The Case for a Significantly Undersized System with Time-of-Use Billing

[SolarFuture]

I would point out here that in some locales (coastal Socal) west facing or southwest facing have an additional benefit - you pick up late generation and do not lose much early generation, since for the best solar months (May-September) morning clouds are very common. They almost always clear by noon.

Excellent point!

[Mike90250]

Welcome Lxm, this Forum does not work well with apostrophes.

it actually does work with apostrophe's' you just need to use a character set that spammers have not been using nearly universally.

[ButchDeal]

it actually does work with apostrophe's' you just need to use a character set that spammers have not been using nearly universally.

or anyone with a modern computer and software.

[J.P.M.]

I would point out here that in some locales (coastal Socal) west facing or southwest facing have an additional benefit - you pick up late generation and do not lose much early generation, since for the best solar months (May-September) morning clouds are very common. They almost always clear by noon.

I'd agree with most of that, but caution that in most cases an all the way shift to west (or very close to a 270 deg. azimuth) if a more southerly azimuth is available orientation may not be the best move, either for output or annual bill offset.

Since most roofs and arrays are stationary, and also probably not strictly oriented in the cardinal directions, some estimate of annual output on an hourly basis plus a reasonably reliable way to compare annual bill offset differences for various orientations is necessary.

While I've done a lot of comparisons using SDG & E T.O.U. tariffs and TMY3 data for Miramar(inland), downtown (airport, coastal) San Diego and also Carlsbad (mostly coastal), for most every case, south facing (180 deg. azimuth)beats west facing (270 deg. azimuth) for annual output every time.

Down at the lbottom line where most folks' real interest lies, that is, as for bill offset, using current T.O.U. tariffs and times things get murky or at least very hard to make general statements about due to the fact that everyone's use pattern will be different. But after looking at a lot of SAM runs, I've come to the opinion that more southerly orientations will usually produce more bill offset than more westerly facing orientations.

Often, the difference is small to moderate, maybe like <10 % or so, maybe a bit less, over a year but with the more south facing orientations producing more offset, even for the downtown and Carlsbad TMY locations and data. Looking at the data, seems to me that more coastal orientations, while they will tend to shift optimum or maximum bill offset orientations to the west by a bit, that shift will probably not be as much as a gut check might lead some to believe.

Every situation and use pattern is different. 'Ya gotta' run the numbers.

[jflorey2]

I'd agree with most of that, but caution that in most cases an all the way shift to west (or very close to a 270 deg. azimuth) if a more southerly azimuth is available orientation may not be the best move, either for output or annual bill offset.

Definitely agreed there. I ran some numbers a few years back and found that, with some basic assumptions for June gloom, the ideal angle was about 205 degrees for maximizing summer generation where I was (a few miles from the coast.)

So in the case of a fixed ground mount array south-southwest is going to be close to ideal. The amount of west orientation can be estimated based on distance to the coast, since June gloom doesn't often extend more than about 10 miles inland. For cases like mine (southwest or southeast exposures) southwest wins hands-down.

[J.P.M.]

Definitely agreed there. I ran some numbers a few years back and found that, with some basic assumptions for June gloom, the ideal angle was about 205 degrees for maximizing summer generation where I was (a few miles from the coast.)

So in the case of a fixed ground mount array south-southwest is going to be close to ideal. The amount of west orientation can be estimated based on distance to the coast, since June gloom doesn't often extend more than about 10 miles inland. For cases like mine (southwest or southeast exposures) southwest wins hands-down.

Thanx for the info.

I'm about 20 air line miles from the coast (zip 92026) @ ~ 1,400 ft. elevation. Still get May gray & June gloom but I'm above most of it by ~~ 10 A.M or so and probably more clear than Miramar before, say, 9 A.M. most of the year.

I crunched some #'s last year using latest T.O.U. rates and times (schedule DR-SES, that is, T.O.U. rate schedule for use with a residential PV system), using the before and after rate and time readjustment for T.O.U. which is how I got to the conclusion that the new T.O.U rates and times made PV systems ~ 20 % to 25 % less cost effective than the old T.O.U. rates and times. But that's off topic.

Using the Miramar TMY3 weather data, and a less than 100 % use offset array size and the concept of treating an array as a revenue generator. What I got was revenue generated per year per installed STC kW. I then varied the orientation to get annual offset revenue for any chosen orientation. The limitations are that the system, whatever size it may be, generates no more than annual usage, and I took no account of NBC.

At that time, for Miramar TMY3 data and using PVWatts because it seems the best combination of reasonably reliable output and ease of use for the most # of users, and the new (at that time), less favorable T.O.U. rates and times, the greatest bill offset per installed STC kW was at an azimuth of ~ 202 deg. and a tilt of 30 deg., or probably pretty close, it seems, to your findings. My SDG & E rates need updating, but the time schedule hasn't changed.

Back then, the revenue produced at 202 deg. az. and 30 deg. tilt was ~ $450/yr. per installed STC kW.

By way of comparison, south facing 30 deg. tilt using the same T.O.U. rates and times produced ~ $445/yr. per installed STC kW.

For the 225 deg. az., the optimum tilt was ~ 30 deg. and that orientation produced $445 /yr. per installed STC kW. A more common 20 deg. tilt @225 az. produced $440/yr.

For the 270 deg. azimuth, the optimum tilt was about 20 deg. and the revenue offset was ~ $408/yr. per installed STC kW.

Again, the rates used were current then ,but need updating, but the T.O.U. times have not changed. I suspect the rate changes have produced some changes in the revenue offsets, but I also suspect those changes aren't enough to change the optimum orientations much or make the 270 deg. azimuth any better as a revenue producer compared to the more southerly orientations.

Being closer to the coast will probably shift the ideal azimuth a bit more to the west, but I suspect not much. The optimum tilt at south and all west of south azimuth may also become a bit lower, due to cloud(y) times usually being earlier in the day, but I'd SWAG not by much.