Some Observations on Photovoltaic Cell Panels

by Oliver Seely

The text and photos on this page are in the public domain. Copying is encouraged!



Revised January 4, 2013

Introduction

For many years I've taught a course on science and technology from a historical perspective. The voracious appetite for energy by advanced societies and particularly the United States figures prominently in the course. I comment each semester that when humans are forced finally to stop using fossil fuels, either because of increasing costs or global pollution, there will still be plenty of energy from the sun, particularly in regions lucky enough to have cloudless days most of the year. One day my wife said to me, "Why don't you put your money where your mouth is?" So I did.

The decision to install photovoltaic cell panels hinges on where one lives. The amount of sunlight as determined by the climate of one's location and the ability to receive the direct rays of the sun when it is shining helps one to decide if it makes sense to install the panels. On the left and right are images showing available sunlight for the United States and the world respectively. Although our location appears to be ideal because of the available sunlight per year, in actual fact our distance from the Pacific Ocean is only 12 km and coastal fog is a problem part of the year. On the average, the amount of sunlight we get is limited to between 5 and 8 hours per day throughout the year (see the blue strip along the coast in the left image in front of the arrow tip).





Here is another image of the continental United States with a legend including units of kilowatt hours per square meter per day.


















A roof sloping toward the south in full view of the sky would be ideal. However, our roof line slopes toward the east and the west. There is a chimney near the peak, as you can see.








The panels have to be installed where there are no shadows during the day, so we installed our 2.5 kilowatt system on the east-sloping roof with an additional tilt toward the south. There are actually 18 panels each rated at 165 watts which brings the system to a theoretical maximum power output of 2970 watts or 2.97 kilowatts. The rating of 2.5 kilowatts may be due to the installing company knowing that the tilt of our panels would be less than ideal. Maximum output occurs around 11 am each day that the sun shines.
















Miscellaneous Observations

A good energy output per day during the summer is 14 kilowatt hours. A REALLY good day brings 15 and there have been a few exceptional days which brought in 16 kw hours. Cloudless winter days yield around 7.5 kw hours. Rain is something else again. Here is what it looked like on 28 December, 2004 all day. And with more than a little rain. Still, 2.20 kWh wasn't all that bad. There was a day two weeks earlier that achieved only a little more than 0.5 kWh. Note the AC power meter. There may be 271 volts being produced but there isn't enough light to offer even one watt of power.





On some partially cloudy days during the early spring a curious observation can be made as the sun periodically peeks through the clouds. Here are two images on April 22, 2006. Except for the occasional gaps in the clouds, this was NOT a typical sunny day in southern California. But take a look at the meter: 2507 Watts! Is it favorable atmospheric conditions which allow a large amount of UV light to arrive at the panels or is it the added illumination of reflected light from the clouds which increases the power output to an "as advertised" level?





Structural modifications of any kind carry the risk of unintended consequences. Our installation caused a leak through the bathroom vent during heavy rain as a result of runoff onto the roof and subsequent splashing into the vent. The vent was modified by adding a shield, as shown. Ugly though it may be, it did the trick as demonstrated by another rainstorm shortly after its installation.

Each morning the controller goes through its countdown, synchronizing the inverter phase with that of the grid. When it locks into the phase of the grid the system goes online and electricity begins to flow from the panels. We estimate that it will take 14 years to recoup our out of pocket expenses. Richard Corkish of the Photovoltaics Special Research Centre at the University of New South Wales estimates that it will take between 3 and 7 years to produce enough energy to equal that amount contained in the non-renewable fossil fuels used to fabricate the panels in the first place. That estimate can be found in his offering, Can Solar Cells Ever Recapture the Energy Invested in their Manufacture?

Panel Maintenance

The panels are guaranteed for 25 years. The good news is that the panels work silently with no moving parts, pumping excess electrical energy routinely into the electrical grid when we use less than that which is generated. At those times the meter turns backward.


The bad news is that over time in an urban area where there is a lot of dustfall, the efficiency drops. In the Los Angeles area several months can go by without rain. These images show what happens after such a period. We have found it advisable to do a once-a-month rinsing of the panels to make them sparkling clean and to bring them back to maximum efficiency.


















Here's an image taken of the panels after two months without rain. My guess is that the drop in power output was somewhere around 10%.

















