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Monday, December 5, 2011

Power to the People: How to Build a Plug and Play Grid Tie Solar Array

 
P1060193.tmb

Background:  In August 2006, we had a 6300 watt photovoltaic system installed on our roof through our state’s clean energy incentive program.  Our system consists of 36 Sharp NT-175U1 175 watt solar modules, and two Xantrex GT 3.0 inverters.  It makes about 7300 kWh per year.
 
In the winter of 2009-2010 our area got over 80 inches of total snowfall.  As a result, some of our solar modules were damaged by the snow.  The damaged modules were on the edge of the roof near the gutter, and we think that the heavy snow accumulations between the roof and the modules went through freeze/thaw cycles that most likely caused the aluminum frames on the two modules to bend.  Despite the damage, the modules were still electrically functional.  Here is a picture of one of  the damaged modules:
 
 
 P1010540.tmb             P1010539.tmb            
Bend in the PV module frame.
       
Sharp Solar Warranty Hassles:  As you can see, the frames bent at a hole at the midpoint of the module, pulling the aluminum frame away from the glass plate of the module.  When my solar contractor inspected the damage, they thought that three of the modules had bent frames.  Our contractor tried to get Sharp Solar to replace the modules, to no avail.  My contractor’s PV module supplier also tried to get Sharp Solar to replace the modules, to no avail.
 
The modules came with a 25 year warranty, so I personally contacted Sharp to see if the damage would be covered by their warranty.  Although Steven Yorita at Sharp Solar was very courteous, he stated that his superiors told him that the damage to the modules would not be covered under the warranty.  My contention was that the frames were not properly designed, and that there should have been a brace in the middle area to prevent the frame bending in that area due to stresses.  Sharp Solar did not agree with me, and refused to honor their warranty saying that the warranty did not cover damage due to snow and ice.  We negotiated for several months, and when it became clear that we reached an impasse in negotiations, Sharp Solar agreed to sell and ship me three new replacement modules for half price—around $350 per module plus shipping, and I could keep the old modules. 
 
In December 2010, before the first 2010-2011 snowfall, my solar contractor came to replace the modules.  While working on the roof, he found that only two of the modules were damaged, so I now had two damaged modules, and a new module left after the repairs, and I stored them in my garage for  the winter.
 
Here is a picture of the contractor switching out the damaged and new modules.
 
P1010541.tmb
The modules on the left were the snow damaged modules.  The module on the right was the new module.  Note that the new module came with a mid-frame brace at the exact point where the old modules deformed.
 
 
How to Deploy the Spare Modules?:  While the three 175 watt panels were stored in the garage, I thought about how to use the extra modules.  I came up with three ideas:
 
1.  I could mount the three modules on the roof of my sunroom, but that would entail putting racks on the sunroom roof, and drilling through my shingles.  We have had some difficulties with leakage from the lag screws that attach the racks onto my main roof for the 36 modules up there, so I was not too keen on penetrating another roof with lag screws. If I used this option, I would use Enphase microinverters, but that would require running a 240 line out to the sunroom from my breaker panel.  In the end, I rejected this option as too much work and too expensive.
 
2.  I thought about building a pergola over my concrete patio, and placing the modules on the pergola, using the microinverters.  This option would avoid drilling holes into my sunroom roof, but I would still have to build a structure, and supply a 240V AC circuit for the microinverters.  This was still a bit more work than I wanted to do.
 
3.  I could build a ground rack, and use the inverters found on eBay that invert the panels’ DC current to 120V AC and backfeed into existing household circuits.  I researched the experiences of people who had used these Chinese inverters, and found that several people had good experiences with the Sun G inverters.   I decided to go with this option.
 
Repairing the Damaged Module Aluminum Frames:  My first order of business was to repair the damaged frames on the two modules that came off the roof.  This was fairly simple.  I simply unscrewed each damaged frame piece (only one of the four frame sides was bent on each module), and placed it in the space between the slab and the bottom of my storage shed.  I lined up the bend with the edge of my shed, and pulled up on the frame piece, straightening it back into its original shape.  I actually bent it back a little too far so some tension would hold the rubber gasket on the module edge snugly in place.  I put the rubber gasket on the module back in place, and reattached the newly-straightened frame piece.  I also tested the modules in full sun, and they were still perfectly functional.
 
