Hourly output of West facing (270 azimuth) panels

[cnil1]

I am in Phoenix and am starting to research solar. I have seen on the Enphase website public section that solar panels that are facing South produce about half of their daily kWH before noon and half after. In other words, the bell shaped hourly production curve peaks at noon.

My question, at what time does production peak with West facing panels? Many or even all of my panels will face West due to roof space limitations and also I would like to shift production until afternoon, because I will be on the APS ET-2 rate plan and I want to offset the most expensive time of the day, which is 12pm to 7pm.

[J.P.M.]

Off south orientations will shift peak production time - to later in the afternoon for westerly facing and before noon to easterly facing with larger off south generally producing more time shift. But, and do not forget, off south orientations will, for most locations, reduce the size of the peak and also the total daily and yearly output, sometimes, depending on local climate, by a fair amount - more off south, more reduction in both daily peak and annual output.

Now, depending on array orientation, local insolation patterns and how the T.O.U. tariff is configured, there is a theoretically optimum orientation (tilt and azimuth) that maximizes annual $$ output from a given array size, different from max. kWh production. That optimum orientation for max. revenue to offset an electric bill seldom exists in reality. More usually it's a matter of comparing $$ production per installed kW for various available orientations and selecting the orientation that produces the most $$ offset per kW (but not necessarily for the most energy output).

Comparing estimates of annual revenue production per kW of system size for various available orientations by combining PVWatts hourly output option with a user's hourly usage history from a POCO on a spreadsheet will often provide useful information. If the available optimum orientation is full, the next optimum orientation can be used, etc.

Commonly, but not always, the optimum orientation in terms of $$ revenue for a lot of current T.O.U. tariffs is closer to an orientation of very approx. 200 - 230 deg. or so. Again, run PVWatts for your location for better estimates.

Often, and in spite of what a POCO may tell you, at least in most of my area, if given a dichotomous choice between a mostly south and mostly west orientation, the mostly south orientation will produce both more output and more revenue than a mostly west facing array of equal size (but the revenue production difference will probably be less disparate). It's never a slam dunk, and given the ease of a PVWatts run or two, the availability of POCO hourly usage data and the ease of constructing a spreadsheet revenue calculator as described above, no reason to assume west is always the best revenue producer.

[easye]

Well. I live in Texas and my west facing panels peak around 2:30. About 1/4 of my panels face west with the rest south. The whole array peaks around 1:45. My west facing panels produce about 6% less than the south facing panels.

[organic farmer]

My photovoltaic array faces due South.

I am located near enough to the Magnetic pole that deviation here is large. I built a large sun-dial before construction of our house, so I could establish True North/South. I wanted our house' Passive Solar gain to be maximized. Later when I was constructing our Solar-Power array I built it to run parallel to our house.

I have read other people who have set a part of their PhotoVoltaic array to face East, to maximize the morning rays. I find that an interesting idea. But I think the total number of sun-light hours a panel can utilize would be fewer hours if it is facing anything other than due South.

I can see the idea behind getting a boost in wattage the morning. But I think the total wattage per day would be reduced.

I agree that:

... Off south orientations will shift peak production time - to later in the afternoon for westerly facing and before noon to easterly facing with larger off south generally producing more time shift

I am not in a state with T.O.U. tariffing. There is no possible 'revenue' from solar power production in my state either. So I never had any reason to consider those parts of a puzzle, that would only apply in some other state.

[J.P.M.]

Well. I live in Texas and my west facing panels peak around 2:30. About 1/4 of my panels face west with the rest south. The whole array peaks around 1:45. My west facing panels produce about 6% less than the south facing panels.

Sounds about right.

[J.P.M.]

My photovoltaic array faces due South.

I am located near enough to the Magnetic pole that deviation here is large. I built a large sun-dial before construction of our house, so I could establish True North/South. I wanted our house' Passive Solar gain to be maximized. Later when I was constructing our Solar-Power array I built it to run parallel to our house.

I have read other people who have set a part of their PhotoVoltaic array to face East, to maximize the morning rays. I find that an interesting idea. But I think the total number of sun-light hours a panel can utilize would be fewer hours if it is facing anything other than due South.

I can see the idea behind getting a boost in wattage the morning. But I think the total wattage per day would be reduced.

