tag line has a typo

suus should be sumus (always)

Posted by: @agentgeorge

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@majordennisbloodnok 

your tag line has a typo

suus should be sumus (always)

Quite right. Amended, and thanks for spotting. Rather proves the point, eh?