Mk2 DIY Solar Panels Part Five | on the Brian blog

diy solar electrical system

System / December 15, 2017

My wife Rebekah and I have always wanted to live a PV-powered life, but the high price of the technology—an estimated $30, 000 to cover all our electricity needs—had always been outside of our budget. Then came microinverter technology, which would allow us to start small and easily expand our system in the future. That, low mortgage rates, a drop in PV module prices, and the 30% federal solar tax credit sealed the deal.

I’m an active member of the Midcoast Green Collaborative, a local volunteer organization committed to creating a sustainable economy in coastal Maine. In 2009, one of our goals was to get feed-in tariff legislation passed. The proposed law was modeled on the successful German feed-in tariff law, which levies a small fee—usually $1 or so—on every electric ratepayer’s bill. The utility uses these funds to pay a premium per kWh to small-scale renewable energy generators. This helps make it cost-effective for homeowners to finance installing a PV or wind-electric system, since the income typically covers the loan payments. Once the loan is paid off—typically in 20 years—you’re set up to become a profitable electric micro-utility!

I testified at a hearing before the Maine Utilities and Energy Committee, where I presented a spreadsheet showing how the financing would work with a 20-year, low interest loan and a 20-year generation contract with Central Maine Power (CMP)—our local utility. I showed that a payment from CMP of 50 cents per kWh would significantly incentivize small-scale residential solar generators.

Testifying led me to do more research on the cost and feasibility of installing my own grid-tied PV system. My first call was to my friend Naoto Inoue, the owner of Solar Market, a solar dealer and installer in Maine. He helped me design the solar heating system for my workshop back in 2001 (see “Solar Heat for My Maine Workshop” in HP89 and “Solar Heat Upgrade: Expanding & Improving an Owner-Installed System” in HP119) and sold me much of the equipment. He mentioned an emerging technology—microinverters—that was changing the paradigm of PV installations. Instead of the modules being wired together to create high-voltage DC that is sent to a large, single inverter, each module is equipped with its own small inverter. The power is converted to 240 VAC right at the module, which can make the system more efficient and the design more flexible. It eliminates the shading issues that can compromise the performance of DC systems when modules are wired in series, since shading part of one module in the series can compromise the whole string. Also, modules of different capacities can be mixed, allowing system growth over time without worrying about module mismatch.

Designing the System

Typically, the first step in designing a PV system is to know how much power you use—or will use. But before that, to reduce your system costs, most people need to work at reducing their usage. Rebekah and I had reduced our energy footprint, both with conservation practices and efficient lighting and appliances. Our past 12 months of electric bills showed an average use of 550 kWh per month. To compare, in 2006, the average U.S. household used 880 kWh per month. Take a look at your recent electric bills and see how your home stacks up—there may be room for improvement! We use propane for cooking; propane, solar, and woodstoves for space heating; and propane and solar for water heating, so our electricity use was mostly for appliances and electronics.

Producing 550 kWh as a design ideal was the starting point. PV systems are usually rated by the total kW capacity of the modules—not the AC power produced. Energy losses come from inverting the DC to AC, wire heating, module soiling and mismatch, and other losses like module production tolerances; so a derate factor—typically 0.77—is applied by sizing tools like PVWatts (see Access). This derate and module temperature losses are used to estimate the AC kWh of a given solar array. However, the Enphase microinverters I planned to use have an efficiency of 0.95 (vs. 0.92 assumed by PVWatts) and will circumvent module mismatch issues. When incorporated into the calculation, the derate factor was 0.81—a significant improvement over 0.77!

PVWatts further calculates the array performance based on system location, power rating, tilt, and orientation. I decided to use 21, 175 W BP 175B modules for a 3, 675 W array. Our derate factor and our array’s westerly orientation showed an annual production of about 3, 600 kWh. PVWatts also showed the array’s estimated average monthly production at 300 kWh, 250 kWh less than our average consumption, and 150 kWh less than our consumption that is billed at a higher rate. The way we are billed, the first 100 kWh is provided at a lower rate (about 12 vs. 18 cents), so there is less incentive to offset that first 100 kWh.

Part of the original impetus to install our own PV system came from my experience of watching proposed feed-in-tariff legislation get strangled to death by well-intentioned members of the Maine Utilities and Energy committee. I testified and then spent many long afternoons in the committee room observing the deliberations. The bill passed, but it had no teeth—the payout was based on historic high wholesale prices and no tariff, thus zero incentives.

In the process of preparing my testimony for the committee, I began to realize that Maine’s net metering law could provide at least some benefits to make the PV system more affordable. I estimated that if we were to install a system that generated all the electricity that we typically use, our annual bill would be reduced to the minimum connection fee—about $8.00 per month. It was actually financially prudent to slightly undersize the system, so that we would not be giving away any surplus. Annual system energy production values are often conservatively estimated and the system may exceed initial estimates. Under Maine’s annualized net-metering agreement, the utility does not pay for a surplus at the end of the year, but only credits the excess power generated in any given month.

Budgeting for the System

The federal tax credit allowed us to deduct 30% of the cost of the system from our federal taxes. We would have normally set funds aside on a weekly basis to pay quarterly estimates, and by avoiding having to pay them in 2009, we saw an immediate reduction in expenses. This year, we will avoid paying nearly $6, 000 in taxes. We also took advantage of the Maine Public Utilities Commission’s Efficiency Maine program, which offers a solar rebate that pays $2 per watt for the first 1, 000 W (capped at $2, 000).

To pay for the system, we refinanced our house. We had an adjustable rate mortgage that could adjust up this year, and we figured that it was a good time to lock in a 20-year fixed mortgage. To keep the money in the local economy, we found a mortgage with a local bank. I watched the economic indicators and changing mortgage rates carefully and then locked in a low loan rate in late April 2009.