The first point in the graph on the left shows the output from panels which had been allowed to collect dust for a month. After rinsing the panels, the output was monitored in 7 minute increments following the first measurement. The "best fit" line crosses the time of the first measurement at an output of 905 watts. The measured output of the dirty panels at that time was 811 watts or around 10% less than that which might have been generated at that time by clean panels.







Here's another one a bit more dramatic. The minutes from 3 to 45 were minutes following 10am, August 5, 2005. It was a cloudless and hot summer morning. There had been no rain for two months. Readings were taken every seven minutes. The first three readings were taken, the panels were then given a cold water rinse and four additional readings were taken. Note the large increase in output for the fourth reading and the subsequent drop-off. I assume that the larger reading is characteristic of the colder operating temperature immediately after the cold water rinse. There appears to be a 15% increase after the rinse.







Recently a visitor to this page complained that my claim of a significant increase in power output after rinsing was not convincing. Here is a third set of data which may be copied and used in any manner you wish.

The data were taken starting at 9:12 am on a cloudless midsummer morning, 2006. The cold water rinse was effected between readings taken at 9:26 and 9:33. The previous rinse had been done about a month before this one. There had been no rain between rinses.
Time Power
output
(watts)
9:12 1838
9:19 1869
9:26 1896
cold
water
rinse
xxxx
9:33 2157
9:40 2079
9:47 2070
9:54 2086
10:01 2124
10:08 2136
10:15 2160
10:22 2174
10:29 2177


Here is my graphical spin on the data. I would venture that there was a 5% increase in power output after the rinse.

How much water?

I was asked recently about water needs for panels installed in some of the desert regions of California. The frequency of rinsing depends on the dustfall of the region, so any projection of needs requires a measurement of dustfall and a comparison with areas for which the dustfall and accompanying energy loss is known, but the rinsing frequency is a judgment call based on that energy loss which one is willing to tolerate. My once-per-month rinsing during the dry season in suburban Los Angeles seems to coincide with an energy loss of 5-10%. The "case study" rinsing frequency (below) seems to be based on an energy loss of 15% to trigger a rinse. A typical rinse of my 18 panels using the method shown in the photo above requires 1.21 cu. ft. of water. The rinse consists of a first pass "to soften up" the layer of dust and bird droppings followed by a second pass to remove the softened residue. 1.21 cu. ft. = 9.05 gallons (U.S., liq.) = 34.3 liters. A careful measurement of volume needed and the noble expectation that one will be able to claim that the runoff will go into one's garden is shattered when one observes the runoff lying in the rain gutter behind a pile of leaves and evaporating slowly. I leave it to the reader to make the calculations needed for his or her application. Suffice it to say that my method is just about the most inefficient one could use. An industrial operation would have an advantage of scale and recycling potential.

Particulate matter

All kinds of stuff falls routinely from the sky: meteorites, cosmic dust and used satellites for starters. At our house in southern California and near an airport we get little spots of yellow gunk which produces opaque spots on the solar panels. It has the consistency of dust particles mixed with oil judging from the way it scrapes off the panels. Fortunately the ultraviolet light breaks enough of the chemical bonds in whatever binds the particles together to allow them finally to sluff off during repeated rinsings. Still, they do represent a potential fall-off of energy generated by one's installation.







Rate Games

When our system was first installed we were billed at a flat rate per kilowatt hour. The meter routinely turned backward during the day. We have since switched to "time of use" or TOU metering. For the first two years of TOU billing, instead of a bill each month at the fixed rate, we received a spread sheet, the bottom line for which didn't have to be paid but once each year. The first year the electric company put us on the TOU-D-2 schedule (the one for big users). TOU metering uses four different rates. Those which are established for the TOU-D-2 schedule are:
Period Cost per
kwh ($ U.S.)
Winter On Peak 0.15
Winter Off Peak 0.11
Summer On Peak 0.335
Summer Off Peak 0.1075


The spread sheet for 06/09/05 to 06/09/06 looked like this:





















The negative bottom line within the spread sheet ($-107.19) is a statement of our credit, but according to California law, there is no compensation to the customer for whatever credit might accrue during the year. That is, the electric company never writes us a check for our contribution to the power grid. We do, however, have to pay an annual TOU meter charge and Customer Charge.

