Renewables: "talking 'bout my generation"

Grandad2b · 24 March 2023 at 3:18PM

I'm struggling somewhat to see how my measured output (according to the inverter) relates to PVGIS. Have I misunderstood what PVGIS does?
The system was installed late January so our first full day was 26/1 meaning we had 6 full days to the month end. From my reading of PVGIS with thin film panels the whole month should give 40.2 kWh. We got 51.
Last month PVGIS predicted 69.86. We got 208.5. Almost 3x prediction.
This month "should be" 139.3. We're already at 245.

I don't think the inverter is over reporting - it only updates online every 10 minutes but there's often a far greater amount being exported according to the IHD than the last online reading shows.

Martyn1981 · 24 March 2023 at 4:51PM

Hiya, did you set the kWp figure on PVGIS, or possibly leave it at a default of 1kWp?

Edit - Not sure of your orientation, but I've popped in 4.44kWp for my location in Cardiff, at a nominal south facing, 35d pitch, and it suggests 240kWh for Feb, and 393kWh for Mch. Hope this helps.

Grandad2b · 24 March 2023 at 6:24PM

I did. I set it to 4.44 kWp which seems to be too much detail since it variously comes back as 4.4 or even 4. I should probably leave it at 1 kWp and multiply the prediction by 4.44.

One thing that I didn't notice mentioned is shading. I'm fortunate to have virtually none (some trees to the ESE about 25 metres from the house so they have very little effect).

Interesting that you get so much more sunlight in Cardiff. Hang on! I put 170 degrees. Should that have been 10?

Edit: just pointed my system almost due North. Very different outcome (and June is lower than both May and July).

Coastalwatch · 24 March 2023 at 7:06PM

3.39 O's today and so far topping consumption for the first time this year.

Martyn1981 · 24 March 2023 at 7:15PM

Grandad2b said:

I did. I set it to 4.44 kWp which seems to be too much detail since it variously comes back as 4.4 or even 4. I should probably leave it at 1 kWp and multiply the prediction by 4.44.

One thing that I didn't notice mentioned is shading. I'm fortunate to have virtually none (some trees to the ESE about 25 metres from the house so they have very little effect).

Interesting that you get so much more sunlight in Cardiff. Hang on! I put 170 degrees. Should that have been 10?

Edit: just pointed my system almost due North. Very different outcome (and June is lower than both May and July).

I think you've found the problem, and it's a trap many of us fell into. For 170d, I assume your system is just slightly to the east of south facing(?), so you need to enter -10, as the azimuth (point of interest) setting. So south is zero, before south (to the east) is negative, and after south (to the west) is positive.

Having fun yet?

Edit - PS. PVGIS is a little pessimistic with the default losses of 14%. So most of us on here tend to get better annual generation, even allowing for the weather.

I've got a bit of shading, and Solaredge systems to minimise the loses, but even so, compared to the PVGIS predictions, I still average over 100%, in fact an average of 105% for the last 10yrs. No complaints.

Raxiel · 24 March 2023 at 7:32PM

4 O's today.

On the topic of PVGIS, my current extrapolated total for the month is somewhere around 87% of the PVGIS total, but thinking about it, is that not normal for spring? The days at the end of the month are longer than the start, so it stands to reason that the total will be biased towards the end of the month. Autumn months presumably start out ahead of the curve and then drop down as the daylight recedes.

By my reckoning, if I get 2.8 O's every day for the last week of the month, I'll close the gap and hit the prediction. At the beginning of March that would have felt a bit optimistic, but after the last week it doesn't seem all that unreasonable to me.

Grandad2b · 24 March 2023 at 7:44PM

Make that -10 degrees. I expect it's somewhere in the instructions that you have to pretend your panels are on the North facing roof - you know; the one that's covered in moss because it doesn't get any direct sunlight.

18kWh and 4.05 O's

Edit: just seen your reply Martyn

I thought 14% was a bit pessimistic so I changed that to 8. We shall see how that compares now.