Am I Crazy?

[J.P.M.]

A diff of 396 kwh/year for a 5kw system seems ball park to me. Enough with your negative vibes jpm. you make these generalizations with no backing.

PVWATTS yielded 7332 kwh (see attached)

My “dart” throw resulted in 7728 kwh a diff of 396 kwh. I’d say that is close.

PROOF: [(5kw) *( 5.5 solar hrs) * (365)]*.77 = 7728

and a snapshot of your beloved PVWATTS results;

same formula was used by Mike90250 back in 11/10/2009 -> http://www.solarpaneltalk.com/showth...hlight=formula

And if you don't understand loss when talking about PV solar systems, let me help - as in mechanical engines there is power loss to get the power from the engine to the wheels, PV systems also have loss in many areas, one of the easiest ones to understand is in the inverter, there is electrical loss in converting DC to AC. Do you need me to define DC and AC too? keep the brain flatulence to min, and remember - Not everyone who tells you what may be unpleasant or seems insulting is your enemy.

I can't write for Mike, but, if I'm reading the correct post, I do not think what you wrote above is quite the same as what he wrote. Nor is your point the same as his in that post of 2009. I believe you're taking his post out of context. I did see his mention in that same post that PVWatts was a better estimator than his simple formula which you incorrectly copied.

I'd respectfully suggest you consider taking a deep breath, stop embarrassing yourself, and consider the possibility you are in over your head from a technical standpoint. It is however, a free country, so do as you wish.

If you continue making unsubstantiated and incorrect pronouncements of a technical nature (separate from opinion) that those of us who are knowledgeable know to be wrong, do not be surprised if those statements are challenged in an attempt to keep incorrect information from impeding progress toward a better energy future, same as other incorrect and misleading statements would be and are challenged.

BTW, to perhaps correct your thinking on one point: I think PVWatts is a decent first cut at a residential sizing tool. It's not gospel and I'm not enamoured with it. I've used stuff that, IMO, are better design tools and, FWIW, written stuff going back to FORTRAN and Visual Basic that is more flexible.

As far as my knowledge of solar energy and, as you say, "mechanical engines and power loss", I believe I can handle that part without your assistance, but thank you for the offer. My guess is I may have forgotten more about the profession of Mechanical Engineering and its technical aspects than you are likely to be aware of for some time to come.

[Ian S]

A diff of 396 kwh/year for a 5kw system seems ball park to me. Enough with your negative vibes jpm. you make these generalizations with no backing.

PVWATTS yielded 7332 kwh (see attached)

My “dart” throw resulted in 7728 kwh a diff of 396 kwh. I’d say that is close.

PROOF: [(5kw) *( 5.5 solar hrs) * (365)]*.77 = 7728

and a snapshot of your beloved PVWATTS results;

same formula was used by Mike90250 back in 11/10/2009 -> http://www.solarpaneltalk.com/showth...hlight=formula

And if you don't understand loss when talking about PV solar systems, let me help - as in mechanical engines there is power loss to get the power from the engine to the wheels, PV systems also have loss in many areas, one of the easiest ones to understand is in the inverter, there is electrical loss in converting DC to AC. Do you need me to define DC and AC too? keep the brain flatulence to min, and remember - Not everyone who tells you what may be unpleasant or seems insulting is your enemy.

This concept of solar hours is kind of intriguing. I've attached a plot showing my production today and wonder how I would determine the number of solar hours from it.
ScreenHunter_01 Aug. 05 20.56.jpg

[Datacruncher]

Simple Formula to calculate PV annual production is close to PVwatts calcs.

You were told wrong. That formula is of little even as a dart throw. "Solar hours" is an undefined term, no account is taken for azimuth or elevation and "LOSS" is undefined.

A diff of 396 kwh/year for a 5kw system seems ball park to me. Enough with your negative vibes jpm. you make these generalizations with no backing.

PVWATTS yielded 7332 kwh (see attached)

My “dart” throw resulted in 7728 kwh a diff of 396 kwh. I’d say that is close.

PROOF: [(5kw) *( 5.5 solar hrs) * (365)]*.77 = 7728

and a snapshot of your beloved PVWATTS results;

same formula was used by Mike90250 back in 11/10/2009 -> http://www.solarpaneltalk.com/showth...hlight=formula

And if you don't understand loss when talking about PV solar systems, let me help - as in mechanical engines there is power loss to get the power from the engine to the wheels, PV systems also have loss in many areas, one of the easiest ones to understand is in the inverter, there is electrical loss in converting DC to AC. Do you need me to define DC and AC too? keep the brain flatulence to min, and remember - Not everyone who tells you what may be unpleasant or seems insulting is your enemy.

PVWATTS yielded 7332 kwh.jpg

[J.P.M.]

Not sure what you mean by "The higher the better" for what? and what is the 10%-25% you mention?

