By Deborah Zabarenko and Ayesha Rascoe

WASHINGTON, Feb 26 (Reuters) - U.S. President Barack Obama's estimate of $646 billion in revenue for the first years of a carbon-capping program to curb climate change is realistic or possibly a little low, policy analysts said on Thursday.

Obama's budget for 2010 projects this revenue, from 2012 through 2019, will fund $150 billion in clean energy technology investments over 10 years and a tax credit to help Americans make the transition to a less carbon-intensive economy.

"I don't think it's overly optimistic at all," said Brian Murray, director for economic analysis at the Nicholas Institute for Environmental Policy Solutions at Duke University.

"It's on the conservative side," said Tim Profeta, the institute's director. Both spoke in a telephone interview.

"From a substantive standpoint, the numbers are based on a good analysis," said David Gardiner, senior adviser to the Ceres coalition of investors, environmental groups and others aiming to curb climate change.

The $646 billion figure, spread over eight years, presumes that a U.S. law to limit carbon emissions will be in place by 2012, and Obama has said he will work with Congress to make this happen.

Obama has said he wants a so-called cap and trade system that would put a price on emissions of climate-warming carbon. Companies that emit more than the limit would have to buy emission permits; companies that emit less could sell emission credits.

It would mean about $80 billion in revenue annually, the Nicholas institute's Murray said, with each ton of carbon emissions priced at $15 at the start of the eight-year period. Because the goal is to reduce carbon emissions, the carbon price would rise over the eight years.