There is the added complication of TOU schedules. TOU-D-2 is for big users. Those users pay a lower average kwh rate, but their flat TOU meter charge and Customer Charge are higher than those for customers on the TOU-D-1 Schedule. The first year, shown here, was on the TOU-D-2 schedule. At the end of the year shown, we changed to the TOU-D-1 schedule in the expectation of breaking even again at the end of the next fiscal year and paying lower annual fixed charges.

Here are the four rates charged on the TOU-D-1 schedule. Note the usurious Summer On-peak rate, which as it turns out, worked in our favor - see below:
Period Cost per
kwh ($ U.S.)
Winter On Peak 0.202
Winter Off Peak 0.142
Summer On Peak 0.504
Summer Off Peak 0.147


The spread sheet for 06/09/06 to 06/09/07 looked like this:





















The credit accrued during this year was $134.00, as shown. The fixed charges came to $41.36, which gave us a considerable saving over the previous year. Most unfortunately, the electric company discontinued the spread sheet for their TOU customers several months ago. Instead we receive a short statement which outlines the applicable charges for that month. To those of us who took the time to learn how to read the spread sheet this decision puts us at a decided disadvantage to be able to estimate how we are doing throughout the year. As if to add insult to injury, the short statement offers the customer a labyrinthine summary of the rate schedule with charges for such things as the " Transmission Owners Tariff Charge," the "Nuclear Decommissioning Charge," the "Public Purpose Programs Charge," the "The Public Utilities Commission Reimbursement Fee", and the "California Alternate Rates for Energy Surcharge, where applicable." All of these fees are charged by the kW hour and I have been told by a representative that there is variability from one month to the next as to which are applied to a specific customer's bill so that even if the customer wanted to create a private spreadsheet, it would be impossible because the rate changes slightly from month to month owing to which of the above charges apply. On the other hand, an approximate rate can be determined by using simultaneous equations between pairs of months in which only one rate "season" was involved: winter or summer. Since on-peak and off-peak rates are different, one can then calculate each rate for that particular pair of months. It isn't exact, but it is close. It does however require a passage of six months through the year (summer through fall to winter) to be able to establish a credible estimate. Stay tuned. I have one month yet to go before I'll have a bead on just what I'm being charged for my electricity.

The large credit accrued has at least one misleading characteristic: it largely represents credit at the highest rate, that is, "Summer On Peak." If a rather small balance shift were to occur toward Net Load from Net Generation for this period, or possibly worse, if the Summer On Peak rate dropped significantly, a customer would risk receiving a large electricity bill. One implication of this subtlety is that an owner of a new solar electrical generation system which generates at somewhat below the level of use, might be shocked to receive a whopping electrical bill based on the inflated Summer On Peak rate ($0.335 per kilowatt hour for the TOU-D-2 schedule and $0.504 for the TOU-D-1 schedule). However, the matter of credit vs. charge cuts both ways. For the periods 2005-2006 and 2006-2007, our energy use off the grid was positive. That is, more energy was pumped in from the grid than was generated by the solar panels (2005-2006, +443 kw hours; 2006-2007, +312 kw hours), but most of this energy came to us during periods of the low billing rates and was offset by net energy generated during the high billing rate period (Summer On Peak). Had we been able to opt NOT to convert to Time Of Use billing, and had continued receiving a bill calculated at a flat rate, we would have had to pay for our net energy consumption. At $0.13 per kw hour (close to the going flat rate), our bills for those two years would have been $57.59 and $40.56, respectively). Customers at the greatest disadvantage are those who install solar panels to generate some small fraction of electricity used and then switch to Time Of Use billing. Partly for this reason, the California State Senate on May 24, 2007 and the California State Assembly on June 6, 2007 passed Assembly Bill 1714 (and approved by the Governor) which allows the owners of new photovoltaic systems during the year of 2007 to opt NOT to have Time of Use (TOU) metering to be installed. The summary of the bill reads, in part: "This bill would authorize the PUC (Public Utilities Commission) to delay implementation of time-variant pricing for ratepayers with a solar energy system, until the effective date of the rates established in the next general rate case of the state's 3 largest electrical corporations. If the commission delays implementation of time-variant pricing, the bill would require that ratepayers required to take service under time-variant pricing between January 1, 2007, and January 1, 2008, and that would otherwise qualify for flat rate pricing, be given the option to take service under flat rate or time-variant pricing."