Pvwatts gave me a production of 7112 kWh/year using a default derate of .77 ; using 5kw system, 15 degree tilt, 192 Azimuth
0.77 is close to the value all the quotes I've gotten.

I was told that the production formula is [(Sys_Size * Solar hours * 365 days) - (LOSS)]

for zero loss that would provide a production value of 10037 kWh/year (using the 5.5 solar hours for my area; pvwatts calcs 5.46)

You were told wrong. That formula is of little even as a dart throw. "Solar hours" is an undefined term, no account is taken for azimuth or elevation and "LOSS" is undefined.

[russ]

Not sure what you mean by "The higher the better" for what? and what is the 10%-25% you mention?

Pvwatts gave me a production of 7112 kWh/year using a default derate of .77 ; using 5kw system, 15 degree tilt, 192 Azimuth
0.77 is close to the value all the quotes I've gotten.

I was told that the production formula is [(Sys_Size * Solar hours * 365 days) - (LOSS)]

for zero loss that would provide a production value of 10037 kWh/year (using the 5.5 solar hours for my area; pvwatts calcs 5.46)

If you read the posts on the topic you will learn the 77% is considered very conservative - the 82% to 84% range is a lot more realistic.

Some salesmen like to go for bigger numbers - more commission for them and to heck with the customer.

PV Watts is an estimation that with a reasonable derate factor should be more or less on over some long time period - say 10 years. There can be substantial annual variation.

[Datacruncher]

Whats the formula for 100% efficientcy with 0 Derate

5% is salesman's BS and nothing more - every local is different. The higher the better as far as the salesman is concerned - I am surprised some fools hasn't gone for 10% or even 25%.

Not sure what you mean by "The higher the better" for what? and what is the 10%-25% you mention?

Pvwatts gave me a production of 7112 kWh/year using a default derate of .77 ; using 5kw system, 15 degree tilt, 192 Azimuth
0.77 is close to the value all the quotes I've gotten.

I was told that the production formula is [(Sys_Size * Solar hours * 365 days) - (LOSS)]

for zero loss that would provide a production value of 10037 kWh/year (using the 5.5 solar hours for my area; pvwatts calcs 5.46)

[russ]

Oh ok, thats the road I was on originally - doing my own spreadsheet 'till I got offtrack by someone plugging pvwatts as the endall. No one is plugging PV Watts as an end all - it is a very useful tool to get you into the ball park for annual production

On the derate factor, all the quotes I had used 23-30% loss (.70 -.77), maybe do to possible shading issues. Probably - the derate used today is typically 82 to 84%

So the Energy Value $ in pvwatts is the estimated cost displaced by the solar system for the first year assuming no rate chgs and all other inherent assumptions. K, got it. The energy shown by PV Watts is the rate of the local POCO per kWh * total kWh per year generated according to the PV Watts projection.

So no canned programs exist that try to factor in rising cost scenarios? all the quotes I had the companies did this calculation using 4.75%/year. Seems rising costs is the key to determining savings in lease scenarios Full purchase and assuming no changes for 20 years seems to most always come out positive.
[ATTACH=CONFIG]4593[/ATTACH][ATTACH=CONFIG]4592[/ATTACH]

5% is salesman's BS and nothing more - every local is different. The higher the better as far as the salesman is concerned - I am surprised some fools hasn't gone for 10% or even 25%.

[Ian S]

Oh ok, thats the road I was on originally - doing my own spreadsheet 'till I got offtrack by someone plugging pvwatts as the endall. On the derate factor, all the quotes I had used 23-30% loss (.70 -.77), maybe do to possible shading issues.

So the Energy Value $ in pvwatts is the estimated cost displaced by the solar system for the first year assuming no rate chgs and all other inherent assumptions. K, got it. So no canned programs exist that try to factor in rising cost scenarios? all the quotes I had the companies did this calculation using 4.75%/year. Seems rising costs is the key to determining savings in lease scenarios Full purchase and assuming no changes for 20 years seems to most always come out positive.
[ATTACH=CONFIG]4593[/ATTACH][ATTACH=CONFIG]4592[/ATTACH]

Yeah PV watts assumes no shading. If you have shading then that can complicate things significantly. Be very wary of predicting future rate increases. In fact for Cali residents who are high users, their costs could actually decline if flattening of the tiered rates is implemented. IMHO, I'd assume no rate increases as a conservative approach. Even if there are rate increases, there could also be solar specific fees instituted that would offset some solar benefits.

In the final analysis, there is no real predicting the next 20 years.

