I seem to recall a comment, I think from Solar Guppy himself, about the production comparison between east and west facing arrays in Florida. PV Watts shows an east facing array producing about 103% of a west facing array in the Tampa area, over the course of a year. I'm wondering if Solar Guppy or anyone else can share any real world numbers, over the course of a year preferably, comparing an east to west facing roof/array in Florida. Those numbers would be very helpful to me.

Thanks very much.

FloridaGuy

I don't have any data other than do I know the influence of afternoon heat and moisture from the Gulf of Mexico and the Atlantic causes afternoon thunderstorms in FL. If I did not have a decent S exposure, I would pick a SE or E exposure over a SW or W exposure any day in FL due to its weather patterns in the summer months.

The difference in production between East and West facing arrays can be even larger. It really depends on what part of (Central) Florida you are in. Just looking at the historical weather data easily shows the impact of afternoon thunderstorms in the summer on potential production. We always try to go South, then East, then West when looking a possible array locations during a design.

Scott.

Most rain is early afternoon on ward during the rainy season, the temperature climbs 20+ degrees as well, both of these factors make a BIG difference in overall harvest from solar based on the direction the array set too. In the Summer, I see about a 30%+ difference between East-South/East and West - South/west orientations for the roof mounted arrays in Harvest. If I had the choice between East, South or West I would pick due east, coolest part of the day, all year long.

Just to clarify. . . you have a ESE and a WSW facing array that you have checked against each other and you are seeing 30% more production on the ESE side in the summer. Is that correct? Do you happen to have a full years worth of comparison data? To me, the annual basis is what matters most.

I just ran the numbers on an SSW facing roof and an ESE facing roof (90 degrees off of each other), each with a 5.4kW array for May 18 - June 18, 2009. This was a relatively wet period which would tend to favor a more easterly facing array. Even so, the SSW facing array beat the ESE facing roof by 1.21% over that time period. If we'd had dryer weather, it would have almost certainly beat it even more. Unfortunately, I don't have a years worth of data on these two arrays that we are tracking.

It is interesting that you say you'd pick East over South on an annual basis as PV watts shows a south facing 5/12 pitch roof producing 113% of an east facing roof on an annual basis. Did I understand you correctly on that? I don't want to trust PV Watts without verification (although it is a good reference) but that is why I was looking for some other sources of actual results over a longer period of time.

PVWatts only looks at isonolation averages, nothing else. For the angle, its just uses basic cos-sine math from the sun angle to makes it's predictions. It does not use temperature variance over the day or account for when the sun is shining vs when its raining, and these are very important details it misses for Florida's semi-tropical weather.

The panel temperatures ( roof mounted ) in the afternoon will exceed 65C-70C in Florida vs being close to ambient in the morning, this alone is about a 10% gain being east facing.

I haven't logged data for a while, I have my lab in the house and see daily the differences ... The SE and SW arrays are 90 degrees from each other and are not identical arrays. Currently the SW ones are fully instrumented, so I know exactly the temperature and irradiance and see every day how the afternoon effects performance.

Its pretty simple, its always cooler in the morning and it usually is clear until about 11-12 am then partly cloudy to full blown thunderstorms.

For 2+ years the 5kw array was out performing the 6400 watt array for most of the summer months ( 4+ months ) in actual kWhr production



I don't know how your logging the data, if the arrays are equal in performance ... in clear weather and ground mounted, SSE and SSW should have identical performance. Lakeland is the epicenter of thunderstorms in the US for 4 months a year, possible a micro-climate, similar to if one is effected by marine fog layers in the morning, I do know its very real here.

Also note, if you split your arrays, some South and some South-East or South West you very likley will have issues with MPPT if you have them all on one combiner box. Almost always all array(s) on the same plane will need to be on their own charge controller or GT inverter in order to properly track MPP and maximize energy harvest. Just FYI.