If your billing is estimated, PG&E works hard to estimate your usage as accurately as possible. PG&E bases the estimated bills on your energy usage for the same period within the last 12 months. If your energy usage from the same period last year is higher than your usage this year, then the estimated amount could be higher. If your usage from last year is lower, then the estimated amount could be lower.
If your bill is estimated, keep payments on schedule to avoid a future large bill covering several months. If the estimated amount is higher than your actual usage, you will receive an adjustment on the amount overpaid as a credit, and it will go toward your next bill(s). If the amount estimated is lower than your usage, any additional amount owed will appear on your adjusted statement.