Have you ever wondered whether installing a grid-tied solar-electric system could be a do-it-yourself project? My wife Carol and I had made some of the easier energy-efficiency upgrades to our home, and even installed a domestic solar hot water system, so it was time to tackle our electricity needs. But the prospect was daunting. Most people will cede that installing a PV system is a much bigger undertaking than putting in a solar hot water system. Plus, it’s more expensive and requires more room.
The learning curve is steep, and I started with Home Power articles, progressed to attending a helpful seminar presented by Power Trip Energy of Port Townsend, Washington, and then reached a decision point: Were we going to hire a professional to install a system, or tackle the design and installation ourselves? Ultimately, the decision was made easier because there are no installers in our area. An out-of-county company gave us a quote of $53,000 for a 5.9 kW, ground-mounted, grid-tied system that would offset most of our household’s yearly electricity needs. About that time, with the national economy taking a nosedive in early 2009, our comfort level with a very large PV investment also dropped. It was time to consider cutting costs by taking on the project ourselves—if a DIYer could actually make it happen.
Thus began several months of intensive research, a step that cannot be taken lightly. Next, I researched Internet sellers of PV components for current specs and prices. Since I’m not an electrician, I had doubts about tackling the complexities of both high-voltage DC and AC systems, but after careful reading of John Wiles’ Code Corner columns and Article 690 of the National Electrical Code Handbook, those elements started to make sense. Even at this early stage, it was evident that the savings could exceed $10,000. By the time we were done, the savings were more than twice that.
This kind of project should only be undertaken by someone ready to do the homework and learn the skills. If you are considering a DIY PV system, be aware of some major hurdles:
Need analysis: Review your electricity bills. Reduce your electricity consumption where possible. Determine how many kWh of electricity per year you’ll need.
Site analysis: Find true south at your site. Analyze your exposure to the sun’s path, and, ideally, seek a location that’s shade-free during the entire year. Determine how many peak sun-hours your location receives in an average year. (For more information on siting a PV system, check out “Solar Site Assessment” in HP130.)
Mounting system: Evaluate the roof’s available space, orientation, pitch, and structural soundness, and the condition of your roofing material. Are there shade-free options for a separate pole-mounted or ground-mounted system? What are the respective costs, installation time, durability, and visual considerations for each mounting option?
System design: What type of PV modules best suit your situation? How can you ensure a good match between the PV array and the inverter(s)? What other components—such as combiner boxes, disconnects, overcurrent protection, and metering—are necessary for a code-compliant, efficient, and safe system?
Rebates and other financial incentives: What federal, state, and local incentives are available? Do they require purchasing locally manufactured components? Are incentives structured as up-front rebates, tax credits, or paid out over time? Do they require that a licensed professional install the system?
Permitting: What permits does your local jurisdiction require for this project? Will you need a building permit for roof work or ground-mount construction? An electrical permit? A contract with the local power company for the grid intertie? Do the permitting authorities require that work be done by a licensed professional?
Equipment and tools: Do you own or have access to the necessary tools and equipment, some of which may not be a part of the average tool kit?
Installation: Do you have a working knowledge of Article 690 and other articles of the NEC necessary to install a safe, code-compliant system? Do you need to hire someone to consult on these issues?
In 2007, we significantly reduced our electricity demand by installing a solar hot water system (see “Do-It-Yourself: Tips for Solar Hot Water Success” in HP123). The next year, we replaced our smaller wood heater with a larger and more efficient cast-iron one. Our horses drink from a geothermally heated water fountain, rather than a resistance-heated tank, during the winter. A new, homemade solar hot air panel has just started reducing some of our winter cordwood consumption.
Rather than bring in propane, we’ve kept all of our other energy loads on electricity. Our home is already connected to the grid, so the reduced cost and increased simplicity of a grid-tied PV system was the logical choice. Our electricity bill history showed that the new system would need to produce an average of 680 kWh per month (about 22.4 kWh per day) to cover our electricity needs. Our local average daily insolation is about 4.8 peak sun-hours. According to PVWatts (see Access), offsetting 100% of our electricity consumption would require a 6.5 kW PV system.