Inverter Selection:  A solar electric system consists of two basic components, the photovoltaic (PV) modules that create direct current (DC) electricity from the sun, and inverters that invert the DC to AC (alternating current).  In my net research, several people had remarked that the Sun G type inverters were somewhat inefficient, and that you should purchase an inverter rated at twice the watts you want to feed into the inverter.  For example, my three 175 watt modules would theoretically supply 525 watts (3 X 175) to an inverter, and some comments I read on the net suggested I would need a 1000 Watt inverter for maximum efficiency.   I found one particularly helpful study on the web that helped me to decide what size inverter to purchase:    http://www.youtube.com/watch?v=o9rWaqbOOMo.  In this YouTube video, sparktastic 1 showed that at full 500W power input, the Sun 500G inverter had about 82% efficiency.  Feeding the full 500 test watts into two Sun 500G inverters only slightly increased the efficiency to 87%. 
 
Seeing that my three modules had a zero percent efficiency sitting in the garage, I opted to purchase a Sun 600G inverter off of eBay for $161.00 (including shipping).  I would have 525 rated watts of panel power feeding a 600W rated inverter.  The DC output voltage of my Sharp 175s was about 44 Volts, so this inverter would accept the voltage from the modules.  To keep the inverter input voltage at 44 Volts, the modules would be wired in parallel so that the inverter input voltage would stay the same, and input DC amps would be additive.  The AC output of the inverter is 90 to 130 volts, and would match my house sunroom circuit voltage.  I planned to backfeed the inverter’s AC output into my house through an outdoor outlet on the back of my sunroom.  It is important to note that this inverter has anti-island protection which means it shuts down in the event of a power failure, and does not energize circuits in my house in the event of a power failure.  This feature protects utility workers who could be working nearby from unexpected exposures to electrical current.
 
One thing that did concern me about this inverter is that it is not UL or other recognized testing lab approved.  To deal with this risk, I decided to mount the inverter to the rack I would build for the modules so that if the inverter failed/burned/shorted, it would not catch my house on fire.  I also made sure that the inverter output current would backfeed through a GFCI (ground fault circuit interrupter) protected house circuit that supplies my outdoor outlets, and that the inverter output would not exceed the circuit breaker and GFCI rating for the house circuit.
 
As far as overloading the household circuits go, I calculated that the maximum current output of the inverter would only be about 3.5 amps AC.  (525 watts X .80 efficiency divided by 120 volts = 3.5 amps).  This output would not stress the 15 amp AC circuit that supplies the outdoor outlets.
 
Here are the specs and a picture of the inverter I bought:
 

Electrical Specifications:

Model

SUN-600G

Normal AC Output Power

540W

Maximum AC Output Power

600W

AC Output Voltage

220V/230V/240V

190V ~ 260V

110V/120V

90V ~ 130V

AC Output Frequency Range

46Hz ~ 65Hz

Total Harmonic Distortion(THD)

<5%

Power Factor

0.99

DC Input Voltage Range (Optional )

10.8V ~ 30V / 22V~60V

Peak Inverter Efficiency

92%

Standby Power consumption

<0.5W

Output Current Waveform

Pure Sine-wave

MPPT Function

Yes

Over Current Protection

Yes

Over Temperature Protection

Yes

Reverse Polarity Protection

Yes

Anti-Island Protection

Yes

Stackable

Yes

 

Mechanical Specifications:

Operating Temperature Range

-10 0C ~ 45 0C

N.W

3.6Kg

G.W

4.0Kg

Dimension

332mm*265mm*135mm


 

 Inverter picture
 
Designing the rack:  Once I received the inverter and repaired the bent frames on my modules, I built a rack to hold the modules.  One thing I wanted for the rack is the ability to seasonally change the angle of the modules to get maximum efficiency.  I found this great article that describes the optimal tilt angles for solar modules, depending on your latitude:  http://www.macslab.com/optsolar.html.
Our latitude is almost 40 degrees north, so I used the charts in this article to determine the three tilt angles I would need for seasonal changes.  Here is a chart I prepared for planning the rack based on our 40 degree north latitude:
 

 Module Tilt Angle (degrees)                                                 Dates

59.6

October 7 through March 5

36.9

March 5 through April 18

12.5

April 18 through August 24

36.9

August 24 through October 7

 

