PV Daily Curves

[Alan Ambrose]

Hi,

 

I've been fiddling around with PVGIS daily data vs. roof orientation - actually the objective was to get an idea of battery sizing. This is for East Suffolk, 30 degree roof pitch.

 

I think I've figured out that ... if you have the option ... that south facing panels really are better than split East/West panels.

 

Maybe that's not a surprise, but I was assuming/hoping that splitting East/West would be very handy in order to flatten and widen the output curve - which would provide more useful power.

 

Well it looks like it does a little bit in summer, but in spring and autumn and winter it seems to not help at all. See below:

 

Alan

 

Edited October 21, 2022 by Alan Ambrose

[Radian]

Pretty obvious really given how the range in azimuth diminishes towards the winter solstice. Now if you split SE/SW you might see some extra benefit in terms of extending the generating day.

[Dillsue]

If youre looking to size a battery then the battery will give you a more useable spread of energy and I would guess youd want your array to generate the max it can by being south facing. As I understand things you only want to split the array to give a longer spread of energy if you dont have a battery or if youve oversized your array.

[Alan Ambrose]

>>> if you split SE/SW you might see some extra benefit

 

See below - the same thing with SE+SW. So apparently not much benefit in deliberately splitting the roof orientation away from South.

 

But ... less harm of course in doing SE+SW so as you only lose 1% / 8% / 16% of best case Southerly output in Summer / Shoulder / Winter respectively. Numbers for straight E+W are 6% / 25% / 49%.

 

>>> If you're looking to size a battery

 

Yeah, for battery I need to model daily consumption data plus some indication of the variation in generation.

 

Alan

 

 

 

 

[S2D2]

If you have a DCC connected smart meter, see here for sizing the battery:

 

 

I was looking at panel orientation rather than E/W split but found when using a battery vertical mounted panels saved the most on energy bills for my specific location/consumption - you sacrifice summer export to generate more in winter. Same principle here with south facing panels generating more in winter, that will have a much bigger impact on energy bill savings than extending the generation window slightly when the system has a battery. 

[Dillsue]

18 hours ago, Alan Ambrose said:

Yeah, for battery I need to model daily consumption data plus some indication of the variation in generation.

So 100% south facing to cram as much in the battery as you can

[Alan Ambrose]

>>> vertical mounted panels

 

Ah, interesting ... PVGIS gives this for south facing vertical. Although 27% less than the optimum 40 degrees pitch roof, it has wider 'shoulders'.

 

Alan

 

 

 

Edited October 23, 2022 by Alan Ambrose

[Marvin]

Oh, and 18 degrees off vertical I thought was best for winter....

[Radian]

 

30 minutes ago, Alan Ambrose said:

>>> vertical mounted panels

 

Of course this places greater demands on the environment being clear of obstacles. I have a South facing fence that I've had some spare modules stood against and while they're doing great at the moment, I see the shadow of the house getting closer every day 😪

[S2D2]

3 hours ago, Alan Ambrose said:

>>> vertical mounted panels

 

Ah, interesting ... PVGIS gives this for south facing vertical. Although 27% less than the optimum 40 degrees pitch roof, it has wider 'shoulders'.

 

Alan

 

 

 

I should probably have qualified further, I was comparing SSE vertical mounted (gable end installation) to 35° WSW (roof install) so a bigger difference than your comparison. Based on your data carry on with the optimal pitch. Also worth noting that if you plan to export at current rates (£0.15) it tends to dwarf any other small optimisations to just maximise total generation.

[Alan Ambrose]

>>> Of course this places greater demands on the environment being clear of obstacles.

 

I see that PVGIS has a feature to handle that if you're a proper nerd - which I guess I am.

 

>>> Also worth noting that if you plan to export at current rates (£0.15) it tends to dwarf any other small optimisations to just maximise total generation.

 

Just go with South/40 degrees and you're done? You bring up an interesting point though which is that given export is worth anywhere between 2x and 6x less than reduced import, then we should focus not on summer peak but maximising the other seasons?

 

Below are graphs from PVGIS of generation by month and generation by season for various roof slopes (all for a south facing roof). The concussions are:

 

+ for max total generation roof slope of 30-50 degrees is good.

+ for max non-summer generation, slope of 40-60 degrees is good.

+ if you weight the value of the winter electricity (to account for better value of those units), 40-50 degrees is good.

 

Maybe I didn't discover anything new  - except maybe a slope of 50 is a bit better overall than a slope of 40.

 

Alan

 

 

 

Edited October 24, 2022 by Alan Ambrose

[S2D2]

28 minutes ago, Alan Ambrose said:

Just go with South/40 degrees and you're done? You bring up an interesting point though which is that given export is worth anywhere between 2x and 6x less than reduced import, then we should focus not on summer peak but maximising the other seasons?

As usual it depends, but probably if your focus is maximising return on investment. Optimising for offsetting your usage is more nuanced and requires you to factor in consumption, as in the thread I linked. This has the benefit that you'll be reducing that usage for as long as the system lasts, whereas export rates can disappear overnight.

 

For my usage and proposed system preferring increased winter generation leads to a better usage offset but optimising for total annual output is more financially beneficial (at current rates and MCS install). It depends on your priorities.