Illustration of 8 KW (DC) GT Solar PV System Install for Residence in Urban Area

[a0128958]

Here's an illustration of a grid-tie (GT) only solar PV system installation for a residence in an urban area (Plano, TX (Dallas suburb), 1/4 mil pop.), designed to take advantage of all available rebates and incentives (Oncor is my utility Transmission Delivery & Service Provider). Construction started this month, Nov.

Day 0 - Design

System Sizing:

• Electric service panel (circuit breaker) box is 200 A. Max circuit breaker size for inverter-to-panel circuit is 200 A * 20% = 40 A (AC).
• Properly derate 40 A at 65% = 26 A max, which, at 240 VAC, is 6.2 KW (AC).
• Assuming a 0.77 default PVWatts DC-to-AC Derate Factor, multiplying the inverse of 0.77 times 6.2 kW, you get max 8.1 KW (DC) for total solar PV panels output.

Equipment Selected:

• Panels - Astronergy CHSM 6610M mono-crystalline, 225 Wp, 37 Voc, 8.5 A Ioc, UL Listed.
• Inverter - SMA Solar Technology Sunny Boy 7000US, 8750 W max PV power, 30 A (DC) max input current.
• DC Disconnect Switch - SMA Solar Technology - integrated with above inverter, accommodates up to 4 x 20 A (DC) strings.

Resulting System Size:

• 8.1 KW / 225 W/panel = 36 panels.
• Each panel is 5.4' x 3.3,' or 17.7 sf in area; 36 panels * 17.7 sf/panel = 636 sf min. roof area (not counting area needed for fire department 'walk around')

Array Design:

• 3 strings, 12 panels each string (later changed).
• 2 strings (24 panels) on the house roof (later changed).
• 1 string (12 panels) on the garage roof (later changed).

Notes:

Using PVWatts Ver 2 and my specific cell location (0218386), and assuming all defaults (77% DC-to-AC derate, panel tilt at Lat. (33.156°), all panels at 180° azimuth, and no shade ever), system should produce 11.1 kWh annually.

During some periods of solar kWh production I'll be able to totally consume it. The rest of the time I'll have to 'bank' into the grid as best as possible. Best available plan appears to be to sell at 7.5 cents to TXU, my utility Retail Electric Provider.

My last-12-month's average rate at the moment is 10.3 cents. My last-12-month's electricity consumption is 29,039 kWh, which is 2420 kWh/mo, or 80 kWh/day.

Here are 2 images showing roof and electric service entrance areas, before installation begins (8:45 AM CST).





Here are 3 images showing the solar PV system equipment as it arrived.





Next: Day 1 - Installation starts.

Best regards,

Bill

[AntronX]

How much did you pay per watt for those panels?

[a0128958]

Day 1: Installation starts

The design was changed after it was noticed that there's more shade on the garage than expected. Eight panels were moved from the southern most end of the garage to the far western edge of the house (left side of picture below). Another 7 panels were moved to an expanded house roof area. This leaves 9 panels on the garage roof. On the house roof (image below) will be 27 panels - 3 strings of 9 each.

The 36 panels will be divided into 4 strings of 9 each. With only 8 of 9 panels on the western most roof area, a conduit will need to be installed on the roof to the large area of house roof panels, to pick up the 9th panel for the string (and to avoid a 3rd roof penetration for wiring) (image shot at 8:35 AM CST).



The Sunny Boy inverter is mounted inside the garage, near the electric service panel. It weighs 141 pounds.

The panel is near the inverter. Room inside the panel will be needed, at the bottom, for a 40 A 2-pole breaker. The low voltage wiring attached to the 3 sets of current transformers, that are part of an existing power monitoring system, separate from the solar PV installation, will need to be installed inside the panel so that the cover can be put back on properly.



Here's the finished rail installation on the garage roof where a string of 9 panels will go. The rail supports are screwed into the roof rafters. Each obstruction installed into or onto the roof has an upper slope deflecting sheet metal water screen. I don't know how these screens are permanently attached to the roof. (Image shot at 8:35 AM CT)



Next: Day 1 continued.

Best regards,

Bill

[a0128958]

Day 1: Installation Starts (Continued)

The mounting rails and hardware are all from Unirac.



The panels are wired in series for each string, then connected to #10 USE-2 wires (same as what's attached to panels) via mating connectors like what are already part of panel wires. The USE-2 wires go to roof top junction boxes (shown below). There is EMT conduit that runs from the bottom of these boxes into the attic. This is the first of 2 roof penetrations.