 
Building the Rack:  The entire unit has two parts:  a boxy support rack structure and a “tabletop” consisting of the three modules connected together by two 2X4s.  Given the large range of tilt from 12.5 to almost 60 degrees, I designed the rack to accommodate a prop rod that would provide all three tilt angles.  This meant that the support rack structure had to be at least 36 inches high to allow for the prop rod to work at all the angles.  I constructed the box frame out of pine 2 X 3s I got at Home Depot, and some 1X3 bed slats that I salvaged from someone’s trash.  I employed a variety of galvanized deck screws.  The frames were attached to 2 X 4s near the top and bottom of the modules using nuts and bolts with lock washers, and the three module tabletop unit was attached to the support frame using door hinges which would allow the three modules to tilt.  Here are several pictures of what ensued:
 
P1060198.tmb 
Note the three PV module hinged “tabletop.”  The bottom supporting box rack was high enough so that a single prop rod could tilt the modules into all three angles. This is the winter configuration at 59.6 degrees.  Door hinges attach tabletop modules to the bottom support rack.
 
 
P1060195.tmb
Holes were drilled in the top of the supporting box frame and in the prop rod for each tilt angle—59.6, 36.9, and 12.5 degrees.  This picture shows the highest angle.  Carriage bolts with wing nuts allow the angle to be easily adjusted seasonally.
The unit was a bit top heavy on the hinge side, so I used ground stakes to anchor the unit into the ground in the back, and to provide electrical grounding for the modules and inverter.
 
 
How to set the tilt angle:  To drill the holes in the prop rods and top of the supporting box rack to get the right tilt angles, I used a handy device I got on eBay ($9.00) called an inclinometer.  These are usually used to align satellite dish angles, but they work well in solar applications too.  After the 3 module “tabletop” was attached to the bottom support rack using the door hinges, the supporting box frame was leveled, and I used temporary 2X4s to hold up the tabletop while I measured the angle with the inclinometer.  When the right tilt angle was achieved, I drilled a hole through both the prop rod and the support rack top bar (with the prop rod at right angles to the support rack top bar), repeating the process for each of the three tilt angles.
 
Here is a picture of the inclinometer in use:
 
P1060610.tmb
You can barely see where the module frame was bent after I repaired it.  The shed in the background provided the weight to lever up on the bent extruded aluminum module frame and straighten out the damage.  The inclinometer is reading about 60 degrees for the winter tilt setting.
 
 
P1060196.tmb
A 3 inch deck screw attached the prop rods to the 2X4 on the tabletop.  The box frame was leveled when I moved it to the yard using concrete block pavers from Home Depot.
 
Mounting the Inverter to the Support Box frame:  I wanted to protect the inverter from the elements because I was mounting it on the box frame and not the house.  I suppose the best way would be to put it inside an electrical box, but these inverters heat up when in use and I wanted some ventilation.  I decided to simply put the inverter inside an inverted polyethylene dishpan from K-Mart.  To mount the dishpan and inverter to the supporting box rack, I used a piece of 1X12 that I had in the garage.  Here is how the inverter was mounted:
 
P1060107.tmb
Inverter, exposed to air but snugly protected by the dishpan.  I drilled holes in the outer edges to keep water from collecting.
 
Making the Electrical Connections:  Because the original PV system on our roof was completed in 2006, all of the damaged PV modules had older MC3 connectors.  The new replacement modules from Sharp came with MC4 connectors.  My solar contractor had to cut off the MC3 connectors from the damaged modules and put them on the new modules.  I had them connect the MC4 connectors to the damaged modules positive and negative wires before they left.
 
To connect the three spare modules to my Sun 600G inverter, I decided to buy some new MC4 connectors so I could make connecting wires between the modules and the inverter.  A local solar supplier was very helpful and sold me 3 male MC4 connectors and 3 female MC4 connectors for a reasonable price (around $25).  I also got some 12 gauge multistrand wire, some DC tube fuses and holders, and some ring connectors from McMaster Carr.   My connector wires consisted of an MC4 connector, a run of wire, a soldered fuse holder and fuse, a run of wire, and a soldered ring connector to fit over the negative or positive post on the inverter.  Six of these connector wires were constructed, 3 with male MC4 connections, and 3 with female MC4 connections.  I did not buy the expensive MC4 connector tool, but soldered all the MC4 connections to the 12 gauge stranded wires.  I had read online that it was a good idea to place DC fuses between the inverter and the modules, so each positive connector wire got a properly rated DC fuse (they were rated at 125DC volts, 10 amps).  Because I wanted all of the connections to be soldered, I used the little plastic fuse holders for tube fuses.  That didn’t work out too well (the plastic sleeves wouldn’t fit over the soldered connection, so the whole fuse assembly was just encased in layers of 3M electrical tape for insulation.
 
