For owners of solar panels, is their own roof the best location for those panels?

I haven't a clue. In some cases, yes, even for a small producer, but this all seems to hang on exploiting tax and subsidy angles, as here:

[ame="http://en.wikipedia.org/wiki/Power_purchase_agreement"]Power Purchase Agreement - Wikipedia, the free encyclopedia[/ame]

Or some type of charitable subsidy, as here:

Austin Utilities - Environment - Solar Choice
SNAP Program - (SEPA)

If the question is, could you sell green tags for your rooftop panels, I guess in theory you could (though the whole thing has to be certified and audited and such). But if you did, then, from your perspective, your panels would be as dirty as the average coal-fired plant -- somebody else would own the "cleannness" of them.

It's also possible that you transferred the right to those green tags to your utility when you signed up for net metering.

Here in Virginia, they actually spell it out in the state regulation. Your deal can be done either way.

"... The eligible customer-generator owns the renewable energy certificates associated with its electrical generating facility, however, at the time that the eligible customer-generator enters into a power purchase agreement with its supplier, the customer-generator shall have a one-time option to sell the renewable energy certificates associated with such electrical generating facility to its supplier and be compensated at an amount that is established by the Commission to reflect the value of such renewable energy certificates. ...."

 

Only because roof top solar panels tend to be mounted close to the roof while ground mounted solar tends to be mounted at least a couple feet off the ground and angled perfectly. But who has the ground space to mount their system? My system will take up 250 sqft of roof space - 24'x10'. I only have a modestly sized yard and mounting it in the yard would mean a LOT of shade issues which would more than offset any production benefits. Up on the roof, there are NO shade issues at all. Of course, on the roof I'm losing about 10% output because I'm able to orient the panels properly. And just spacing the panels out 6" from the roof minimizes any heat build up.

If you are talking about a 1000w rated array, that is under lab conditions (STC - standard test conditions). My 95% was only talking about how efficient the system is at taking the DC power from the panels and converting it to AC power. What you should really be using are the PTC (PVUSA Test Conditions)

The system I am designing should produce about 90% of it's PTC rated power and that is accounting for the inverter, sub-optimal mounting and heat losses (According to the CSI EPBB Calculator which is supposed to take all those in to account).

This page shows all the typical inefficiencies found in a solar system:

PVWATTS: Changing System Parameters

Line losses are minimal. If you are losing any significant amount of power down the line from your roof to your circuit breaker box, you didn't spec your wire large enough.

Also keep in mind that the heat the panels block on the roof isn't necessarily wasted if you use air conditioning - the panels will also reduce your heat load and reduce the amount of air conditioning you need as well.

Very few systems will be close to optimal - but as the saying goes "Perfection is the enemy of progress!" We should be installing solar everywhere it is good enough, otherwise we'll never get anywhere.

(icarus, I know you are familiar with most of these issues, but I was being verbose to help others who may be less knowledgeable about solar - no offense meant at all)

 

None taken,, but thanks.

Just like the MPG or HP rating of cars can be misleading as we all know, the STC rating of PV can also be misleading to those who may not be well informed.

The emphasis on the "total watt" size of a system, often gives the false impression that if, for example I have a 2000 watt system on my ouse, I can offset 100% of 2000 watts of load, and that is almost never the case.

The axiom that I often use in the off grid world (because the relationship between net/net capacity and loading is so often so critical), is that people too often over estimate the amount of sun (and harvest) that they will get or are getting. At once and the same time, they also underestimate their loads, net/net. The result is that too often folks end up with (off grid) systems that under perform relative to expectations, but usually are right on if you look at the numbers.

I believe a reasonable number to use for a grid tie system is something in the 75% range. That is, for every 1000 watts of STC name plate rating, you should be able to harvest 750 watts out the inverter in full, good sun.

As a side note, it is pretty interesting to watch the volt/amp/watt meter in my charge controller. I was doing some testing yesterday, and with "nearly" clear sky the panel in question was putting out EXACTLY 80% of STC rating, just the number I would use to do a calc. If it was crystal clear and cold it might jump to 100-110% for a short time, especially if there was an edge of cloud event, but even at ~30f, panel temps rise enough to get them close to STC temp outputs.

You are also right, that roof tops are perfect for most folks for a variety of reasons, many you noted. That said, even 6" of airspace under a PV is not enough to keep it as cool as it might be in "free air". The conversation was, would you be better off in some real sense investing in PV off site, where it could be more efficient for a variety of reasons, and the answer is clearly yes. The problem is the way the tax credit and rate structures are calculated, it forces one to install on site rather than remotely,, and that I think is a shame!

I also agree that you can't toss out the good waiting for the perfect. You will wait forever! Just look at the health care debate!

