Eight months after voters approved a renewable-energy standard for Colorado, utilities and interest groups finally are making progress hammering out controversial details.

Utility firms Xcel Energy and Aquila Inc. say they seek minimal regulation and maximum flexibility in complying with Amendment 37's mandates.

Renewable-energy advocates and environmental advocates, however, prefer specific requirements for implementation.

The controversy was headed for a flare-up in a public hearing set to begin today before the Colorado Public Utilities Commission.

But in a recent series of confidential meetings by utilities, renewable-energy advocates and environmentalists, the parties appear to be approaching an agreement.

"Overall we're pleased with how the discussions are going," said Xcel spokeswoman Margarita Alarcon.

"I feel optimistic that we're making pretty good progress," said Rick Gilliam, senior energy-policy adviser for the Boulder environmental-law group Western Resource Advocates.

Amendment 37, approved in November by a 52 percent majority, requires Colorado's largest utilities to obtain 10 percent of their power from renewable sources by 2015.

The two-page ballot initiative, however, lacked details on timetables, payments to renewable providers, penalties for noncompliance, and whether the program should be managed by utilities or independent administrators.

Most of the renewable power is expected to come from wind farms, although the law requires 4 percent of the renewable portfolio to be solar power.

Today's PUC hearing begins at 9 a.m. and includes public comment from 2 p.m. to 6:30 p.m.

However, the principal parties in the negotiations have agreed to delay their testimony until late August, when they hope to have an agreement. The hearing will be in hearing room A, office level 2, at the commission's offices, 1580 Logan St.

"We will still proceed, and anyone is welcome to comment," said PUC spokesman Terry Bote. "But it'll pretty much be a nonevent," with the major players on the sidelines.

Staff writer Steve Raabe can be reached at 303-820-1948 or sraabe@denverpost.com.