Customers of the Pasadena Water and Power Department can expect a significant difference in the amount of their next bills. Electricity costs, beginning this month, will drop. Average residential customers can expect to see a saving of $9.90 on their April-May bills, city officials announced. Commercial electricity customers using an average of 100,000 kilowatt hours per month can expect a saving of $1,980 over the same period. The reduction results from declining oil prices and the city's recent renegotiation of its natural gas contract with Southern California Gas Co.