 

Just curious for those that also have the PV panels.....do you monitor the inverter information at all? I've got a monitoring system in place, that if I chose to use the electricity to keep the computer on all day (which I did for about 6 months) will do continual updates at set intervals and upload to the internet. It's really quite interesting, and if you had sufficient information in regards to weather, could produce some really cool graphs / charts I think.

Anyone that wants more info on the program, feel free to PM me. I don't typically go back and look for threads specifically, but if they pop up in the "recent activity" section then I will take a look again.

 

Power Purchase Agreements (PPA's) ~ Funny you should mention this. We began our plans to go PV back in September of 2008. We researched the paltry Federal incentives for individuals. They were virtually worthless. But for PPA's you could write off 30% as a tax credit on Fed income taxes. You have to be "In the business". That's not a big deal. We'd simply "add" that function through our corporate identity, and our personal identity would be the customer. We began to structure the deal, and it was a pain.

The economy had collapsed in 2008( [ame]http://en.wikipedia.org/wiki/Emergency_Economic_Stabilization_Act_of_2008[/ame] ), and the Fed's decided it'd help to prop it up with lots of incentives, including the 30% renewable energy credit to individuals ... but ONLY folks who held off until 2009. So we disolved that element of our business, but our utility bill back under our personal identity and held off our PV until January 2009.

Any person or group of people can work as a PPA. The trick is to have enough capitol to sponsor a dozen or so systems. Once they're up & running (in short), the customer has no electric bill, & simply pays the PPA a fixed lease fee. As power cost continue to go up, the PPA makes more & more money. So if you know a half dozen or so folks who feel ambitious, there's an income source out there just waiting to get started.

A couple folks with PV have sent me web links that show operation of their systems accessable via the internet. Pretty cool. You can follow production per day ... week month year or live during a given day (or no production due to night). I'll see if I can find them & come back with an edit.

Ground versus Roof: Those are individual variables for each person / system. Some folks have acres of land, but it's sloped down 20% facing north. Others (like us) have roofs that don't face ideal directions. And there's no such thing as angled perfectly unless you have a tracking system that ensures the panels are continually perpendicular to the sun ... oh, and none of your neighbors within 200 feet can have any trees. That's rare. Eh, you likely know all that. But as for mounting close to the roof? Our 'sun-power' panels are mounted about 8 or 9" above the roof - so there'll be plenty of cooling area. That spacing is probably standard.


Edit:
Bah! Can't find the local folks, using the coolest setup(s) - but here's an example of web accessable PV logging (give the flash program a moment to load):

http://dean0.mysolarlog.com/?template=flash

I think that not all manufacturers of grid tied systems offer the compatable hardware that interfaces with the software. You'll have to check with your installer or manufacturer.
.

 

Definitely - when I first learned of the difference between STC and PTC ratings, it was enlightening, but not too much of a surprise.

Yes, I think those are pretty close to the numbers that PVWATTS recommends using (IIRC, they suggest 77% as a ballpark figure).

How does an "edge of cloud" event increase power output?

Definitely not, but I'm not sure it's as bad as you might think.

Hah, sometimes I wonder if anything will ever get done.

Just about all the systems will upload the data to the internet to the inverter's companies servers so you only need to keep your internet router turned on.

One cool thing about the Enphase microinverters is that you can monitor each panel's output individually. They have a bunch of reference systems installed which are fun to look at: https://enlighten.enphaseenergy.com/systemslist

 

I have two ground mounted PV arrays (2.88 and 2.4). Personally I prefer to have the panels mounted on the ground. Easier to clean, remove snow and cooler although, when you put your hand on the underside of the panels in the summer they are very hot.

 

Drees,

As the sun transitions from behind a cloud, particularly a puffy cumulus type cloud, the reflections from the edge of that cloud, coupled with the full sun can have a considerable net increase in insolation, although short lived, a matter of a few seconds to a minute or two. It doesn't have a huge effect on total harvest, but it is important to use in calculating Vmax of any array.

Just for edification,,, STC is Standard Test condition. STC in PV solar is this: The PV module power ratings are for Standard Test Conditions (STC) of 1000 W/m2 solar irradiance and 25oC PV module temperature.

That is know irradiance and know Pv temp.

PTC is defined as the following: (From solarhome.com)

"What is the PER WATT PTC rating of the solar panels that the dealer/installer is trying to sell you ? In an effort to create a level playing field, the state of California as well as many other states require that all solar panel manufacturers submit their product's operating specifications. These specifications are compared to real world performance ratings that have been determined by an independent laboratory called PVUsa before they are approved for the state's program. PVUsa uses more stringent conditions than the manufacturer uses and assigns a PTC rating or (PVUsa Test Conditions rating)

So a solar panel that has a STC or (Standard Test Conditions rating) of 170 Watts might have a PTC rating of 149 watts. The PTC rating along with the efficiency rating of the inverter is what the state uses to determine the cash rebate. So the higher the PTC rating that a solar panel has, the higher the cash rebate that goes into your pocket. So needless to say not all solar panels are created equal.

