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Solar Photovoltaic (PV)
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 MichelleC
(@michellec)
Active Member Member
Joined: 7 months ago
Posts: 5
Topic starter 10/03/2025 2:25 am  

Is there some table or an online calculator that I can look up expected solarPV kWh generated by month, for my latitude/longitude & azimuth & tilt? Need the figures by the month. Thanks, Michelle



   
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solarPV online calculator
 Gary
(@gary)
Prominent Member Member
Joined: 3 years ago
Posts: 350
10/03/2025 8:44 am  

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



   
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Majordennisbloodnok
 Majordennisbloodnok
(@majordennisbloodnok)
Famed Member Moderator
Joined: 4 years ago
Posts: 914
10/03/2025 10:18 am  

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.


105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and SPH5000 inverter
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; sumus solum profundum variat"


   
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 Old_Scientist
(@old_scientist)
Honorable Member Member
Joined: 1 year ago
Posts: 235
10/03/2025 8:24 pm  

Posted by: @majordennisbloodnok
↑

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
↑

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.

 

 


Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.


   
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 MichelleC
(@michellec)
Active Member Member
Joined: 7 months ago
Posts: 5
Topic starter 12/03/2025 11:14 am  

@majordennisbloodnok what degradation per year is typical with aging?



   
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 Judith
(@judith)
Prominent Member Member
Joined: 2 years ago
Posts: 381
20/03/2025 6:21 pm  

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.


2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with it) open system operating on WC


   
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 AgentGeorge
(@agentgeorge)
Trusted Member Member
Joined: 7 months ago
Posts: 23
05/04/2025 8:37 am  

@majordennisbloodnok 

your tag line has a typo

suus should be sumus (always)



   
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Majordennisbloodnok
 Majordennisbloodnok
(@majordennisbloodnok)
Famed Member Moderator
Joined: 4 years ago
Posts: 914
05/04/2025 11:33 am  

Posted by: @agentgeorge
↑

@majordennisbloodnok 

your tag line has a typo

suus should be sumus (always)

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

 


105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and SPH5000 inverter
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; sumus solum profundum variat"


   
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