N.H. Rev. Stat. Ann. § 79:4-a

The taxes which are not paid when due pursuant to RSA 79:3 shall bear interest at the rate of 18 percent per year computed from the due date. Interest and penalties on the tax shall be collected by the tax collector and deposited in the general fund of the town. In addition to the interest due, a penalty for failure to pay may be assessed against the owner as provided in RSA 21-J:33.