[Datacruncher]

pvwatts $/kWh meaningless for a complex tiered

Pvwatts only asks for a cost per kWh. That's kind of meaningless for a complex tiered or TOU plan. You have to take the monthly production data from pvwatts and work with your own usage data and utility rate structure to estimate your new monthly electric bill after solar. Make sure to use something like a 0.84 derate factor otherwise pvwatts will underestimate production. Maybe someone has done this for your utility but I wouldn't hold my breath. If you have your own usage data for 12 months and the utility's detailed rate structure, you can incorporate the PV watts data on a monthly basis to calculate your new electric bill month by month for a year. I did it for my situation but it resulted in a messy spreadsheet that nonetheless yielded a pretty good estimate. But then I had access to a friend's production data as well.

Oh ok, thats the road I was on originally - doing my own spreadsheet 'till I got offtrack by someone plugging pvwatts as the endall. On the derate factor, all the quotes I had used 23-30% loss (.70 -.77), maybe do to possible shading issues.

So the Energy Value $ in pvwatts is the estimated cost displaced by the solar system for the first year assuming no rate chgs and all other inherent assumptions. K, got it. So no canned programs exist that try to factor in rising cost scenarios? all the quotes I had the companies did this calculation using 4.75%/year. Seems rising costs is the key to determining savings in lease scenarios Full purchase and assuming no changes for 20 years seems to most always come out positive.
Furture Costs at 608kwh_per_month.jpgSCE_Rate_History.jpg

[russ]

JPM, please don't reply to any of my comments on this website anymore. Sorry, I find your comments simply generalizations and you provide no help other than criticism or go RTFM. As the highly rated PVWATTS plugger that your are, you could have easily told me what the column of dollars meant. But no, you said "The answer to your first question is in the PVWatts documentation". Thanks for NOTHING! Then earlier you criticized my own crunching of data with out calling out any specifics, only to say I'm reinventing the wheel and mine looks like it came out "square". Again thanks for NOTHING! I would have welcomed any specific input that something is wrong. Really, you can reply to this but PLEASE do not reply to anymore of my comments unless you have specific information that helps, no more RTFM or "use PVWATTS". I do not have time for you long exacerbated comments that do not help 1 BIT.

Two points -

1) Do NOT tell anyone what they can or can not do on the site.

2) Your so called data crunching is blatant BS. You want the answer handed to you on a platter.

[Ian S]

I have all the details, but I do not see a PVWATTS screen that asks for it, the GUI seemed to only have two very basic windows of information - all it asked for was system size, tilt, and Azimuth:. Now I just went to run it and I found a diff version that looks even more crude. http://rredc.nrel.gov/solar/calculat.../pvwattsv1.cgi Where is the version located? The newer looking one was at http://pvwatts.nrel.gov/pvwatts.php but even that one does not ask for monthly usage and actual rate details.

Pvwatts only asks for a cost per kWh. That's kind of meaningless for a complex tiered or TOU plan. You have to take the monthly production data from pvwatts and work with your own usage data and utility rate structure to estimate your new monthly electric bill after solar. Make sure to use something like a 0.84 derate factor otherwise pvwatts will underestimate production. Maybe someone has done this for your utility but I wouldn't hold my breath. If you have your own usage data for 12 months and the utility's detailed rate structure, you can incorporate the PV watts data on a monthly basis to calculate your new electric bill month by month for a year. I did it for my situation but it resulted in a messy spreadsheet that nonetheless yielded a pretty good estimate. But then I had access to a friend's production data as well.

[Datacruncher]

You mention solar hours per day but that's an average over a year and gives you no info on monthly production. The latter is important because most utility rate structures are set up on a monthly basis and can give rise to situations where 100% offset design can result in significant monthly bills while having a year end true up where a large number of kWhs produced by your solar system are sold back to the utility for a pittance. Unfortunately, if you want to explore this, you need your own detailed monthly usage and the actual rate details from your utility. Then you can use the pvwatts monthly production estimates to work through a simulation of how 12 months of solar will impact your electric bill.

I have all the details, but I do not see a PVWATTS screen that asks for it, the GUI seemed to only have two very basic windows of information - all it asked for was system size, tilt, and Azimuth:. Now I just went to run it and I found a diff version that looks even more crude. http://rredc.nrel.gov/solar/calculat.../pvwattsv1.cgi Where is the version located? The newer looking one was at http://pvwatts.nrel.gov/pvwatts.php but even that one does not ask for monthly usage and actual rate details.

[Ian S]

I will use PVWATTS and give it a try. I don't see what my SAVINGS will be. Need your help. So it spits out a column of numbers (see attached jpg) called ENERGY VALUE for a 12 month period listing Jan-Dec. What are these numbers? the $ I will spend on electricity or the $ I will save on electricity. Then what about 20 years? All I want or anyone else is HOW MUCH WILL I SAVE OVER 20 YEARS? JPM, Please help me understand the results of your critically acclaimed PVWATTS. thanks

I input $2.8/watt as my DIY system I am looking at is a 10 panel 250w/panel system (2.5kw) costing roughly $7000: $7000/2.5kw = $2.8/watt

The energy value from pvwatts is a pretty crude estimate that uses a flat cost per kWh from the utility to determine the savings your solar system will produce. It's probably only useful if your utility has a very simple rate structure and a very generous implementation of net metering.

