A measurement of energy equal to 4.359 743 81 x 10^{-18} J. I have no particular clue as to when one would use a Hartree Energy , but it exists. The symbol is E_{h}, and the formula to derive the unit is E_{h} = e^{2}/4(pi)(sigma)_{0}a_{0} = 2R_{(infinity)}hc = (alpha)^{2}m_{e}c^{2}.