BOSTON — Sunshine may be free, but the future of a key solar power bill remains cloudy on Beacon Hill.

This week, House and Senate lawmakers ended their formal sessions for the year without reaching a key deal to raise caps on the state's "net metering" program that allows electric customers and local governments to sell excess solar power they generate back to the electrical grid in exchange for credit.

The Senate passed legislation earlier in the year to raise the caps to help the state reach its goal of 1,600 megawatts of installed solar capacity by 2020. That's enough to power about 240,000 homes each year.

House lawmakers waited until Tuesday — the second-to-last day of formal sessions — before passing their own version of the bill.

Like the Senate, the House called for a 2 percent increase in net metering caps, but the chambers disagreed on several other provisions including the reimbursement rate for large solar projects, with the Senate supporting a more generous rate than the House.

With little time to hammer out a single compromise version from the two bills, lawmakers left for holiday break without getting final legislation to Gov. Charlie Baker's desk.

Baker had urged action on raising the caps, pointing out that a federal solar energy tax credit program runs out at the end of 2016.

Like lawmakers, Baker also wants to raise the private and public net metering caps 2 percentage points each.


Critics say the Legislature's inability to get a final bill approved could end up jeopardizing proposed solar projects already in the development pipeline that are relying on the net metering benefits.

Renewable energy advocates say 171 communities across the state have already reached the cap, including communities in much of eastern Massachusetts served by National Grid.

Baker has said even without changes to the net metering cap, however, the state is on track to hit the 1,600 megawatt goal by 2018 — two years early.

While individual homeowners are exempt from the caps, the inability of lawmakers to raise them is making it harder for larger projects and residents of apartment buildings to benefit from the program, advocates said.

The cap is calculated as a percentage of each company's highest historical peak load — the most electricity consumed by their customers at any one time. Private facilities are currently capped at 4 percent, public facilities at 5 percent.

Utilities have said they're concerned about lifting existing caps, arguing that customers without solar panels are helping foot the bill for those who use net metering, saying that they're not helping cover the cost of the wires and poles they are using to send power back to the grid.

That tension between supporters of solar energy and traditional utilities was on display this week with solar backers charging utilities with trying to stall or water down strong pro-solar legislation.

A review of state lobbying records by the Associated Press in July found that companies specializing in alternative energy have also begun flexing their political muscles on Beacon Hill.

The review found that in 2005, just two dozen power companies reported spending a total of about $1.2 million trying to woo state lawmakers. Few were primarily involved in renewable power.

By 2014, the number of energy companies filing lobbying reports doubled, as had the amount spent on lobbying, nearly $2.4 million.

Of that, more than a million dollars was spent by solar, wind and hydropower energy companies, energy efficiency firms and others in the renewable energy industry.

House and Senate negotiators will have the rest of the year to stitch together a compromise bill to take back to lawmakers in both chambers next year.