More than 12 million energy utility customers in California pay less than the national average for natural gas and electricity, and the wealthiest residential consumers tend to pay the highest monthly bills on average, according to a study done by the California Public Utilities Commission (CPUC) planning division.

Average monthly gas utility bills year-round for residential customers were about $34 last year, compared to a national average of $64/month, the CPUC study found. For electricity, the average residential bill was $93/month, compared to a national average of $114/month, according to the CPUC staff calculations.

The CPUC report is part of a Big Data initiative begun last year to break down energy, water and broadband data into more bite-sized increments. A geographic information system (GIS) platform was used to determine average bills and consumption patterns by zip code.

For gas, data came principally from the three major gas investor-owned utilities (IOU): Pacific Gas and Electric Co. (PG&E), San Diego Gas and Electric Co. (SDG&E) and Southern California Gas Co. (SoCalGas). Other data came from various sources, such as the CPUC energy division, the California Energy Commission (CEC), and California Energy Efficiency Statistics.

There now are 11.1 million residential gas customers served by the three IOUs, with SoCalGas serving more than half of them (6.1 million), or 55% spread over 660 different zip codes.

In 2015, 87%, or 9.5 million, gas utility customers, had average monthly gas bills of under $40, the study found. Less than 1% (0.5%), or 51,000 residential customers, have natural gas bills averaging more than $80/month in the residential sector.

"Similar to the electric bill pattern, examining gas bills by income levels [among residential customers] reveals that the average bills increase as the median  income level increases," the CPUC report said. "The difference in average bills becomes even higher in more affluent areas with income levels above $150,000 annually."

Twelve of 15 zip codes with the highest and lowest gas bills in winter and summer turned out to have the highest average gas bills and median income levels of more than $100,000. In contrast, 13 (winter) and 12 (summer) zip codes with the lowest average gas bills have median income levels under $50,000 annually.