In the end, we went with a 5.7 kW system which, according to PVWatts, should cover about 87% of our yearly electric usage. This was largely dictated by PV module prices, since the biggest cost in a solar-electric system is the PV array. We were able to find a pallet (28 modules) of “blemished” PV modules (cosmetic problems only, with full functionality, output, and warranty). Additionally, these Evergreen modules have a positive-only (+2.4%) production tolerance, which should increase the yearly energy production to above the PVWatts estimates, since PVWatts assumes a -5% production tolerance.
The next choice was the inverter, which requires careful planning. An array voltage that’s too high can damage the inverter, while low voltage or amperage will run the inverter less efficiently, and can cause the inverter to drop out. Inverter sizing calculations must take into account that array voltage output drops in hot weather and is higher in cold weather, and can decrease over the years. (See “Grid-Tied Inverters Buyer’s Guide” in HP133 for tips on selecting an inverter.)
PV module manufacturers supply some of the information you’ll need, such as module specifications and how much module output varies with temperature. Some data you’ll determine based on your local temperature extremes and mounting system. And other data will come from the inverter manufacturer. Fortunately, most major inverter manufacturers provide online calculators to help you figure out string sizes for their inverters. If you’re designing a system, you need to understand all these variables, and you’ll want to check the online results with your own calculations—I found that online string calculators gave useful results, similar but not always identical to hand-calculated figures. (For more information, see “String Theory: PV Array Voltage Calculations” in HP125.)
We determined that a single PV Powered PVP4800 inverter would fit our needs. It tolerates lower environmental temperatures (-13°F), so no enclosed or insulated/heated/actively vented space would be needed. On the rare days when the temperature drops below -13°F, the PVP4800 will shut down and then safely resume work after the sun has warmed things up a bit.
In addition to the PV modules and inverter, there are many other components needed, such as disconnect switches, fuses, breakers, wires, lightning arrestors, conduit and a combiner box. Find a component dealer that offers the info you need, and staff willing to help select compatible components. There are other good ones, but we decided on Affordable Solar.
If the PV modules are not going to be roof-mounted, you’ll still need to decide between a pole mount and a ground mount. Poles allow the use of trackers, and make it easy to adjust for seasonal elevation changes—but the installation complexity and cost goes up accordingly.
A tracked array was not a cost-effective solution for our site. In the late afternoon, the sun disappears behind a hill, cutting back the energy that could be gained by using a tracker, which is easiest to justify when there is horizon-to-horizon solar access. In the end, we decided on a fixed, ground-mount. Compared to an adjustable ground-mounted system, having a fixed ground mount at 45 degrees would only incur a 4% energy loss per year. Plus, a ground mount would not require the specially fabricated and more expensive horizontal pole-top racking system that our deep snows would dictate. It was also something I could build myself and for less than a pole-mounted system.
Next, I needed to look for shading problems in the solar window. The sun-path software offered by the University of Oregon Solar Radiation Monitoring Laboratory told me what the sun’s elevation and azimuth would be year-round. I scoped out those sun positions using a Suunto KB-14 optical compass and a Suunto PM-5 clinometer, finding that another dozen trees would need to be removed. With our firewood bays well stocked and our vegetable garden less shaded, the result was a solar window of at least six hours per day, year-round.
Once I’d determined the array location, I laid out the ground mount, which involved basic surveying skills and equipment. Our backyard is sloped, and rather than engage in complex math and sloped construction, I leveled the ground. Each post sinks at least 3 feet below the original grade, and all posts benefit from 1 to 4 feet of additional fill above the original grade. In retrospect, having moved some 40 cubic yards of gravel, rock, and dirt for this project, perhaps I should have done the complex math instead.
If you’re designing your own ground mount, CAD software can make the job easier. I’ve used various versions of TurboCAD Deluxe over many years on numerous construction projects, including this one.
I built the ground mount with large treated timbers—a construction method that I’m familiar with after building our sheds and pole barn, and I had the needed heavy equipment. Having learned from prior mistakes, I knew to build only on undisturbed ground and to give adequate vertical support to prevent the structure from sinking under snow load. Rebar inserted horizontally through the vertical posts helps secure them inside 2-foot-diameter steel culvert collars filled with 5/8-inch gravel. The timbers are attached together with 1/4-inch steel plates, 6-inch channel iron, and 5/8-inch grade-8 steel bolts. All timbers are diagonally braced with tension wires tightened with 1/2-inch galvanized turnbuckles. Even though the timbers were pressure-treated for long-term ground contact and largely sheltered by the array, all exposed wood was protected with waterproof sealant.
