PORTLAND — Central Maine Power Co. wrote off $3.8 million in bad debt last year after nearly 20 percent of its residential customers failed to pay electric bills on time.

While only 11 percent of home phone customers at NYNEX money during that period, its write-offs totaled $8.2 million.

Smaller utilities were not immune from collection problems. At the town-owned Madison Electric Works, nearly six of 10 customers owed the company money in 1994.

The figures were compiled by the state Public Utilities Commission, which made a first-ever comparison of utility credit and collection programs. A goal of the study was to see how effective Maine’s electric, phone and natural gas companies were in pursuing debts.

CMP, whose collection techniques include letters, phone calls and payment plans, ranked well, with average write-offs that were among the lowest of all power companies.

The company is authorized to cut off power during the daytime to customers who ignore their bills.