For the current year in progress, a change in the Summer On-peak rate from $ 0.504 per kWh last year to around $0.35 per kWh this year has occurred for reasons which at this writing are not clear. That will put our domestic system at a decided disadvantage because of the excess of generated energy for the Summer On-peak periods. That is, the cumulative credit received for that period will be less than that granted last year.

All that having been said, any annual credit, whether reflecting rate disparities between summer on-peak generation and winter off-peak consumption or a surplus of generated energy throughout the year will end up making the electric company your favorite charity. "Drat and Blast!" you say. What is to be done? A customer not entirely sympathetic to the hollow-eyed plea for a spirit of giving from the executives of our public utilities needs to find alternative consumption strategies so as to bring the surplus or the credit down to zero. There are a number of amusing and intriguing possibilities which I leave to the creative genius and resourceful diligence of the reader to discover and to put into practice.

Get a check from the electric company?

Starting in January, 2011, in California, people who generate excess electricity were allowed to sell it to their electric utility. That is, for the first time in history, the tops of our roofs began to have profit potential. But before you go out and plunk down the better part of your family fortune to have solar panels installed, the fine print tells you that a customer must have both a $ credit at the end of the year and have generated more kwH than were consumed to get a check from the electric company. Moreover, since the advantages of rebates and tax credits are forsaken if an installation greatly exceeds one's need for electricity, very few customers will ever see a check at the end of the year. That's all right, I hear you say, I'll install more panels than I need and not expect to get the rebates or tax credits on the extra ones. I'll cover my roof with those extra solar panels, sell all of the excess electricity and become rich!

It turns out that the electric company practically stonewalled that idea in testimony before the California Public Utilities Commission by presenting the argument that there are many expenses connected with delivering energy to the customer; those added expenses justify that the amount paid to the customer be discounted. The operative term in California is "feed-in tariff," or the amount you'll get for feeding in your generated electricity to the grid. This writer guessed, in 2010, that the discount would result in a payment 40% of that which we are charged for electricity. Drat and blast! The members of the California Public Utilities Commission caved in to 28% of the average rate paid to the utility for consuming its electricity. My check at the end of the 2010-2011 fiscal year was $8.42, amounting to 3.7 cents per kilowatt hour, based on an average rate of 13 cents per kilowatt hour paid by customers in southern California.

An amusingly diabolical opportunity?

Still, many clouds have silver linings, and here is one to think about. Consider a computer which monitors your photovoltaic system and estimates on the basis of daily use and generation and past knowledge of annual use and generation how much extra energy you can produce each day. Then there is this guy who shows up each month with an empty black box, unhooks a full black box from the previous month, attaches the empty black box, writes you a check and leaves. He's jolly and wears a white jump suit just like the man who delivers bottled water, except that this guy brings in something empty and leaves with another one which is full. Your assignment, dear reader, is to figure out what will be in the black box to make you rich! Nice idea, huh?

The Down Side of Photovoltaic Panel Installations on Your Roof

You're the owner of your own private utility, for better or for worse. Most people won't be interested in assuming that role, just as most people aren't interested in being ham radio operators, model plane flyers or parachute jumpers. As an owner of your own private utility, it is your job to keep it operating, so you have to have a system which will give you a "heads up" when things go wrong.

What do you make of this, viewed in the middle of a cloudless sunny day? It is a non- functioning inverter, that's what it is. Toward the end of last month I climbed up to take a look at my panels and noticed a layer of dust about a month old, so I did a routine rinse of the surface. Then, almost as an afterthought, I took a look at the readout of my inverter and saw what you see here. There was nothing! What did it mean? I contacted the repairman recommended by the Sunny Boy web site and set up a visit for the following day.


















