Shading late in the day

[ChabSin]

We're planningp an installation on a South facing roof here in California. There are some fir trees directly to the West, which would cast a shade at the end the day (last hour or two, I suppose mainly in the summer).

Where can I find a tool which will help me calculate the effects on production? I understand that production is anyhow lower during that period of the day. But by how much? Is all shading the same (i.e. does a taller tree further away have the same effect as a lower tree closer by)?

Trying to decide if we should place the panels horizontally (better production with shading from that direction, but fewer panels will fit). Or whether to bother trimming the trees.

Thanks in advance for your patience!

[citabria]

I think you are looking for a solar pathfinder...it appears they can be rented. You may need to google for local contacts.

[J.P.M.]

For a lot of reasons, some of which have to do with not losing production due to panels becoming dirt pans and so losing production that way, horizontal orientation is not a good idea outside the tropics and not a very good one even then. As a moderator to lost shading it's no idea at all.

As for shading tools, one is something called the "Solar Pathfinder". See the net for details. Kinda pricey, but can be rented. Another software tool (free) is actually a modeling tool called SAM from NREL. Not for the inexperienced, it'll get you to incorrect conclusions unless you know what you're doing. There are other tools and models of varying cost and complexity.

As for shading penalty, my array is in zip 92026. I have shade on my 195 deg. az. array from about 1600 hrs. solar time onward most of the year. My output penalty from that shading is about 3.5 % of the total output on a yearly basis. Worrying about late afternoon shading may or may not be worth the expense/hassle.

For the most part, shading from trees or the penalty from trees is probably and mostly irrespective of whether the trees are deciduous or connifers and is more f(open area of the obstruction). But any shade from tress/bushes will probably have a very noticeable affect on array power production. If an array has a shadow cast upon it, it'll lower power production more than the percentage of array shaded, and more than most folks know or understand.

[Kendalf]

One other thing to note is that while production is low towards the end of the day, with TOU that is also the most expensive peak period, so loss of production late in the day is probably more costly than loss of production early in the day. How much loss of course depends on just how much shading there is.

[ChabSin]

I appreciate your responses, particularly J.P.M.

I already have an idea of how late in the day the shading begins. I suspect that the SAM tool will indeed be too complicated for a novice like myself to use, and figure out what the impact of the shading would be.

But I was hoping there is a general rule of thumb out there for how production late in the solar day compares to production in mid day. This would at least give me an idea of how much is at stake here for me.

To Kendalf's point: you are, of course correct. However, it's my understanding that even in a system with no shading production is lower than consumption at that time. The idea is to produce a surplus during those peak production hours which also coincide with peak rates (in my case: beginning 2pm clock time) which will cover the shortfall.

[J.P.M.]

I appreciate your responses, particularly J.P.M.

I already have an idea of how late in the day the shading begins. I suspect that the SAM tool will indeed be too complicated for a novice like myself to use, and figure out what the impact of the shading would be.

But I was hoping there is a general rule of thumb out there for how production late in the solar day compares to production in mid day. This would at least give me an idea of how much is at stake here for me.

To Kendalf's point: you are, of course correct. However, it's my understanding that even in a system with no shading production is lower than consumption at that time. The idea is to produce a surplus during those peak production hours which also coincide with peak rates (in my case: beginning 2pm clock time) which will cover the shortfall.

For any use my stuff may have, you're most welcome. Just be sure to question everything everyone says or writes, including me. There's a lot of uninformed B.S. and repeated hearsay masquerading as experienced information.

As for a general rule of thumb (which are best used for measuring thumbs), there is sort of a relatively easy way to get a reasonable guess at yearly total production for any hour of 24 hours by running PVWatts for your situation, using the hourly output option, and sorting by hours of the day for each day of the year and summing the totals. That will do nothing for a shading est., but it will tell you what the max. shading % is likely to be for any hour of the day totaled over 365 days by assuming complete shade for that hour (or hours).

For example, for my array, ~ 11.2% of the PVWatts modeled annual output is between 2 P.M. and 3 P.M. Approx. 8.3 % comes between 3 and 4 P.M. Approx. ~ 4.3 % comes between 4 and 5 P.M ,and approx. ~ 1.3 % comes after 5 P.M, for a total of ~ 25 % of annual production coming after 2 P.M., but only about 5.6% comes after 4 P.M.

Now, I've got my array dialed in pretty good and as best as I can est./SWAG, I lose about 3.5 % of annual production to late afternoon shading after ~~ 4 P.M. solar time, which would mean that after 4 P.M. the tree to the west of my array causes a loss of ~~ 3.5/5.6 ~ = 60% of the incoming irradiance. I'd get rid of the tree but it ain't on my property.

Bottom line, after 3 or 4 P.M., in many places/locations solar array production is not a large portion of the hourly production of a reasonably oriented array.

As for T.O.U. and facing an array west with the idea of benefitting from mid to late afternoon boost in $$ offset, as best as I've been able to figure out, at least around my area and for the latest SDG & E T.O.U. tariffs, schedule DR-SES, due west facing arrays are losers with respect to how much of an annual bill they will offset compared to more southerly facing arrays of just about any tilt. For example, a south facing 20 deg. tilt array will produce about $453 per year in electric bill offset per installed S.T.C. kW of array size. A west facing 20 deg. tilt array will produce about $419 per year in electric bill offset per installed S.T.C. kW. I don't have the per hour annual bill offsets for those orientations (yet - but I will now that the question seems to have come up), but I suspect between the slightly higher potential output for westerly facing arrays and the greater per hour T.O.U rates after 4 P.M., that some additional $$ penalty for westerly shading on a westerly facing array would be likely, but since the contribution for hours after 4 P.M. over 365 days is rather slight to begin with, I bet it won't be much more in terms of bottom line $$ loss in bill offset revenue than would be incurred for a mostly south facing array.

BTW, and FWIW for anyone still reading this mental spoor, the optimum orientation and tilt for my location is about 202 deg. az, and about a 30 deg. tilt. That orientation produces about $463 annual bill offset, and so not much more than a south facing 20 deg. tilt.

Take what you want of the above. Scrap the rest.