What do you mean by 'optimal'?
Are you after the greatest yield per year (kWh/year, greatest payments), or maximum power at a fixed time (kW @ xx:xx, least imports).

It also depends on your local weather regime (temperature, wind speed and direction, cloud cover) and other external influences (trees, buildings).

There is lots of talk about using power diverters to say store the electrical energy as thermal energy. Tow points to think about here, you can't store more than your store can hold and you get the FiTs payment as compensation for sharing some of the power with the grid.

To find out, within a bit, what you can expect, do some scenarios on PVGIS, it is generally within 5% of what you can expect.

Do you know when your greatest demands are?

Posted By: Ed DaviesWhat Steamy says. Also, it's worth making sure you understand the local horizon. If there's an obstruction to, say, the south east then orientating the panels towards the west would increase total energy harvested.

PVGIS's automatic horizon is quite good (well it was for us) but (I assume) it's based on ground height data in its maps so if there are significant buildings or trees it could be way out. However, there's also an option to specify a custom horizon file (a list of horizon angles around the compass).

Can you measure those times, or shift load to when there may be higher output?

There was some US research which suggested a SW orientation, but I think it was from Arizona or some other place where much of the demand is for air conditioning.

In the UK, many days start clear and cloud over later. In most UK locations, I suspect panels oriented somewhere east of south will collect the most energy - though not necessarily at the time you need it.

Actually it will depend very much on location.

If the PV site is next to the sea or in a rural valley it will often experience hazy or misty mornings. These mists normally burn off by around 8 or 9am but do hit the performance somewhat.

(This is somewhat offset by by the greater yields of systems next to the Sea due to reflected light)

My experience (based on 13 years of customer data) is that, wherever your house is, a system facing slightly SW will get slightly more kWh per year than a Due South facing system. This is probably due to long summer evenings giving maximum amount of light and compensating for shorter winter days.

This is dependent on roof pitch and technology type however. Sanyo/Panasonic panels mounted at 35 deg pitch and around 20 degrees West of South have consistently proven to provide more than the same panels mounted at 45 deg pitch on the same orientation. Similar roof pitch and orientations with Monocrystaline panels (JA and Solar Century panels mainly) produced much less.

Some Poly panels (Yingli mainly) have worked better than the Mono in the SW orientations - a discussion for another time.

Back to the point - it depends on your local climactic conditions and your requirements - maximum kWh per year or maximum usage of power produced on site?

Regards,

Simon

Personally I would go East -West rather than South and double the number of solar panels using a controller that can handle 2 strings. The advantage being early morning and late evening power generation. Unfortunately I dont have that option as my barn faces due South.

Agree with almost all the above - esp ST's initial response.

There's a good case for going all-out to maximise mid winter collection from low-elevation sun and let the rest of the year look after itself. In winter, when the sun is out, it's just as powerful and worth collecting maximally, at given elevation-angle, as summer sun.

I guess it depends on your starting limitations/assumptions. If it's a a simple pitched roof with two equal sides, are you going to assume that it's all usable for PV or just the better side? Say you could fit 4 kW on each side, would you stick to 4 kW or go for 8? If you decide only to use one half, or if it's a mono pitch roof, then it's simpler.

Ed

<blockquote><cite>Posted By: SteamyTea</cite>Did you not put some modules on the north side as well John.
Have you got them on yet, if so, what sort of yield you getting?</blockquote>

Still waiting for the agri building to be built due to legal issues but once built it will have majority of solar panels on South aspect with all roof lights relocated onto North aspect with the remaining area on the North fitted with solar. Will also incorporate a solar skin to keep the panels cool and the hot air utilised for drying woodchip.

More complicated than just needed to know of grid or storage connected.

It is the hours times the mean intensity times the system efficiency that will give you the yield (the kWh).
Peak production is variable thought the day, year and weather dependant.
Horizontal (or close to it) modules gives good yield, but poorer maximum power (in the UK). Thing is with a PV system, trying to optimise maximum power is difficult. I hardly want to wait till the system is dishing out 3 kW before I put my kettle on.

Didn't pick this one up before

Posted By: fostertomIn winter, when the sun is out, it's just as powerful and worth collecting maximally, at given elevation-angle, as summer sun.

Posted By: SteamyTeaNot quite.
It is a lot less powerful as it has more air to travel though.

I said 'at given elevation angle', which is what determines the amount of 'air to travel though', regardles of winter vs summer.

If your panels are facing south [¹] then there's much more energy available from the sun at an elevation of, say, 8 to 12°than for the same elevation range in the summer. In the summer when the sun is in that elevation range it's basically behind (to the north east or north west) of the panels.

[¹] Assuming you're in the northern hemisphere, of course.

Depends what and how you are measuring surely

Posted By: fostertomdjh and Ed, I agree.
But there's a belief around that sun is fundamentally weaker in winter - indeed I found that intuition hard to abandon.

I Am Not An Astronomer but IIRC the sun is closer to the earth in December than it is in June...

FWIW If I were to install due S facing PV here (i.e. on Skye at 57N) with a view to maximising winter production (e.g. off-grid) I'd go for a slope of around 60 degrees. Steeper could give a fraction better production in December/January but only a little (& only a little better of not much anyway at that).

Posted By: fostertomOK - W/m2 on a plane perp to sun vector at given sun elevation, clear sky. Any other variables?

But the sun is never at the same elevation at different times of the year! Or at least, it's at 14° or so at noon around midwinter, but it's only at 14° a considerable time before and after noon in summer. So it's not a very useful comparison.

The instantaneous power is not what decides whether it's worth collecting anyway. It's how much energy you can collect over a day that matters, and typically with the panel positioned for optimal total collection over the year.