A Word About Fuses:  There are two excellent papers by John Wiles at New Mexico State University that I consulted on the confusing topic of fuses in PV systems.  These papers are "Focusing on Fuses"  at http://www.nmsu.edu/~tdi/pdf-resources/cc67.pdf and "To Fuse or Not to Fuse" at  http://homepower.com/article/?file=HP125_pg106_CodeCorner.  The first article helped me to understand how fuses are rated, and how to select the correct fuse for a PV system.  The second article focuses on the question of whether to fuse or not.  After reading the second article I determined that I needed to add a fuse to each of the positive leads on the PV modules.  Fusing is necessary in a parallel system where the combined output of the other  modules (or strings) in parallel could backfeed into a failed module.  Here is an explanation of the process I used as described in the article for making the decision of whether to fuse or not to fuse, and how to select the right-sized fuse. 
 
From reading the labels on the back of my Sharp NT-175U1 PV modules, I learned that the short circuit current (Isc) for the modules is 5.40 amps.   The label also says that the fuse rating on the modules is 10 A.  Since I have three modules in parallel, if one of the modules fails, the remaining two modules wired in parallel could theoretically force 13.5 amps (5.40 amps X 1.25 (NEC's worst case max output)  X 2 modules) into the failed module.  Since 13.5 amps exceeds the 10 amp fuse rating on the module, each of the modules needs at least a fuse rated 1.56 times the Isc (8.4 amps), but no higher than 10 amps. In my case I would need a DC rated fuse of 8.4 to 10 amps.    To meet this requirement, I chose a fast blow, Bussmann ceramic DC fuse rated at 10 amps, 125V  DC from McMaster Carr  (Part # 71385K34).  Here is a link to the fuse I got from McMaster Carr.  http://www.mcmaster.com/#catalog/118/922/=gh3rxn
 
It should be noted that not all parallel systems need fuses.  In my next project I’ll be feeding two parallel-wired Evergreen 220 watt panels into a Sun 600G inverter.  The labels on the back of these panels state that the short circuit current is 8.22 amps, and the fuse rating on the module is 15 amps.  Since there is only one module that could backfeed into the other in a fault situation, the maximum backfeed would be 8.22 amps X 1.25, or 10.27 amps.  Since this value is less than the 15 amp fuse rating on the panel, I don’t need to add fuses to the positive leads of this array.
 
In the end, the electrical setup looked like this:
P1060108.tmb
Fuses were encased in electrical tape.  (Not so great)
The inverter output wire that came with the inverter was a standard three pronged power cord.  I connected this to a 12 gauge wire, 25 foot long extension cord and plugged it into an outdoor outlet.   There were three sets of positive and negative leads with MC4 connectors coming from the modules. For parallel wiring, each module’s positive output went to the positive post on the inverter, and each negative output to the negative post on the inverter.
 
Here is a view of the high-tech inverter protection and assembly from the front of the unit:

P1060202.tmb
 
How well does it work?   Before turning on the unit I oriented the modules to solar south.  The magnetic declination in our area is about minus 13 degrees, so the unit was aimed at 13 degrees west of compass south.  Find your magnetic declination here:  http://magnetic-declination.com/.    Since it was already mid-October, I raised the module tilt to its winter setting of 59.6 degrees.   I connected the unit for the first time in mid-October 2011.  Using my Kill-A-Watt meter, here was the output I got on a nice clear day:
 
P1060199.tmb
Using my Kill-A-Watt meter, I actually saw the output get up to 397 watts AC at one point.  This would put the total efficiency of the unit at 76%. (397W/525W).  I’m pretty happy with the results of my project.   The unit should produce about 700 kilowatt hours per year, or about 8% of our yearly electricity use.  I have not seen the AC current output go above 3.5 amps.
 
What I learned:  If I did this over, I would not have built the rack so high.  I built it three feet high so I could use one set of prop rods.  If I did this again, it would be half the height, and I’d simply have three different sets of prop rods for each tilt angle.  This unit is kind of high (around 6 feet) when the modules are tilted up at their winter angle.
 