When shopping for a system you will rarely see two systems that use solar panels with exactly the same wattage ratings. For example, one system might use (20) 175 watt panels and have a total DC Watt rating of 3,500 watts and another system might use 170 watt panels and have total DC watt rating of 3,400 watts. So how do you compare the each system's true CEC performance and the amount of rebate per watt when two different wattage rated panels are used.

Simply visit the California Energy Commission's website at http://www.gosolarcalifornia.ca.gov/equipment/pvmodule.php and look up each individual panel's PTC rating. For example the 170 Watt panel might have a PTC rating of 152.5 Watts and the 175 Watt panel might have a PTC rating of 154.9 Watts. Simply take the 170 Watt panel's PTC rating of 152.5 and divide it by 170 watts and you'll get a ratio of .897. Next do the same for the 175 Watt panel. 154.9 divided by 175 Watts gives you a ratio of .885. Obviously the higher the ratio, the higher the per watt CEC rating.

Along with a high PTC rating it is also important to check what the manufacturer in guaranteeing that you'll receive. Again, if you're buying 190 watts but the manufacturer is only guaranteeing that you'll receive 171 watts due to a poor negative tolerance rating, then the PTC rating is meaningless."

PTC is a newer test standard that has evolved with primary impetus coming from California.

 

You can probably use the SG View software and some simple wiring to get the information to your computer. Try looking here:

Monitoring Software

I use it on my computer because it saves all the data into a spreadsheet. Makes it simple for me to track my actual electric usage every month, plus provide information to the "broker" for my RECs. I know for me, I have the GE system and I only had to wire up 3 wires using a Serial connector. You would also need a Serial - to - USB converter unless you're running Windows XP or older more than likely. Most PCs and Laptops don't even have a serial port on them any more, at least from what I've observed. I've even heard of people that buy cheap laptops just to monitor their inverter figures continually.

I used to monitor 24/7 and have it upload to my website a graphic of that particular instance. I've since chosen not to use the extra energy and just update it when I turn on the desktop.

This is actually dependent upon which inverter you have. I believe the GE systems were most prominent, and so you would need to do some wiring to get the data from the inverter to the computer. I'm sure some of the newer ones have this feature built in, as long as you can get the connection to your router. I load the data to my computer periodically.

 

For those that want a real time, real world solar installation, and how it might work on your house at your location, PVwatts is a great easy to use tool. I prefer V1 as it is easier to use IMHO:
PVWATTS v. 1

You plug in your location, the orientation of your roof, the size of your desired PV array, and it will calculate the amount of power it will reasonably be expected to generate in a day/month/year. It will also calculate the value of that power, compared to your local utility rate.

All with a few mouse clicks.

 

I was bench testing a charge controller today, and experienced some pretty interesting edge of cloud events,,,just to illustrate the above question.

With a single panel, a 60 watt 12 vt nom. panel, steady state with PV warmed up, early afternoon, (Warm panel puts out less than a cool panel,, see above) Panel was putting out a very steady ~50 watts. After noon fair weather cumulus began to fill the sky. Right away, the PV began to put out ~52 watts,, a 4% increase. (this was just with scattered cloud, no right next to the sun) When the sun dipped behind the cloud the output dropped to near zero as expected, but as the sun begins to come out, the output rises to ~ 50 watts again as the sun comes out from behind the cloud. Watching the meter, the output continues to climb so that when the sun is a few degrees from the cloud, the highest wattage I saw today was ~55 watts for about 30 seconds, a 10% boost.

Now the effect is even more significant in our Northern Canada location. There, super cold panel temps,,,~0f and huge reflection off of ice and snow (we are located on a island so the lake is winter frozen, covered with snow) and the numbers are even higher. In fact, I have seen on my battery based PV system numbers that exceed STC numbers by more than half.

 

Nice data. Would be interesting to get some data based on how much air flow the panel has, by varying mounting height. What's your setup like - how are you able to test a single panel?

For example, stick the panel flat on the ground (pavement or dirt to somewhat simulate roof conditions), 3", 6", 12" and 3'.

A thermometer taped to the back of the panel to measure temps, and then measure output.

 

Drees,

I have a bunch of individual panels lying around. In this case, I just wired the output into a charge controller, (remember battery based) so the 17.4 Vmp panel charges (through the charge controller) a 12 volt battery. I just leaned it up in front of the truck and wire the controller to the battery.

