The 30-year-old former Chelsea forward, enjoying a fine second season at the Berlin club, took his tally in the league to seven goals.
Kalou put the finishing touches to quick breaks by the visitors in the 33rd and 60th minutes, only to see Hiroshi Kiyotake pull one back with a 70th-minute penalty.
Ivorian striker Kalou then converted a spot kick in the 87th minute to secure Hertha's second consecutive away win as they moved on to 20 points, ahead of Schalke 04 on goal difference and one point behind third-placed VfL Wolfsburg.
Hanover are 14th on 11 points with the pressure mounting on coach Michael Frontzeck.
Leaders Bayern Munich, on 31 points, face VfB Stuttgart on Saturday and a win would see them open an eight-point gap over second-placed Borussia Dortmund who are in action on Sunday against Ruhr valley rivals Schalke.
(Reporting by Karolos Grohmann; Editing by Tony Jimenez)
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