Pacific Gas & Electric’s announcement last week that two Bay Area companies hope to build the nation’s largest photovoltaic solar electric plants near San Luis Obispo is evidence of the dynamic future of solar power – and of Congress’ confounding failure to foster it.

The projects’ combined 800 megawatts – the size of a small nuclear plant – will produce enough power to light up 239,000 houses, about 2 percent of PG&E’s power needs. They will help the utility meet the state law requiring that by 2010 it line up alternative energy sources to produce 20 percent of its power. They will establish solar photovoltaic energy as competitive with wind and other alternative energy sources, including solar-thermal power plants.

But both solar power companies have made it clear they cannot move forward with their projects unless Congress extends the investment tax credit for solar energy.

The 30 percent solar tax credit is set to expire Dec. 31. Eight times this year, Congress has failed to extend it. Opening up offshore to further oil drilling will not yield a drop of oil for more than a decade. Passing the tax credit would spur energy production now.

But House and Senate Republicans continue to block the sun, holding the credit hostage to permanently enacting President Bush’s deficit-exploding income tax cuts. Of the two major presidential candidates, Democrat Barack Obama voted for the tax credits. Republican John McCain, while lately expressing support, has missed all votes in the Senate extending them.

San Jose-based SunPower Corp.’s 250 megawatts would be nearly 20 times the size of the company’s largest plant to date. Its 12-by-8-foot photovoltaic collectors would be mounted over 3 1/2 square miles of non-prime farmland in a valley free from coastal fog. Hayward-based OptiSolar’s very thin sheets of silicon solar panels, laid out on an adjoining nine square miles, would produce 550 megawatts.

Increasing numbers of homeowners and other retail electric customers of PG&E have taken advantage of federal and state solar tax credits to install photovoltaic cells. By seeding the market through the tax credit, Congress has helped to lower the price of solar cells and to make them more efficient. That’s one reason that large-scale photovoltaic solar projects can now compete in the wholesale electricity market with other alternative energy sources.

America is vulnerable to oil price spikes and unstable petrocrats. Extending the credit is good for the valley’s economy and America’s security.