Meanwhile, I took a look in the junction box where the wires from the panels connect to the wire which leads to the inverter. The removal of the inspection plate and a brief investigation revealed the damage you see here. The plastic shield of the twist connector had melted, leaving the conical compression connector which holds the wires together. The single red wire leading out of the connector to the inverter had melted close to its entrance in the connector. The melted break isn't visible here because of the angle of the photo. Much relieved, I called the repairman and told him that the problem was not a malfunctioning inverter, but simply a broken (melted) connection. The voltage coming out of my panels is around 350 V.D.C., so I chose to let the repairman do the work. He arrived the following day, made the repair and I was back in business. But the owner of a PV system must realize that a non-functioning system is money out of his (her) pocket; PV systems need to be monitored and maintained. This kind of problem cries out, at the very least, for a visible indicator in one's house when things are operating properly and perhaps a readout showing the kwH generated throughout a day. Why did it happen? The jury is still out, but an inspection of the interior of the inverter revealed about a dozen dead grass hoppers. Perhaps an insect in the junction box was electrocuted. Perhaps oxidation over five years of operation began to produce an excessive amount of heat in the connection which finally caused the breakage. Stay tuned.









Solar Silliness

When we installed the panels on our house roof I had the expectation that we could allow them to sit there without a worry or care and to generate electricity during daylight hours for the next twenty-five years when the guarantee runs out. That they ought to be exposed to unshaded sunlight was obvious, but my early discovery that in order to achieve maximum output they need also to be rinsed periodically was an early lesson in the maintenance of solar panels. I have been more recently surprised that these two points are not fully appreciated by everyone, not even some "experts."

As more people install PV systems it stands to reason that some will make informed choices and others will not. It is with more than a little amusement then that one can find some rather large but ill-conceived installations carried out by people one would think should know better. Here is an ambitious private installation of approximately 35 kW on an apartment house in Santa Monica, California, consisting of both vertically and horizontally mounted panels. The vertical panels face southwest and do not receive direct sunlight until late each morning. Moreover, neither the vertical panels nor the horizontal panels at the right are tilted toward the south at the angle of latitude. The shadows cast by the 3 palm trees and the eucalyptus tree (right) for the better part of the day almost certainly will have an attenuating effect on the energy output; how much would be a function of the internal series/parallel circuitry but could be determined with a simulated equivalent unshaded system. There does seem to be a cleaning schedule in place judging from the blue crystalline appearance of the panels' surfaces, at left.

The shading of one's solar panels by a neighbor's trees can rise to a litigious level if one lives in California. The Solar Shade Control Act, signed by the governor in 1978, bans trees or shrubs from shading more than 10 percent of a neighbor's solar panels between 10 a.m. and 2 p.m. and includes shading on panels installed after the trees were planted if the trees grow to such a height to produce shade which exceeds that which is allowed by the law. A recent celebrated case invoking that law involves neighbors in a community near San Francisco. Neighbor A planted eight redwood trees, B, between 1997 and 1999. Neighbor C installed a 10 kW photovoltaic solar panel system, C, in 2001. Redwood trees, B, grew until their shade, D, exceeded that which is allowed by the Solar Shade Control Act. In December 2007, Santa Clara County Superior Court Judge Kurt Kumli ruled that six of the trees can remain and that the two generating the most shade must be removed. It was reported on July 23, 2008 by KGO-TV that Governor Schwarzenegger has settled the conflict by signing a bill which states that a tree which casts a shadow onto a neighbor's solar panel will no longer have to be cut down, as long as the trees were planted before the panels were installed.













The California Department of Transportation building in Los Angeles (right) has a system of panels sandwiched in a casing of bullet-proof glass on the south face, but notice in the close-up that each rank of panels shadows the one below. Moreover, there is no cleaning schedule for the glass surface. If one could depend on frequent inundations blowing from the south then these panels would be periodically cleaned, but that kind of weather doesn't happen in southern California. We have lengthy periods without rain and when the storms do come they're more often in the form of vertical drizzles which will very definitely clean the uppermost rank of panels but do little good for the ones below.











The Los Angeles Convention Center has a system which was installed by the L.A. Department of Water and Power. The panels were placed around the periphery of the building well below the roof line (I would estimate 4-7 meters). The panels which are mounted on the east and west sides receive no direct sunlight for about half of each day. The ones mounted on the west side and shown in the photograph at the right are in the shade until early afternoon.


















A system consisting of 3872 300 watt panels (Schott ASE-300-DGF/50) yielding a rated power output of 1162 kilowatts was recently installed on the campus of CSU Fresno over Parking Lot V. The general contractor for this installation was Chevron Energy Solutions. The owner of the panels is MMA Renewable Ventures with which the campus has entered into a 20-year power purchase agreement at a starting rate of $0.16 per kilowatt hour and a 2% annual inflation adjustment. An examination of current rates paid by big users of electricity makes a rate of $0.16 per kilowatt hour appear to be a bit pricey. Note that there seems to be a slight tilt toward the south of 1-2 degrees, possibly with drainage in mind.