While I didn't measure PV temp, but I certainly could on a stable sun day. The best way to do it would be to have multiple panels in different orientations so that they wouldn't be subject to the heat (or cool) from a previous test condition.

Also sticking a panel "flat on the ground" and then raising it X" would be a bit problematic as the panel wouldn't have a very good attitude for proper solar harvest.

What I could do is place a single panel on a roof section directly, and then raise it. I may be able to try it in the next little while.

My system is a kind of odd set up. I have a number of panels on the roof, elevated ~6" off the roof deck. These are on an ideal angle for the latitude for year round harvest. (The house was designed with this in mind) I also have another series mounted on the front wall of the house, under the eave. These panels are hinge mounted so that I can change the angle with the season. By mounting them on the wall, they don't accumulate snow. Normally this isn't a big issue as most panels will warm quickly and will melt and shed snow except in very snowy locales, ( and they can be swept easily) But when we leave for multiple months in the winter, we need the panels to keep the batteries topped up, so the wall panels always get sun, regardless of how much snow might be on the roof mounted panels.

As I noted, a panel will heat up quite quickly as long as some of it is exposed. Even at -30 if one corner of the panel is exposed, the entire panel will melt off 6" of snow in a day or so. The problem is when we leave, we can get major snow, and it can bury the entire array, and as long as it's buried it won't melt.

 

I've watched out inverters reach peak (even in mid summer) just as clouds clear. It makes me wonder. I'd be interested to hear from folks at high altitude (say colorado for instance). I wonder what a 200 watt panel could do on a crispy 40 degree day with full mid day summer sun at 7,000 feet elevation . . . what kind of over production percentages could folks see. Heck, for that matter, there's a 20,000 foot plus peak in Equador. Now THAT'd give some cold temps with maximum sun!

.

 

I don't know, but my hunch is that altitude is in our normal world (0-~10,000' above sea level) would have a negligible effect on Pv output. A much bigger factor is local environmental issues, fog, haze, smog, particularly high humidity low level haze. Clear, cold has, as you know, a dramatic effect on output. On a clear (no cloud) day I might get 70% of name plate, but when it is dry and clear with no haze, I see over 80% quite regularly.

In my case, my Pv is located overlooking the water. Summer reflection gives me a small but noticeable boost, but reflection off of the frozen lake coupled with cold temps gives me a HUGE boost. (We are at ~1800')

 

Fully Agree,
Not to mention the lines maintenance of thousands of miles of lines, we need to generate the power where it is needed. And it can be other things besides PV You live in a windy location then a small turbine up on the hill. What I think would be interesting is a neighborhood approach If you live in say Alaska, BC Washington State etc etc where there is good running stream up the hill from your neighborhood then then Micro Hydro is an option that is becoming increasing popular on lifestyle blocks here in New Zealand... I am not talking about damming rivers, (big scale hydro has its own issues) What they have here are things that are the size of say cooler that has a intake pipe and a outlet pipe so is not a major impact on the stream environment at all and you have 24/7 power! beats wind and solar hands down. Different solutions in different places.

 

It's been moved. I think this is the same program in its new location:
NREL: PVWatts - PV Watts Version 1 Calculator

 

Both approaches are good.

I havent' read this whole thread, so maybe it's been covered, but one factor arguing against large facilities is use permits for such big installations, at least in the U.S. In Europe, German farmers did really clean up, thanks to government subsidies and the ability to checkerboard their fields with PV panels without significantly impacting their typically intensive cultivation methods. In the States, it's hell to get permits for a large installation, or to run a new transmission line to connect it.

In contrast, homeowner installations use wasted space on the roof (Many states won't allow the enforcement of CC&Rs against PV.) there, the hookup is via the existing service drop. and the homeowner has an incentive to keep an eye on the system.

Also, "distributed generation" on a "microgrid" scale is a sensible part of the long term plan for the smart grid.

On the other hand, economies of scale support more big solar, out where the sun shines a lot. But the economics seem to favor enormous fields of mirrors, with turbines on towers like the huge installations in Spain and what's planned here for Ivanpah. There's a trend away from PV and mirrored troughs for large scale solar from what I can see. So there's nothing to stop from people from taking the money they'd use on rooftop solar and buying shares in the companies that are building the big plants.

(Except for the mental image of what a big quake would do to one of those huge complexes.)

 

So what? My goal with PV panels wasn't to personally use the electricity, per se, but to offset my energy usage with something clean. I don't mind if someone else actually gets to use it, since it's still reducing pollution by my desired amount.

If you stored the electricity in batteries, you'd actually see an energy loss and the amount of reduced pollution would be diminished.

Also, I'm not sure about other utilities, but most in Colorado offer the option to buy "green" energy by paying a little extra. I think most of it comes from wind farms.