A leading financial incentive that’s driving solar power’s growth at home is a 1980s-vintage rule called net energy billing. It requires utilities to pay small energy generators the full retail price for all the electricity they send into the grid. Those payments help recover the investment in solar-electric panels, which can run $10,000 or so at an average home.

But net energy billing, often called net metering, is controversial nationwide. As solar’s popularity grows, utilities say those payments are shifting the cost of serving homes with solar panels onto other customers. Clean-energy advocates counter that the value of this energy actually is greater than the cost. Both sides cite figures and studies to support their claims.

The Maine Public Utilities Commission now is examining this issue. It was triggered this year by part of the net energy billing rule that says the PUC must review the practice, once peak power production reaches 1 percent of Central Maine Power’s installed capacity.

Uncertainty over the future of net metering is having a chilling effect on Maine’s small solar installation industry. Business worth millions of dollars stalled last spring after Gov. Paul LePage vetoed a wide-ranging solar energy bill that included a plan to update net metering. The Legislature failed to override the veto by two votes, pitting LePage and his mostly Republican allies against Democrats.

In June, a group of roughly 30 solar installers, businesses, municipalities and other interest groups asked the PUC to limit the scope of its review, so that lawmakers could take up the issue again next year. They fear that the three PUC commissioners, all of whom were appointed by LePage, will gut net metering.

Their fears were heightened last month when LePage’s energy office filed comments in the case. His office wants to lower the level of compensation for net metering, recommending that current customers get the full retail price for no more than three years. CMP says in its comments that it doesn’t oppose a “reasonable period for grandfathering.” It suggests a maximum of 10 years.

Clean-energy advocates and their political supporters say that failing to fully grandfather these accounts would break faith with residents who installed solar under the current net metering rule.

Advocacy groups including the Natural Resources Council of Maine, as well as national and local solar installers, also say CMP and LePage’s office make unsubstantiated claims about the high impact of net metering. For instance: CMP says lost revenue reached $1.3 million last winter. Solar advocates are asking the PUC to dig into these numbers. They also note that CMP and LePage have failed to provide an analysis of the benefits of solar to ratepayers, laid out in a study done for the PUC and recently updated.

Regulators in roughly two dozen states are reviewing or recently have adjusted net energy billing laws, with vastly different results.

In Nevada, decisions to cut the compensation rate for net metering and add new fees have led to a business exodus, court suits and a recently failed attempt to put the issue before voters. In Massachusetts, Gov. Charlie Baker, a Republican, signed a bill last spring that amounts to a short-term fix for net metering. It raises the generation “cap” from 4 percent to 7 percent for small-scale solar, while lowering the compensation rate for larger projects.

Maine’s PUC has a couple of choices. It could do nothing, and let lawmakers take another crack at a resolution. A more likely option would be to propose some changes. That would lead to a comment period and public hearings this fall, with a decision expected by year’s end.