However, in the image at the right which has had its brightness reduced and contrast increased, the effect of such drainage where morning dew and occasional drizzles are the only sources of precipitation for several months running is a distinct residue which builds up over the cells at the lowest elevation of each set of panel segments. It is not clear at this writing if there is a program of routine rinsing in place.





Here's one worth mentioning. This 205 kilowatt array is in Washington, D.C. and is said to comprise 891 230 W solar panels. Sure enough, 891 x 230 / 1000 = 204.93 kilowatts. But Washington, D.C. is at latitude 38 o 53' north which means that at the very best, the rated power output of horizontal panels will be attenuated by an average factor of
cosine(38 o 53') = 0.78 , decreasing the figure above to 159.6 kilowatts. Judging from the image at the right, it would appear that there isn't the slightest indication of tilt so as to allow the panels to self-clean in the annual rainfall of 39.3 inches. (100 cm). The average solar energy in Washington, D.C. is about 73% that of southern California, so it could be argued that horizontal panels will gain a little from the diffuse sunlight through the frequent cloud cover over Washington, D.C., but most likely the gain will be more than offset by the loss due to the lack of tilt on sunny days. Moreover, one would expect the dustfall on these panels to turn to mud on the surface, not unlike some of the other examples in this section, until the panel guys arrive to give them a power rinse. Where is this ill-conceived installation, I hear you ask? It is on the roof of the headquarters of the . . . wait for it . . . U.S. Department of Energy.


This installation may be found above the top level of a parking structure on Holliston Avenue at Caltech in Pasadena, California. It consists of 1404 170 watt Suntech STP170S-24/Ab-1 panels, giving a total rated power output of 238.68 kilowatts. The sign in the photo at the driveway claims 199 kilowatts. It was installed by EI Solutions. Note that the panels are mounted horizontally. What is not clear from the image is that the only practical access to the panels for periodic rinsing would have to be by hydraulic lift on the east and west sides. The installation runs nearly the length of the structure and the limited access to the panel surface at the north and south ends would make periodic rinsing of the entire panel surface impractical from those access points. A representative of Suntech Energy Solutions points out that where the realization of installations such as this, including the execution of "power purchase agreements" by investor groups, are concerned, optimizing energy output is only one of a variety of considerations. The others are the level and conditions of any production rebate, time-of-use energy tariffs by the electrical utility, financing requirements for the area available and the stated objectives of the client. That is, given the sometimes conflicting agendas encountered when putting together an investor group to realize an installation such as this, other exigencies have to be considered.


A Case Study

A large system (557 kW) was recently installed on the campus of CSU Dominguez Hills by Sun Edison. There are 3279 panels, each rated at 170 watts, bringing the maximum rated power output to 557,430 watts or 557.43 kilowatts. The panels have been mounted nearly horizontally over Parking Lot 1. At our latitude of 34 degrees north they ought to have been tilted toward the south by 34 degrees if the objective is to maximize the generation of energy. At noon at our latitude on the summer solstice the sun is 10.5 degrees from the vertical. At noon on the winter solstice it is 57.5 degrees from the vertical. Assuming 0% loss if the panels are pointing directly at the sun, horizontal panels suffer a power loss of 1.7% and 46.3% at noon on the summer and winter solstices, respectively, for an average annual loss of 24%. On the other hand, under Time of Use (TOU) billing (discussed above), the On-peak period is from 10am to 6pm when the rate charged is higher and if the objective is to maximize one's $ credit the panels ought to be tilted appropriately in a southwesterly direction. Even though we often get brilliant sunlight in southern California from 7am to 10am, that time period still falls in the category of Off-peak.