I agree that:


I am not in a state with T.O.U. tariffing. There is no possible 'revenue' from solar power production in my state either. So I never had any reason to consider those parts of a puzzle, that would only apply in some other state.

All reference to compass orientation uses true north - magnetic deviation has nothing to do with it unless users are unaware of the necessity of magnetic compass corrections. There are any number of very accurate ways to to determine true north without a magnetic compass. I've outlined a couple in prior posts.

The actual optimum orientation of a solar energy collection device with respect to both azimuth and tilt will vary by location and weather with, for example, geographic features like hills or structures and local common/predominant weather patterns (morning fog/seasonal wind/cloudiness patterns/etc., often and usually shifting the optimum orientation off dead south by a bit. Example: my optimum azimuth for year round max. production is about 186 deg. My property out in the desert has hills about peak about 3 miles to the west that shift the optimum array azimuth to about 178 deg. or so.

Many or, I suspect, parts of most states and areas of the country have, or will have, some form of T.O.U. tariffs in force or available as an option.

I'd also expect that the schedules and peak pricing for such tariffs can be expected to be fluid and changeable as users shift their usage patterns in response to the price squeezes of peak time rates, causing POCOs to adjust peak and other times.

As Gavin Newsom (CA Lt. Gov., speaking about gay's right to marry) said: "It's gonna' happen, whether ya' like it or not."

Regardless of your situation, T.O.U. is the handwriting on the wall for many, if not most parts of the U.S. FWIW, that's probably a good scenario more than bad. With T.O.U., users will have more control over how big their bills will be. They already have a lot of control now, but mostly either don't know it, or know how to turn stuff off, or don't choose to exercise their options. T.O.U. will usually allow some additional flexibility and choice with respect to some control over how much they pay for what they do use - not complete control, just more control as f(when the power is used). That some /most may choose to look at such an option as an impossibility because it's perceived as an inconvenience is a matter of more choice.

[cnil1]

Well. I live in Texas and my west facing panels peak around 2:30. About 1/4 of my panels face west with the rest south. The whole array peaks around 1:45. My west facing panels produce about 6% less than the south facing panels.

Thanks! That's what I needed... a 2:30 peak would be great for me. APS rates are about $0.24 / kWh from 12:00 to 7:00 p.m. and $0.06 outside this range. Therefore, even though a West facing panel would produce, per PVWatts, about 17.5% less annual kWh, because of the later peak, I estimate that 80% of the output would offset $0.24 electricity, rather than 50%, if South facing.

[SunEagle]

My photovoltaic array faces due South.

I am located near enough to the Magnetic pole that deviation here is large. I built a large sun-dial before construction of our house, so I could establish True North/South. I wanted our house' Passive Solar gain to be maximized. Later when I was constructing our Solar-Power array I built it to run parallel to our house.

I have read other people who have set a part of their PhotoVoltaic array to face East, to maximize the morning rays. I find that an interesting idea. But I think the total number of sun-light hours a panel can utilize would be fewer hours if it is facing anything other than due South.

I can see the idea behind getting a boost in wattage the morning. But I think the total wattage per day would be reduced.

I agree that:


I am not in a state with T.O.U. tariffing. There is no possible 'revenue' from solar power production in my state either. So I never had any reason to consider those parts of a puzzle, that would only apply in some other state.

Your decision is a wise choice.

If the total panel system face either East, South or West the shape of the production curve would be pretty much the same. The major difference would be the peak being higher on the South facing array then either the East or West.

When you then compare the total kWh production to the cost/kWh that the POCO charges you a Western facing array may be a better direction or a combination of a South and West split system especially if you have a TOU rate structure.

It all comes down to where you live and what your POCO rate structure is. From there you can make a financial decision as to what is a better way to orient your solar array.

[J.P.M.]

If the total panel system face either East, South or West the shape of the production curve would be pretty much the same. The major difference would be the peak being higher on the South facing array then either the East or West.

.

As a matter of some opinion/interpretation, and respectfully, I don't totally agree with the above statement. Speaking only about clear day system output, the peaks of the daylong curves of system output as f(time of day) will be skewed east or west, depending on the degree of off south orientation of the array. So, maybe 10 deg. off south orientation probably looks mostly like a south facing output. At a 90 or 270 azimuth - not so sure they'd look the same as south facing and sure wouldn't look like one another - somewhat symmetric about solar noon maybe, but not the same. The differences will also become more pronounced as the tilt angle increases.

