And the cost of solar panels represents only a part of the overall installation cost, typically 30% to 40% for homes.

According to the Solar Energy Industries Association, the United States had 1.6 million solar electric installations in September 2017 capable of generating 49 gigawatts, or 1.8% of U.S. electrical demand. This year the number of installations is expected to reach 2 million and 4 million by 2022, when solar is expected to supply 5% of U.S. demand.

Because of the widening cost advantage, Harris expects solar panels on homes, apartments and commercial buildings will become commonplace — soon.

"I can't tell you if that is going to happen in two years, five years or 10 years," he said. "I see a lot of potential for Tech CU and for the entire credit union industry to be a significant player in solar financing."

Nevertheless, Harris said the tariff set back solar's price advantages by about two years.

Carol Chernikoff, chief lending officer at Alternatives Federal Credit Union in Ithaca, N.Y., said the tariff discourages members who have been considering solar, but were on the fence.

"This tariff – I don't know who it benefits. It will discourage a lot of people from moving forward with solar just on face value," she said.

Yet, Chernikoff, Harris and other lenders still have faith in the resilience of the sun based on powerful trends continuing:

• The price of panels, measured in cost per kilowatt of capacity, is falling.
• Utility electric rates are rising.
• The installation of solar panels is increasingly accepted and is now contributing to home values.
• The tariff steps down from 30% to 15% over four years and then goes away.

And if solar is now becoming a major frontier for household spending, it is also becoming one that increasingly depends on lending.

Two years ago, a survey found half of installers offered third-party financing, such leases or power purchase agreements, in which a third party owns the equipment atop the home, and the homeowner is obligated to purchase the solar power at a rate substantially less than the utility's rate.

In January the 587 solar installers polled found only 20% offered leases or power purchase agreements, according to the survey by EnergySage, a Boston company that provides an online tool to homeowners for comparison-shopping for solar.

Sunlight Financial, a New York company that provides an online conduit for loans sold through its network of solar installers, is also seeing a shift to loans and away from leases and power purchase agreements. As systems have become more reliable, homeowners have been more willing to assume the risk of ownership, and in the process enjoy greater rewards, CEO Matt Potere said.

"Because they own their home, they prefer to own the system that's bolted to their roof as well," Potere said.

Residential solar is highly fragmented with installers trying to connect with a dispersed population of eligible homeowners. Interested homeowners with limited knowledge need to consider multiple installers and systems and financing options.

Some online companies are trying to provide the web platforms to make those connections. EnergySage's interface is designed for homeowners, helping to guide them through the process and connecting them with vetted installers and lenders.

Sunlight Financial gears its online application to installers, allowing them to offer customers a way to finance the installation often with positive cash flow from the first year.

In January, Tech CU said it will commit $400 million a year to a loan program offered through Sunlight Financial, enabling it to fund about 15,000 loans, or about $26,700 per loan.

Alternatives FCU ($103.7 million in assets, 10,109 members) is also expanding its solar lending. Its upstate New York members place a high value on environmentally conscious consumption and numerous solar installers have operated in the area for years.

Installers had been working with remote lenders who would finance the amount that equaled the rebate. The financer would charge the installer for each loan origination, and the customer would be allowed to pay zero interest if they paid off the loan within 12 months. If not, they would pay the accrued interest, often at rates of 18% to 28%.

About six years ago, Alternatives FCU's community focus groups showed there was an interest in better solar financing in the area.

Chernikoff first developed a second mortgage that allowed borrowers to make interest-only payments for the first 18 months, then paying as a fully amortizing loan. The loans are up to $100,000 or 90% of loan to value net of all outstanding liens. Terms are up to 15 years.

The borrower has to qualify for the full amount of the loan, but the setup anticipated they would receive a rebate within the interest-only period. Until then, they could pay the discounted rate. After the rebate arrived, they could pay down the principal and retire the debt early or pocket the cash.

For a 10-year loan of $20,000 at 4.49%, payments would be $207 a month if principal was paid from the start. If the borrower makes interest-only payments, the payments would be $75 a month for the first 18 months, and $236 a month for the remaining 102 months.

In 2016 Alternatives introduced unsecured consumer solar loans. Rates are currently 6.49%, or 5.49% for those with a credit score above 680. The limit is up to $25,000 per employed borrower.

In mid-2016, Alternatives FCU teamed up with Renovus Solar, which had been installing solar panels in upstate New York since 2003. Renovus pays the first 12 months interest on the loan covering the value of the tax rebates.

Last year Alternatives originated 115 solar loans for $1.5 million, up from 50 loans for $584,000 in 2016, when the program began.

"Our consumer solar lending just went off the charts," she said. "We're a small community development credit union in a town of 100,000 people. It's huge for us."

In California's Silicon Valley, Harris said providing solar loans is "on brand" for Tech CU. However, the credit union didn't have a direct connection with people who were looking to install solar panels.

Although its members had homes with plenty of equity, they typically viewed home equity loans as a source of emergency financing, Harris said.

In late 2016, Tech CU began providing loans through the online platform provided by Sunlight Financial.

"It just made sense," Harris said. "They had this great desktop solution for originations, but they didn't have any money to lend out."

The loans are consumer loans secured by the equipment. The loans allow homeowners to immediately pay less money each month on their loan and utilities combined, than under there higher pre-solar bills.

"So they're cash flow positive and they still have their HELOC available," Harris said.

Tech CU originated about $340 million in loans through Sunlight Financial in 2017. "I'm expecting $350 million—or $400 million best case— in 2018," Harris said.

The bulk of Tech CU's loans are $15,000 to $25,000, but it recently financed a $200,000 system on a large home. It offers 15-year terms, but most homeowners prefer 20-year terms. Rates are 2.99% to 5.99% depending on credit risk.

"What we've seen on the credit history of these borrowers is very, very strong," Harris said.  "We have yet to take a loss on a solar loan."