The rack, which secures the PV modules to the timber structure, was home-built using Unistrut. This erector-set-like hardware comes in various sizes and gauges, with assorted connector and fastener options. Structures in this area should be designed for maximum snow loads of 59 pounds per square foot, and Unistrut’s P5500 galvanized channel had the load strength to span the horizontal timbers. After cutting each module support channel to length, the leftover pieces were used at the top and bottom of the rack. This creates a stronger lattice, protects the module edges, and provides bracing and lifting points for sliding the modules into place. The racking is bolted to vertical angle-iron that is bolted to the ground-mount beams.
Once the rack was in place, the next step was to install the PV modules. The Evergreen modules have pre-drilled mounting holes at points that can support 60 lbs./ft.2 loads. Special precautions were necessary to avoid galvanic reaction between the aluminum module frames and Unistrut’s galvanized steel. I used stainless-steel bolts, nuts, and washers, and inserted 1/4-inch-thick shims between the module frames and channels. The drilled shims (made from aluminum and steel) are each double-wrapped in electrical tape, with additional layers applied to both the module frame and the channel.
The DIY ground-mount cost $4,059, less than half the $10,006 quoted by a commercial installer for two steel pipes set in concrete, with commercial pole-mounted racks.
Next came mounting the 135-pound inverter and other electrical components (combiner box, disconnect switches, meter base, and breaker box) on a plywood panel attached to the timber structure.
With the mechanical work done, it was time to wire everything together. Connecting the PV modules was easy—their pre-wired MC-4 lockable connectors simply snap into each other. The two 14-module series strings come together in a MidNite Solar MNPV6 combiner box, where each is protected by a 20 amp, 600 VDC fuse.
Wiring was complicated by the need to keep voltage drops (line losses) small to minimize energy loss. The modules came with #10 AWG wire, so I also used #10 wire for the short wiring run to the combiner box. The 165-foot underground run (from the inverter location to the home service-disconnect location) needed larger #4 AWG wire to keep the 240 VAC line loss less than the 2% recommended in the PVPowered installation manual. I used #4 wire for the many short connections on either side of the inverter, except for the inverter itself, which would only accept smaller #6 wires. The terminals in the disconnect boxes on either side of the inverter allowed transitioning the #6 inverter wiring to the #4 long run wiring.
PV equipment grounding is important to ensure human safety and to minimize risk of equipment damage from lightning. I grounded my steel racking with heavy-gauge bare copper ground wires (#4 and #6 AWG). The wires are secured to the structure by 15 steel clamps and electrically connected with one lay-in lug, then clamped with acorn nuts to two widely separated 8-foot-long copper-plated grounding electrodes pounded into the earth. An additional run of #6 AWG bare wire goes to every PV module, secured by lay-in lugs. If lightning affects the modules and gets into the DC wiring, it should short to ground through a lightning arrestor. On the AC side of the inverter, two more grounding electrodes and another lightning arrestor are wired in, plus another pair of grounding electrodes at the service disconnect.
When your new PV system coming on-line feels like it’s just around the corner, it can be frustrating to have everything held up by inspection delays. One mistake I made was assuming that the electrical inspector would be familiar with PV installations. In retrospect, it would have been better to offer him more detailed information up-front. For instance, he wanted documentation (other than the label on the unit) proving that the inverter was UL listed. Then, when he decided to require ground-fault protection, it was up to me to persuade him that the ground-fault indication/detection (GFID) system was built into this inverter at the factory. He also required adding traditional breakers between the inverter AC output and the production meter. This was to protect the long underground wire run, even though it already benefited from three other types of overcurrent protection at this end: two DC string fuses, an inverter that can’t produce more than 21 amps AC, and panel strings that max out at 9.2 kW. Sometimes it’s easiest to just add whatever components the inspector wants, rather than debate the finer points of the NEC.
I ran into similar glitches with our local power company for the grid interconnection. Their unfamiliarity with PV systems resulted in some uncertainties and difficulties at the management level. To this day, Pacific Power’s computer still has trouble digesting our meter data, which delays each monthly statement. In contrast, their front-line employee, who installed the net and production meters, helpfully answered questions, responding promptly. My hope is that both the inspection and grid-tie steps will become easier for others once this technology becomes better understood in our region.