Module tilt angle makes a big difference.  I tilted the modules at several angles in the sunlight, and having the correct angle at the right season makes a significant difference in output.
 
I would figure out a different way to handle the tube fuses in my connector lines.  What I did with electrical tape works fine, but is not very elegant.
 
I would use a clear dishpan/storage container to protect the inverter. The inverter has indicator lights to show it is working.  There are three green LEDs that light up in sequence from left to right.  The faster they move, the more AC output from the inverter.  There is also a red LED to show that the unit is not receiving grid electricity, and has shut off to prevent islanding.  The use of a clear storage container allows easier monitoring of the LED status lights.
 
I wish there were some low-cost 120 volt output inverters that were UL approved, and that were made in the USA.  The inverter I used is made by the Ningbo Hi-Tech Park Sunshine Technology, Co. Ltd. in Ningbo, China.  The inverter carries a CE certification for electromagnetic interference.  Here is a link to the company’s website:  http://www.chinesegrid.com/I’ll be interested to see how long the Ningbo Sun-600G inverter lasts.  
 
Not counting the solar PV module costs, I spent about $260 on the inverter, wood, screws, wires, fuses, MC4 connectors, concrete pavers, plastic dishpan, ring connectors, and hinges.  Our local electricity rate is about $0.20 per kWh, so at an annual output of 700 kWh for this unit, it will take about 1.9 years to recoup my materials costs.  Of course, if you have to buy the modules the payback would be a lot longer.  In my case, the modules were gathering dust in the garage.  
 
I calculated the 700 kWh output using the PV Watts solar calculator:  http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/.  The basic PVWatts calculation for a .525 kW system in our area gives an anticipated output of 665 kWh per year.  The article above that discusses changing the tilt of the modules says that changing the tilt angle seasonally (4 times a year) increases the output by about 5%.  Thus the anticipated output of this system is approximately 698 kWh (rounded up to 700 kWh).
 
Costs aside, this three module project prevents 1000 pounds of carbon dioxide per year from going into the atmosphere (1.428 pounds per kWh in the RFCE region from EPA’s eGrid chart): http://www.epa.gov/cleanenergy/documents/egridzips/eGRID2010V1_1_year07_SummaryTables.pdf).
 
I wanted to write up this project so you can see that it is really not all that difficult to do a small scale solar project.  As the prices of PV modules drop (they dropped 40% in 2010-2011), it will become even more affordable to produce some or all of the electricity you use from solar.
 
More Information:   I recently ran across this company in Great Britain that commercially sells the same type of plug and play system:  http://solar-power-station.co.uk/.  Click on the specifications for the systems they are selling, and you’ll see that they too are clearly using the Ningbo Sun G inverters.    I wonder when this solar power station will come to the US? 
 
The Brits are selling their 200 watt system for 795 GBP ($1270).  In March 2012 I am able to find solar modules in the US for about $0.80 a watt.   The system I made as described here in my blog is 525 watts.  Using a cost of $0.80 a watt for the modules ($420) and $260 for inverter and materials,  the cost of my system would be $680.  At a retail electric cost of $0.20 a kWh in my area,  my system’s payback time assuming an annual output of 700 kWh is about 5 years—without rebates, REC sales, or tax incentives.

November 5, 2012 Update:  This DIY solar plug and play project has been operating without problems for over one year now (October 2011 through November 5, 2012).  It has reliably produced electricity, and it operated well through a very hot summer.  I have been impressed with the performance of the Sun 600G inverter so far.  The only change I’ve made to the project is that I shortened the rack height in half to make it more stable and less obtrusive.  Here is a picture of the system after I cut the rack height down. The modules are at their summer tilt angle of 12 degrees in this picture:

18 comments:

  1. Hi Ken,
    Great article. This looks like a fun project to start with to learn some of the basics.

    Forgive me if this a dumb question, but since you have a picture of the arch, are you located in St. Louis area? (I am in Ofallon, MO area).

    Thanks!
    -bryan

    ReplyDelete
    Replies
    1. Hi Bryan. Thanks for the comment. I learned a lot doing this project too. I don't live in the St. Louis area now, but I grew up in Ballwin, MO. I still love the Arch and the Cards!