What pvwatts will give you is a decent estimate of monthly production for a location near to you given your solar system physical specs. You mention solar hours per day but that's an average over a year and gives you no info on monthly production. The latter is important because most utility rate structures are set up on a monthly basis and can give rise to situations where 100% offset design can result in significant monthly bills while having a year end true up where a large number of kWhs produced by your solar system are sold back to the utility for a pittance. Unfortunately, if you want to explore this, you need your own detailed monthly usage and the actual rate details from your utility. Then you can use the pvwatts monthly production estimates to work through a simulation of how 12 months of solar will impact your electric bill. Even that will be an estimate subject to weather variability and the suitability of pvwatts data for your location. But it's probably the best you can do in the absence of a neighbor who already has actual data for a number of years.

[J.P.M.]

JPM, please don't reply to any of my comments on this website anymore. Sorry, I find your comments simply generalizations and you provide no help other than criticism or go RTFM. As the highly rated PVWATTS plugger that your are, you could have easily told me what the column of dollars meant. But no, you said "The answer to your first question is in the PVWatts documentation". Thanks for NOTHING! Then earlier you criticized my own crunching of data with out calling out any specifics, only to say I'm reinventing the wheel and mine looks like it came out "square". Again thanks for NOTHING! I would have welcomed any specific input that something is wrong. Really, you can reply to this but PLEASE do not reply to anymore of my comments unless you have specific information that helps, no more RTFM or "use PVWATTS". I do not have time for you long exacerbated comments that do not help 1 BIT.

As you wish. Good luck.

[Datacruncher]

JPM please do not comment anymore to me. THanks.

Congrads. You just had liftoff.

The answer to your first question is in the PVWatts documentation. A lot of other answers to questions you will likely come up with are there for the reading and learning.

Go to PVWatts and hit the help screen.

Read the input. All of it.

On the economic questions:

How much you will save over a period of time is a problem in life cycle costing, a part of something called process economics. Two common goals of such an analysis are to estimate how much a proposed course of action (in this case, getting solar) will cost, usually in "today's" $$'s vs. how much it will save over X # of years, or to compare different ways to invest money by making estimates about the future with the usual goal of selecting (estimating) the most cost effective option, thus maximizing the return on investment.

With respect to how much you will save over 20 years: As much as we all like one word or one sentence simple plug/chug answers, how much something saves or costs in the long run, or how much one may save by any particular course of action is a function of many inputs and one's general situation - in the case of how much a solar electric system will save or, in an analysis of comparison of alternatives whether after, say, 20 yrs. you would have been better off investing the financial resources in real estate, the stock market, or whatever, is a function of many inputs.

A FEW of those inputs are: How your POCO billing structure works, how much energy you use, whether you buy/lease/ppa, cash/finance, interest rates, terms of any loan, tax considerations, resale value, estimates of future inflation, energy rate inflation, How long the analysis, etc,etc, with as many variables as the user thinks important or as few as necessary or as the patience of the user can tolerate. Same for the alternative investments with different parameters (such as estimates of stock market returns, for example).

The cost or economic analysis part of solar knowledgeability is some of what I was alluding to in my epistle of last nite. Some things just take some work. I can't do a whole lot for you there. You've got to do the heavy lifting.

That many folks choose not to spend a few hours to understand some relatively easy economic facts of life to help make a better choice in the solar go/no go decision, or to optimize a go solar choice and perhaps save thousands while at the same time bitching and crying the blues with every over-dramatized POCO rate increase rather than doing something (anything) that looks like proactive attempts at reducing their profligate energy use is something that, as hard as I try, escapes my attempts to understand it using common sense and logic. I guess that may be where my problem lies.

So, Datacruncher, as I suggested previously, get a copy of Duffie & Beckman. Add to that advice read/study the chapter on solar process economics. That's a good start to finding answers to your questions.

JPM, please don't reply to any of my comments on this website anymore. Sorry, I find your comments simply generalizations and you provide no help other than criticism or go RTFM. As the highly rated PVWATTS plugger that your are, you could have easily told me what the column of dollars meant. But no, you said "The answer to your first question is in the PVWatts documentation". Thanks for NOTHING! Then earlier you criticized my own crunching of data with out calling out any specifics, only to say I'm reinventing the wheel and mine looks like it came out "square". Again thanks for NOTHING! I would have welcomed any specific input that something is wrong. Really, you can reply to this but PLEASE do not reply to anymore of my comments unless you have specific information that helps, no more RTFM or "use PVWATTS". I do not have time for you long exacerbated comments that do not help 1 BIT.