Once your PV system is granted PTO, you will get statements rather than bills from the utility. Each month will show how much electricity your PV system generated and how much electricity you have received from the utility. The difference will either show up as a credit or a balance.
You do not have to pay the balance that is being carried each month, though you may choose to, but the balance will carry over each month for a year until true-up. If your PV system is unable to cover all your electricity needs and you have been adding to your balance each month, that can mean a sizeable true-up.