Solar PV direction / time of year / angle

[sparkypenguin]

Hi all,

I am designing a Solar PV / Battery / Export system and I will have panels on 3 different roofs, E, W and N, S not possible.
I have trawled the net and found a couple of calculators and articles about efficiencies and angles etc but non take into consideration all aspects.

This is what I have found:-

Month	Approx. PV production %
JAN	3.0%
FEB	4.5%
MAR	8.8%
APR	11.0%
MAY	12.0%
JUN	13.5%
JUL	14.0%
AUG	11.5%
SEP	9.0%
OCT	6.0%
NOV	4.2%
DEC	2.5%

Month	Optimum Tilt of Solar Panels by Month
JAN	71
FEB	63
MAR	55
APR	47
MAY	39
JUN	32
JUL	39
AUG	47
SEP	55
OCT	63
NOV	71
DEC	78

So these are very broad figures and do not take into account the 3 roof directions which will surely have different PV production / optimum angles due to different sun heights.
Also these angles will change throughout the year.
Hence I am trying to calculate the optimum angle for each roof as I am sure that the 3 should be at different angles.
I want to be able to see how much each roof will produce by day / month and also the effect of changing the angles of the panels.
This is also important as I am planning on having electric heating and therefore will use much more electric during the winter months which may mean that I am better having a higher angle that will produce more during the winter and sacrificing some of the summer production that will just get exported.
I may also consider some form and mechanical device that will allow me to alter the angle if I decide it is worth it.
Therefore I'm hoping someone can point me to a calculator of some sort that takes all of this into account and allows me to mess about with numerous scenarios.

Any help would be much appreciated.
Regards,
Mark.

 

[bladerunnerpv]

Hi, have a look at PVGIS (https://re.jrc.ec.europa.eu/pvg_tools/en/)
IIRC it will work out the optimum pitch angle and it will calculate kWh produced monthly, but only for an individual roof. You can manually enter the pitch too.

Also free, there is easy-pv too (www.easy-pv.co.uk). You'll be able to set up some panel configurations on your roof and work out the yield as a total p.a., it will work out grid and PV import and export for you, and has nice graphs of daily usage, which are based on a nominal year I think.

PV panels tend to be fitted flush to the roof and at the roof pitch, not sure if that's a planning restriction, but not seen anything else. North facing PV is unlikely to be worthwhile in terms of R.O.I.

only seen tracked panel systems for ground installations, is that an option for you?
https://www.spiritenergy.co.uk/kb-pv-solar-trackers

tbh the sun is so low and relatively weak and spends much less time above the horizon in winter that I expect you'll have a very hard time running elec. heating from PV in winter.

Other option is an battery storage filled from overnight economy elec, but not a cheap option due to LiFePO4 battery costs. You'd need to check the small print from elec. suppliers, most of the good deals are aimed at EV users. Note that batteries don't work so well when they're cold.

System Viability all depends on how many kWh per day you are needing for the elec heating.

Rule of thumb I've seen is that E or W facing panel output is about 80% of S-facing for uk

Enjoy your designing

 

[sparkypenguin]

@bladerunnerpv
Thanks for the detailed response, very much appreciated

I do believe that I would have to obtain planning permission if the angle resulted in the panels exceeding a certain distance from the roof.
And as my property already has its permitted development removed I am going to have to involve planning regardless.

I'm still doing the calcs to determine if I wish to have the N facing ones, hence the need for the calculator / extra info.
I am planning on having a substantial battery system (batteries in an insulated garage) so the overnight economy is an option.
Plus if I'm already having them installed on 2 roofs then the cost of adding a third will mainly be the panel cost only.
Also I am doing a lot of the work myself as it's a new build that I am building.

Ground mounting not an option so I was hoping someone had developed a roof top mounting system but so far no joy.
I may have to design one myself.

I did a bit more digging and found a graph that shows the effect of changing angles throughout the year, not for my location but v interesting, especially the significant improvement when tracked (green line):-

The graph below shows the effect of adjusting the tilt. The turquoise line shows the amount of solar energy you would get each day if the panel is fixed at the full year angle. The red line shows how much you would get by adjusting the tilt four times a year as described below. For comparison, the green line shows the energy you would get from two-axis tracking, which always points the panel directly at the sun. (The violet line is the solar energy per day if the panel is fixed at the winter angle, discussed below.) These figures are calculated for 40° latitude.

