Calpine ( CPN) fell 10% in heavy trading Monday as investors worried about the energy company's long-term power contracts with California.

A state utility oversight board was expected to file with federal regulators Monday, seeking to void long-term contracts reached during last year's energy crunch that it deems too pricey. Calpine is the biggest holder of long-term contracts in the state, though the company says its deals are among the state's lowest-priced. Power contracts account for the lion's share of Calpine's revenue.

Calpine shares, which were down 73 cents to $6.38 around midday Monday, have lost more than 80% of their value over the last year as investors worried about the debt-heavy company's finances and the state of the energy market.