I do agree that, somewhat depending on tilt, the greatest instantaneous power output will probably be for a closely south facing array, +/- a few degrees.

In all likelihood, the biggest impact an off south orientation will have most of the time is to lower the total system energy output. Different and somewhat separate from that, the biggest impact per installed kW that a PV system will have on lowering an electric bill - that is fewer $$ out the door to the POCO per installed kW - is a function of both how a T.O.U. tariff is constructed and just how a specific array orientation's output as f(time) interacts with that tariff.

[bcroe]

.

As a matter of some opinion/interpretation, and respectfully, I don't totally agree with the above statement. Speaking only about clear day system output, the peaks of the daylong curves of system output as f(time of day) will be skewed east or west, depending on the degree of off south orientation of the array. So, maybe 10 deg. off south orientation probably looks mostly like a south facing output. At a 90 or 270 azimuth - not so sure they'd look the same as south facing and sure wouldn't look like one another - somewhat symmetric about solar noon maybe, but not the same. The differences will also become more pronounced as the tilt angle increases.

I do agree that, somewhat depending on tilt, the greatest instantaneous power output will probably be for a closely south facing array, +/- a few degrees.

My experiments also show quite a difference in the power shape of E, W, and S daily production curves. All start and stop at the
same time, with "tails" reaching the time of the unfavored direction(s). Here is a recent curve from 3 test panels; ignore the top curve.

Yes the S orientation has the highest peak, and more overall energy. But keep in mind, E and W arrays can be compensated
by use of extra panels, WITHOUT expanding the rest of the system in many cases (not micro inverters).

I also am surprised at finding, solar noon here can vary over a 17 minute range each day, in a year. Bruce Roe

[J.P.M.]

I also am surprised at finding, solar noon here can vary over a 17 minute range each day, in a year. Bruce Roe

Assuming you're referring to the change over a 24 hr. period when the sun crosses the local meridian, I suspect you mean 17 seconds per day, not 17 minutes. ? The actual max. daily change in the time of solar noon is about 21 sec., around the middle of Sept.

[bcroe]

17 minutes was the total range over a year. Bruce

[silversaver]

I think South is better than East or West; but in reality, a little toward West like SW 195 might offer you the max production. I personally prefer SW 220 deg because POCO offering TOU here and that will offer you the max credit on your account.

[HX_Guy]

On the same subject, keep in mind that APS has proposed to raise their off-peak rates by 32% in next year's rate case. While the strategy to do a "peak shaver" may be good today, in my opinion the best long term solution is still a full offset system...especially if you can get grandfathered into the plans they have today.

And that $0.06 off peak rate is really more like $0.075 when you add the taxes associated with usage...so the new rate would be roughly $0.10.

[J.P.M.]

On the same subject, keep in mind that APS has proposed to raise their off-peak rates by 32% in next year's rate case. While the strategy to do a "peak shaver" may be good today, in my opinion the best long term solution is still a full offset system...especially if you can get grandfathered into the plans they have today.

And that $0.06 off peak rate is really more like $0.075 when you add the taxes associated with usage...so the new rate would be roughly $0.10.

Which brings up a point in my mind that while it's good to have a working knowledge of what's blowing in the wind for the probability and politics of rates, if for no other reason than to be able to get a better guess at the most cost effective system with a view to the long term, there is a certain logic of simply biting the cost bullet and getting a 100% (or +) system and saying screw it. As you note, one caveat, among many, is to lock in grand parenting if possible. Another might be to be very aware of the costs of any decision in terms of both NPV $$'s as well as lost opportunity costs that come with tying up the extra funds needed for the oversize, and not just knee jerk it.

[organic farmer]

On the same subject, keep in mind that APS has proposed to raise their off-peak rates by 32% in next year's rate case. While the strategy to do a "peak shaver" may be good today, in my opinion the best long term solution is still a full offset system...especially if you can get grandfathered into the plans they have today.

That is an awful big rate increase.

How much of the day is 'off-peak'? Does this apply to all electric customers?

Doesn't TOU rates only apply if you have a 'smart meter' ?