 

[binky]

so, if you had variable tilt of the panel array, you are also going to have to leave suffcient gaps between rows of panels to prevent shading from one panel to another. That in turn means you effectively have less roof space to use as even something like a 15deg tilt means you need a gap of around 400mm between rows. Ergo, any efficency gain is almost certainly wiped out by losing array size, and the cost and complexity of building such a system I seriously doubt would merit the expense, time and hassle.

N roof, ok it's just the cost of the panels, according to MCS guidelines, panels facing north on a 40deg pitch will generate around 400KwH per annum, per kW of panels fitted, which is not a lot! And most of that will be during the summer of course.

 

[michael8554]

Interesting that the violet winter output is only a little more in December than with the panels at best summer angle,

The areas under the curves would be revealing

 

[Bob Smith]

Hi

you may have come across this thread
https://electricianforum.co.uk/thre...the-generation-in-kwh-of-the-pv-panels.55061/
@bladerunnerpv mentioned some of the links in his response
Most calculators will allow you to change the tilt to play around.

As @binky said you'll need a mechanism to change the tilt (I assume you will not go up there yourself to change it). While I appreciate you are building your house and could integrate such a mechanism into your roof design. Is it worst the cost?

In addition, you have a limitation that anything cannot be higher of x cm (sorry I forgot the value) from the roof's highest point.
Furthermore changing the tilt will only improve so much compared to a system that actually tracks the sun.

Even if you can calculate the optional tilt for each month of the year, the fact you can't control the level of radiation (how much cloud coverage) means that you will be investing in a technology which sometimes won't bring you any extra benefits.

I am personally doubtful of the return on investment of having a variable tilt for panels on your roof.

Saying this, I am going to have panels on my gable. It is very easy to access and I am considering wall mounts I can adjust the tilt manually. I was thinking of changing it 4 times a year. However, in my case, the wall mounts don't seem to cost more than the fixed wall mounts.

You are also mentioning that you will use electricity to heat your home. Do you know the expected kWh needed?
If you were to have fixed tilts, what is your expected generated electricity? I do not know how big is your house, if I compare with my 5 bedrooms and the fact I 'll have 14kWp installed (5kW SE, 2.5kW West on the gable and 6,5kW NW), I am expecting between 9 and 11MWh generated per year. Now in the Winter month, I will generate maybe 1MWh, this will definitively not be enough to provide enough energy if I were to use electricity to heat my home.
Furthermore, even if the kWh is sufficient, the instant power could be a limited factor that will force your to still import electricity (or use from your battery) (if your home needs 5kW to run and your panels provide only 2kW, 3kW must come from the grid or the batteries or both)

 

[sparkypenguin]

@Bob Smith @michael8554 @binky @bladerunnerpv
Thanks to everyone for their input , it's very much appreciated .

I now think I have enough opinions and new information to push me towards not having any form of angle changing system, it's just a pity that I could not have a ground based tracked system as they appear to significantly increase the energy produced but not an option for me!

I may still fit the 3 roofs at different angles depending on my findings once I have used some of the suggested websites.


A few comments below just to tie up any loose ends on the thread.

Interesting that the violet winter output is only a little more in December than with the panels at best summer angle,

The blue line is the full year angle so not sure if this is actually the same as the summer angle?

The areas under the curves would be revealing

Totally agree the area would be very helpful and I suppose not too difficult to work out if put onto grid paper.

You are also mentioning that you will use electricity to heat your home. Do you know the expected kWh needed?

Not yet as I'm going to decide my heating system design once I know the max PV I can produce, store and use throughout the year.
Hence my need to understand angles, times of the year etc...
My house is very, very well insulated so I am hoping that the energy required will be less than a typical build.
Also I am going to have a large battery so should be able to top up with an economy rate during off peak times and sell back during peak times.


Thanks again to all!
Mark.

 

[Bob Smith]

Good luck with your project @sparkypenguin

 

[binky]

My house is very, very well insulated so I am hoping that the energy required will be less than a typical build.
Also I am going to have a large battery so should be able to top up with an economy rate during off peak times and sell back during peak times.

I have a friend who built a passive haus, it comes with some small complications like air handling units, but, he has no heating system bar a single electric radiator for the very coldest weather. Insulation is definetly the way forward.

 

[Bob Smith]

About time the UK catch up with the rest of Europe wrt to house quality!!

 

[binky]

About time the UK catch up with the rest of Europe wrt to house quality!!

Can't do that, it affects the profits of tory party donors