This DIY project involved many steps and technologies that I hadn’t used before. All the parts had to be researched and ordered ahead of time, with each major component having implications for the others. Tackling a solar-electric installation isn’t impossible, but it does require that you be comfortable taking on the duties of the overall project manager, plus responsibility for making each minor step work.
The skills and experiences you start out with will determine what additional knowledge you need for a successful installation. For me, it meant lots of reading; the two most helpful books were The New Solar Electric Home by Joel Davidson and Fran Orner, and Got Sun? Go Solar by Rex Ewing and Doug Pratt. Article 690 of the NEC is mandatory reading (an expensive publication to purchase, but available for free at our public library). It’s rather dry, so help yourself by also reading the commentary in one of the NEC handbooks (I bought the McGraw-Hill version). When those resources left me with specific PV questions, John Wiles was kind enough to answer several e-mails. The manager at our local electrical supply house was very patient in suggesting practical solutions for some of the wiring issues that came up during installation—he was interested in the PV system and repeatedly offered advice and helped with problem solving.
Beyond hands-on knowledge, flexibility is also important to a PV project. Because it’s not a one-size-fits-all situation, you’ll need to adapt the system to site specifics including available space, shading, and energy needs. Keeping that same flexible attitude when using ground mounts and racking may also open up opportunities for savings. I used www.craigslist.org to locate many used items at attractive prices, including 40 feet of 24-inch-diameter culvert, 20 feet of 6-inch channel iron, creosoted railroad ties, 2-inch angle iron, and a sack of galvanized turnbuckles. Craigslist also turned up some of the tools I used for this project, including a tall orchard ladder and a band saw.
Part of a successful DIY project is knowing your limits, and calling for help as needed. Some of the heavy lifting was beyond my capacity, so I was grateful for help with setting the largest timbers. Some of the high-voltage wiring and dealing with electrical inspectors was beyond my experience, so I consulted with an electrician before undertaking these tasks.
In the end, after five months of planning and building, it was exhilarating to do the final circuit testing and then apply power to the inverter, a true “we have liftoff” moment. In the 109 days since we commissioned the system, the summer’s average daily production of 28.2 kWh has exceeded our average daily use of 17.9 kWh, leaving a surplus of 10.2 kWh per day to draw on during the darker winter months. By the winter solstice, we’ll have six months of data, and a pretty good idea of what portion the PV system will provide of our overall needs. Meanwhile, our heightened awareness of our electricity use has already resulted in reducing our consumption. We’re using our two solar ovens more, timing clothes washing to synchronize with peak solar water-heating, adding power strips to turn off rarely used electronics, updating our computer’s old CRT monitor to a more efficient LCD monitor, and using more efficient irrigation practices to reduce pump run-time.
Will this PV project prove cost effective? I think so. The total up-front cost came to almost $30,000. We will be receiving the 30% federal income tax credit, the Washington State production payments of 15 cents per kWh, any future green tag payments (presently unavailable here), and the savings on our power bills. Over time, we’ll get updated energy production figures for this system, and we’ll see what increases in grid energy rates we are avoiding. To date, data predict a 14-year payback for a system that should last twice that long. The clear environmental benefits were enough to draw us into this project, but it will probably also save us money over the next two to four decades. We’re even considering doubling the system, which could provide enough additional energy for an electric car.
Bob Inouye lives with his wife Carol in the foothills of the Cascade Mountains in Washington. The couple happily shares 180 forested acres with horses, mule deer, elk, turkeys, beaver, salmon, and steelhead.
PV System Components:
Evergreen Solar • www.evergreensolar.com • PV modules
PVPowered • www.pvpowered.com • Inverter
Affordable Solar • www.affordable-solar.com • PV system components
Power Trip Energy • www.powertripenergy.com • PV seminar
University of Oregon Solar Radiation Monitoring Laboratory • http://solardat.uoregon.edu/SoftwareTools.html • Free sun-path calculation software
PVWatts • http://rredc.nrel.gov/solar/codes_algs/PVWATTS/version1/ • PV system production estimates