Heating your home with natural gas costs a little less and electricity costs a little more under a set of rate changes that took effect Tuesday for residential customers of Pacific Gas and Electric Co.

Don't get used to it, though: Both rates are expected to go up soon.

PG&E said lower wholesale prices are to thank for the 5.3 percent reduction in its average residential gas rates between the end of December and the start of January.

Meanwhile, the San Francisco-based utility said higher renewable energy costs and state-approved system upgrades are behind a 2.5 percent increase in average residential electric rates over the same period.

This means the January gas bill for an average PG&E residential customer in Bakersfield will be $4.96 lower than it was in January 2011, while that customer's January electric bill will be $2.31 higher, the company said Thursday.

But rates rarely stay the same for long. As soon as next month, PG&E's residential gas rates are set to rise 2 percent to pay for a "pipeline safety enhancement plan" approved by the California Public Utilities Commission.

Then, in May, PG&E's residential electric rates will likely increase by 2 percent as well, the company said this week in a news release. That adjustment would cover infrastructure improvements needed to modernize California's power grid and deliver more renewable energy, it said.

"We know our customers care more than ever about their energy bills during these difficult economic times, so we continue to focus on keeping rate increases as modest as possible while raising enough revenue to continue improving our safety and reliability," Tom Bottorff, PG&E's senior vice president of regulatory affairs, said in the news release.

"These revenues help us serve customers by reducing the frequency of electrical outages, improving the responsiveness of our call centers, providing more convenient services and, above all, continuing to upgrade the safety of our gas and electric operations."