# . . holes in my 'solar' math?

Posted By sgcsalsero, Nov 4, 2008 at 10:18 PM

Not open for further replies.
1. #1

### sgcsalsero Feeling the Heat 2. ```NULL ```

Mar 15, 2006
448
1
Loc:
ClevelandRocks
Can anyone poke any holes in assumptions/data below? Thanks

Data Points
1) Daily electrical usage of Buildings = 5000 KWH
2) Utility per KWH charge = .10 cents
3) Estimate of daily average KWH of solar radiation
per m2 for geographic area: = 4KWH/m2***
4) Estimate of south-facing roof area = 4000 sq feet Building 1
= 1500 sq feet Building 2
Note: Roof angles are close to ideal
5) Estimated 5000 sq feet total, or 450 sq meters total;
4*450 =1800 KWh average daily solar energy potential hitting roofs

Efficiency
Commercial solar cells are not 100% efficient; more like 10% efficient. As an aside, this should change in the near future (lab results now have 40-60% efficient cells).

Total Theoretical Yield Per Day
A large installation typically is on the order of only 10’s of KW. For example: a “40KW” system occupies 4000 sq feet and costs \$300,000 (~\$8 per installed watt) and yields about 125 KWH per day**

Break Even Point Analysis

Total Cost Avoidance over Year
125KWH daily production x .10 per KWH utility charges = \$12.50 daily avoidance
\$12.50 x 365 days = \$4,563

Twice as efficient system = 250KWH daily production = \$9,125 avoidance

‘Payback’ on investment
Current Efficiency
\$300,000 divided by \$4,563 = 65.75 years

Doubled Efficiency
\$300,000 divided by \$9,125 = 33 years

Doubled Efficiency with 33% discount (tax rebates, gov’t incentives, etc.) on total cost
\$200,000 divided by \$9,125 = 22 years

**http://www.dovetailsolar.com/Commercial SOLAR PV Price Sheet.pdf)
***http://store.altenergystore.com/Solar/descfiles/kits2go/kitoffgrid1kw/Us_pv_annual_may2004.jpg

2. #2

### steam man Minister of Fire 2. ```NULL ```

Jan 19, 2008
652
5
Loc:
Northern Maine
I can't imagine what your trying to design for but I can think of a few points to consider.

1-The assumed .10/kw-hr seems low now. Mine is now .185/kw-hr. Plan on rates only going higher-much higher over those time frames. This would help payback.

2-I think you would probably start replacing panels long before those time frames are up. There is going to be cost to keep it going.

3-What would be the return if you figured in a return on investing the money conservatively and buying the electricity? You should subtract that to see what it is really worth.

Mike

3. #3

### peakbagger Minister of Fire 2. ```NULL ```

Jul 11, 2008
2,575
544
Loc:
Northern NH
Have you tried the PVWatts program? It does most of the grunt work for calculations and takes into account climatic conditions for most areas. If you dont like their assumed factors you can edit them. It is regarded as the standard for the solar industry and has been proven to be a quite reliable estimator of solar potential.

Dont get caught up chasing the efficiency game, assuming you have a place to mount the cells, its lowest dollars per installed watt that is the most important with siting a close second. Most of the roof mounts out there look great, but the lack of adjustment for seasonal angle and the higher temperatures up on the roof compared to a rack system offsets any efficiency savings. It is also a heck of a lot easier to clean off the snow from a rack and the steep angle in the winter keeps the snow from building up on the panels .

The rack in the picture may not have the most efficent cells, but its payback is less than 2 years as I bought the cells at a garage sale. The tilt angle is reset 4 times per year and takes about 3 minutes.

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4. #4

### webbie Seasoned Moderator 2. ```NULL ``` Staff Member

Nov 17, 2005
12,193
1,086
Loc:
Western Mass.
Those calcs are probably in the ball park.

But, yes, we're paying about 18 cents here....so there are other factors. That cuts the payback in 1/2.

Still, I'm a believer in very large scale systems (in general) to solve our energy problems. I looked into PV for my house, and concluded that it would not be worth my while......too much shade, etc.

Maybe in two years when the new generation of cells comes along.