The panels will be elevated approximately 6" off of the roof surface. This should reduce attic temperatures. Each attachment to the roof and penetration of the roof is sealed.



Polyurethane construction adhesive is what's used to do all sealing of rail attachments and roof penetrations.



Directly connecting to the roof-top junction box, via roof penetration, is flexible conduit.



Next: Day 1 - Installation starts (continued).

Best regards,

Bill

[a0128958]

Day 1 - Installation Starts (Final Segment)

The flex conduit connects to an attic junction box where #10 THWN-2 wires carrying DC current come together (2 each for 4 strings, plus ground). Since this is a 1-story structure, the attic is large, making all locations in the attic easily accessible to run conduit for DC wiring.



Each rail has an interconnecting #8 bare copper ground wire connected to appropriate grounding fasteners. This ground makes its way to the inverter. No separate ground rod will be installed at the inverter.



The second roof top junction box, for the remaining 3 panel strings' wires, along with the associated roof penetration and attached flexible conduit, is shown here.



Here's an overall summary view of the first day's work. (Image shot at 10:40 AM CST).



Next: Day 2 - 2nd day of installation.

Best regards,

Bill

[a0128958]

How much did you pay per watt for those panels?

The general rate in the Dallas area is $6/watt, installed and complete. It could be a little more for much smaller than 8 KW (DC) systems.

Rarely, if ever, is the panel pricing broken out separately. As is the case for the inverter, too.

In Texas, an Oncor (Utility Transmission Service & Delivery Provider) rebate is only available when a certified installer is used. DIY installations are not able to get the rebate.

The combination of the rebate plus tax incentives cuts the project cost about in half. I.e., an 8.1 KW (DC) system, at $6/watt, is $49K gross. Out of pocket cost, after rebates and tax credits, is about $24K.

In my case, I'm doing a lease purchase with a monthly payment set by the system's design output. For the kWh that I consume from the solar PV system I'll save 30% over what I would have otherwise paid w/o having the system.

Hope this longer explanation helps. The short answer to your question is 'I don't know.'

Best regards,

Bill

[icarus]

I don't know what time of day the pictures were taken, but it sure seem like there is lots of shadowing, even shading over great huge expanses of your roof. Unless I am seeing it wrong I would worry about how much harvest you are really going to get.

It concerns me that after the hardware was purchased, the shading was discovered and design changes were made. I would hope that folks look carefully before purchasing hardware so they are not stuck with a system that doesn't work as well as expected.

I hope I am wrong with my assumptions,

tony

[a0128958]

Day 2 - Second Day of Installation

Today was spent doing 3 things: installing mounting rails on the roof to support 19 panels, finishing installing conduit in the attic for all DC wiring, and installing remaining DC carrying conduit to the inverter and AC carrying conduit outside of the garage.

Here is shown the flex conduit (left) connection to the DC Disconnect switch (9 wires total, each #10 THWN), and the conduit (right) that goes from the inverter to the AC (fused) Disconnect switch outside (four #8 THWN wires - 3-wire single phase 240 VAC and ground).



Up in the attic EMT conduit runs to the right to pick up the 7 wires from the remaining 3 panel array strings. 1" flex conduit runs to the DC Disconnect switch.



It was mostly an all day effort to install the mounting rails for 19 more panels. (Image shot at 8:35 AM CT)



Overall, after two days, lots of progress. Mounting rails are complete now for 28 panels, and all conduit runs up to the outside AC Disconnect switch are complete. (Image shot at 8:40 AM CST)



Next: Day 3 - Third day of Installation.

Best regards,

Bill

[a0128958]

... what time of day the pictures were taken, but it sure seem like there is lots of shadowing, even shading over great huge expanses of your roof.

... It concerns me that after the hardware was purchased, the shading was discovered and design changes were made. I would hope that folks look carefully before purchasing hardware so they are not stuck with a system that doesn't work as well as expected.

Tony, thanks for the note.

All of the images so far, except for one, were shot between 8:30 - 9:00 AM Dallas time. All of the roof is in shade at this point. The last shot for Day 1, was shot at about 10:40 AM, showing all of the house in full sun except for the garage (where there's one string of 9 panels).

I went back and added the time of day an image was shot for appropriate pictures.

We have had so much rain this Fall season that it's been hard to find sunny days to check measurements. The installing company came out at least 5 times to check measurements. They found the shading problem and modified the design appropriately.

Best regards,

Bill

[niel]

do note that grounding the rails is not usually required, but grounding the pv frames are required and only in the approved spot(s) on the frames with something like this,
http://store.solar-electric.com/gbdbtsopagrl.html