Increased assessments at nuclear power stations accounted for a major portion of the increase in assessments against utility companies in Virginia, the State Corporation Commission said yesterday.

The commission said total assessments climbed 25 percent over a year ago to nearly $4.9 billion. The increase reflected the higher assessment ratios required by law, which by 1981 will be 100 percent of market value, said Samuel Burruss, director of the Public Service Taxation Division.

Typical of the increase was Surry County, where local taxes on Virginia Electric and Power Co.'s nuclear power plant account for a major portion of county revenue.

In 1977, the Surry Public Service assessment, primarily against Vepco, was $31,4 million with a ratio of 7.7 percent. This year the figure was 88.3 percent with a resulting valuation of $358.4 million.

The impact of Vepco's nuclear installations also were felt in Louisa County where the first of four atomic plants has gone into operation. Louisa's Public Service properties were assessed at $439.9 million this year with an assessment of 79.2 percent of market value.

Last year, the property was valued at $425.5 million with a ratio of 88.8 percent. For the second year in a row, Louisa County topped all cities and counties on public service property valuation.