5. #5

### sgcsalsero Feeling the Heat 2. ```NULL ```

Mar 15, 2006
448
1
Loc:
ClevelandRocks
No I haven't, and yes I will, thanks for the other input as well

6. #6

### jdemaris New Member 2. ```NULL ```

Oct 11, 2008
452
0
Loc:
Central New York State
Math does not tell the whole story unless your only motivation is to somehow save money on your montly power bill. That being said, my system can pay for itself in 15-20 years. One slight problem is - I'll probably be dead by then.

I've got a 5250 watt array, grid-tie with enough battery backup to run our house for 4-5 days. And, unlike most grid-tie systems, mine WILL run and use the solar panels and invert when the grid is down.
I spent - out-of-pocket around \$8000 after all the incentives and tax rebates are figured in.
Our electricity is listed as 13 cents, but actually comes to 18 cent per KWh with all the charges.
My system makes on average, around 400 KWh per month. The panels will outlast me, but the electronics, I'm sure will need some repairs along the way. Solar panels don't often die, they just get gradually weaker over time. Many have remained almost fully useful after 30-40 years of use.

I cannot predict what grid prices will be down the road. I installed for other reasons. A small degree of security in case of disaster, problems with our foreign owned electric companies, war, terroist attack, etc. Also, I like the portablility. If I ever move the system comes with me and it allows me to move to an area that is totally off-grid.

And yeah, some of it is a crap-shoot. By law, FEMA can take over private solar for the "public good" when needed. So, it's nice when a system is not highly visible and maybe, they won't come and find you when you'd really like to use it for yourself (how un-liberal of me).

7. #7

Nov 19, 2007
213
0
Loc:
CO 9000ft
8. #8

### sgcsalsero Feeling the Heat 2. ```NULL ```

Mar 15, 2006
448
1
Loc:
ClevelandRocks
9. #9

### jdemaris New Member 2. ```NULL ```

Oct 11, 2008
452
0
Loc:
Central New York State
Some of the new developments sound pretty neat, but . . . it won't matter much unless the price per watt of the panels comes down. Since, in many installations, space is unlimited, the true limiting factor is money. On a satellite out in outerspace in geostationary orbit, it's a different story. Solar panels (photovoltaic) have been hovering around \$4-\$5 per watt for years, and slowly keep inching up.

On the subject of that guy's process that allows the panels to absorb/use more light, I wonder what happens during the winter? As it is now, panels overproduce and exceed their own ratings during extreme cold weather and a lot of snow cover reflecting light all over.

On a side-note, I see you've got an 056 saw. I've got several and I used to repair Stihls. I just recently discovered that Stihl discontinued many replacement parts for the 045-056 series. My ignition module crapped out in my 056 Magnum II. Called the local dealer, and then called Stihl headquarters in Virginia Beach. No more parts. Out of frustration and an immediate need for a big saw (my others were apart), I went out and bought a 64 CC Dolmar with a Makita label on it (and love it so far).
Then, I got searching around on foreign Ebays - and what do you know? I found a woman in Germany selling brand new igntion modules for 045s and 056s. I bought two - came to around \$130 each when converted from Euros to dollars.

10. #10

### sgcsalsero Feeling the Heat 2. ```NULL ```

Mar 15, 2006
448
1
Loc:
ClevelandRocks
Thanks the 056 is a '79 and runs great, I'll keep that tip handy when it craps out, which hopefully it doesn't.... eventually in the 'plan' is to get a 7900, btw the 6400 is upgradable to the 7900 (search arboristsite) . . I've read too many good things about the 7900 (pound for pound one of the best out there ) and very pleased with 5100s

11. #11

### jdemaris New Member 2. ```NULL ```

Oct 11, 2008
452
0
Loc:
Central New York State
My 056 is around the same year. I've also got an 040 and S10 from the early 70s. I was the Stihl mechanic at a John Deere dealership at the time when I bought the 056 new. Believe it or not, I never so much as put a spark plug into it until this year - an it's been used hard. Many sprockets and chains, but no engine parts. I cut a lot of huge hard maple and red oak. Great saw. That being said, I have to admit that this Makita Dolmar can cut almost as fast and is considerably lighter. Seems to be a very rugged saw. One of the reasons I bought it was for what you mentioned - upping the ccs with a bigger jug and piston. Also, it's all metal, whereas many of the newer Stihls and Huskys have plastic all over, even for the crankcases.