But it gets worse than that. The lack of tilt means that there is no natural gravity runoff for rain or rinse water. If it appears to you that from the acute angle of view in the photo above the surface color is something other than the typical metallic blue of a silicon photovoltaic cell, you would be right. It appears (at this writing in the fall of 2006) that there has been no rinsing service to maintain maximum output. The surface has been allowed to collect the dustfall of greater Los Angeles since installation around four months ago during which time there has been no rainfall. It is not clear at this writing what the dark spots in the middle of several of the panels represent, but the buildup of dirt certainly doesn't bode well for the overall output of the panel array. It is also not clear at this writing who suffers the greatest disadvantage (the university or the power company) if the power output drops significantly due to lack of maintenance. Only knowledge of the specific billing arrangement worked out in the contract would reveal that information.

Do real data support the depressing conclusion expressed above? Well, yes, generally. On February 28, 2007, a cloudless day from 10am to early afternoon, the system on our rooftop peaked at 10:51 am with an average power output of 2271 watts over the 15 minute interval (7 minutes on either side) which bracketed the maximum of 2284 watts. Taking the theoretical maximum power output specification of these panels, the 2271 watt average translates to [2271/(18 x 165)]x100 = 76.5%. On that same day the power of the university system peaked at 12:15 pm, showing a power output of 319,841 watts. Carrying out an equivalent calculation one gets [318,841/(3279 x 170)] x 100 = 57.4%, a value diminished, I would offer, by the lack of tilt of the panels at the angle of our latitude. We are stymied at this point from looking more closely at these figures and trying to establish how much the diminished value is caused by the lack of tilt and how much by dustfall because the tilt of the domestic roof-top system is itself not ideal. One would need to observe the output of at least one 170 watt panel the normal vector of which is pointing directly at the sun at the time of maximum power by the array of 3279 panels to establish a credible attenuation of power owing both to tilt as well as dustfall.

Here is the one-year line chart of energy generated vs. date for the university system.





































Note the two discontinuities identified by the arrows. They represent the increased output following rinsing. That we are experiencing the driest year since records have been kept starting in the latter part of the nineteenth century, we've had many cloudless days. All maxima on the chart above are representative of energy output on cloudless days. Taking the highest adjacent maxima before and after cleaning, we have 10/27/2006 and 10/28/2006, 1644 kwh and 1930 kwh. The lower value is 85.2% of the upper value. Again on 3/15/2007 and 3/16/2007 we have 2222 kwh and 2599 kwh respectively. The low value is 85.5% of the higher value, suggesting that the event which triggers rinsing by the maintenance crew is a 15% drop from maximum expected value. The very low energy outputs and those at zero are unexplained. They are either outages of the panel system for part or all of the day or there was a failure of the data collection system. No explanation is available at this writing.

Conclusion

So as to gain maximum advantage from an installed system of photovoltaic panels, the following preliminary conclusions can be made. Most unfortunately, if you are not a resident of the State of California, only (2), (3) and possibly (1) make any sense. Read on:

1. If you are connected to a grid, install a system sufficiently large to generate as much energy as you consume during summer and winter periods, because if your rate varies by time of day and by season, and you take advantage of generating more energy than you consume during summer daylight hours, when the rate is the highest, and consume more than you generate during long winter nights when the rate is lower, there is no guarantee that such a rate schedule will remain to your advantage over the long term. In any case, you need to start thinking about a Plan B to use up the energy credit you build up throughout the year and possibly to install more panels if you find yourself suddenly having to pay for electricity.
2. Tilt your panels toward the south (in the northern hemisphere) or toward the north (in the southern hemisphere) at the angle of your latitude.
3. Regularly rinse your panels to keep them clean and to maximize their output.
4. If the panels meet all of your electrical energy needs, that is, if energy consumption is close to energy generation, then the decision to switch to "Time of Use" metering makes sense only if the Winter Off Peak rate is so much lower than the Summer On Peak rate that some Plan B for using up the accrued credit becomes financially appealing.
5. Don't opt for "Time of Use" metering if your panels produce somewhat less than your electricity requirement during the winter, but more than you use during the summer because a slight change in rate of one period vs. another can make the difference between an annual energy credit and an unwelcome electricity bill. Moreover, if the electric company eliminates the method you have used to track your credit/debit status by introducing a "new and improved" electricity statement and/or a change in rate for one or more periods without prior announcement, you'll be, in our vernacular, up a creek without a paddle.
6. If your panels produce only a small fraction of the electrical energy you use throughout the year then do NOT switch to TOU metering. Doing so would subject you to the inflated "Summer On Peak" rate which at this writing is on the order of three times the flat rate.

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