Catalyzes the phosphorylation of isopentenyl phosphate (IP) to isopentenyl diphosphate (IPP). Functions in an alternate mevalonate (MVA) pathway leading to IPP, a key precursor for the biosynthesis of isoprenoid compounds such as archaeal membrane lipids. ATP + isopentenyl phosphate = ADP + isopentenyl diphosphate kcat is 16.3 sec(-1). Homodimer. Belongs to the isopentenyl phosphate kinase family.