If your tax bill qualifies and if you choose to pay your taxes in installments, then you must pay each installment by the due date or the Total amount of your remaining unpaid taxes, special assessments, special charges and special taxes (if any) will be delinquent. (s. 74.11(7) or 74.12 & 74.85, Wis. Stats.)
All delinquent taxes are subject to interest of 1% per month (fraction of a month counts as a whole month) from February 1 until paid. (s. 74.47), and an additional 0.5% per month penalty.