      Delete
  2. Hi Ken, nice experiment. I am looking to install a 350W off-grid system into my greenhouse to power fans and pumps. I am having trouble finding anything for the $.80/watt that you have claimed. The cheapest module I could find on the Internet was around $1.70/watt. Can you suggest a supplier?

    ReplyDelete
    Replies
    1. Keep an eye out here: http://www.sunelec.com/. They have $1.00 per watt modules right now, but there were some cheaper a few weeks ago.

      Delete
  3. You might have a look at these guys:
    http://www.ecobusinesslinks.com/solar_panels.htm

    Gary

    ReplyDelete
  4. You might want to explain how to keep the wind from knocking it over. I'd pound in four green fence posts from Home Depot, or maybe use a few "ground screws".

    ReplyDelete
    Replies
    1. Important and good suggestions Kevin. I used 3 foot ground stakes from Home Depot pounded into the ground with an 8 lb. hammer, and screwed the base of the wood frame into the stakes. We had a big storm come through a few weeks ago when the modules were tilted up at the 60 degree winter angle, and the unit held steady with 50 mph wind gusts hitting the back of the unit.

      Delete
  5. Nice article, thanks
    I use several GTIs in // on a LiFePO4 Energy Storage & Power Supply system (see my blog)

    Christophe
    http://myelifenow.blogspot.com

    ReplyDelete
  6. Thanks Christophe. The system is still working well, although I cut the height of the wood base in half to make it more stable, and to keep it out of my neighbor's view.

    I enjoyed looking at your blog. Ken

    ReplyDelete
  7. I recently built my first PV panel, from scratch, following a DIY
    manual I purchased.

    I built just one panel, 36, 0.5V "B" grade cells, & I am getting
    15-18V consistently on the voltmeter. This is my "test" panel to see
    how well it does before I build a complete system. I am not a
    tradesman, so I wanted to make sure it worked first, and now that it
    is producing electricity, I am very frustrated that I cannot utilize
    it.
    I followed the instructions to a "T", but the inverter "blew up" when
    I completed the connection to the AC side. I attemtping to do a grid
    tie only.

    The "support" part of my DIY manual does not seem to be responding, so
    I am searching for advice from people like yoyrself who have done it &
    appear to be having success.

    Any advice you can give would be greatly appreciated.
    Thanks
    Tim

    ReplyDelete
    Replies
    1. Hi Tim. Here are some things to check: Do the voltage and current outputs from your panel match up with the inverter input specifications? Does the AC output voltage from your inverter match the house branch circuit voltage? Ken

      Delete
  8. Hi Ken,

    I am getting ready to do an installation of two 220W panels -for a total of 440W- and one of these inverters. However, even though you showed the fuses, I didn't see any DC disconnect. My local utility requires a DC disconnect between the panels and the inverter.

    What would be the criteria for selecting a DC disconnect?

    Regards

    Rafael

    ReplyDelete
    Replies
    1. I would just insert a toggle switch suitable for the solar module's DC voltage and amperage output into the positive connector wire between the panel's positive MC4 plug and the positive inverter post.

      Delete
  9. just curious how the inverter is holding up?

    ReplyDelete
  10. The inverter has been doing fine so far. The one in the project described here has been continuously operating for 1.5 years without problems. I built another system using two Evergreen 220W modules and bought another Sun 600G inverter. That inverter has been operating continuously for about a year without problems. Both of them stay outside mounted to the support rack inside inverted dishpans, and that seems to be adequate protection from the elements. Thanks for asking, and I'll keep giving periodic updates.

    ReplyDelete
  11. Nice Post!! pretty informative..thanks for providing such a nice post.
    China Inverter

    ReplyDelete
  12. A question about the warranty...25 years sounds like the power output guarantee. Normally that some manufacturers will have something like <10% drop in first 10 years then <20% drop in the whole 25 years and that is in power. Unless you experience a >10% power output drop at standard conditions, I can understand why Sharp was pretty adamant about it.
    Other things that could've been interesting to look at is the certification of the modules - did they have a) extended static load test rather than the very standard 2400Pa, say 5400Pa (snow load standard), b) dynamic load test certificates.

    Sorry to be longwinded, just a few curious thoughts...

    ReplyDelete
  13. I actually added your blog to my favorites and will look forward for more updates. Great Job, Keep it up.
    china inverter

    ReplyDelete