Estimation of hysteretic energy demand is the first significant step in energy-based seismic design of structures. The present paper extends a modal pushover analysis (MPA)-based energy demand estimation method to include the effect of P- in structures. Efficiency of the extended procedure is tested on three standard steel moment resisting frames by comparing estimates based on this method with results from nonlinear dynamic analyses of MDOF systems for several earthquakes. In addition, three non-standard frames, with artificially increased susceptibility to P- effects, are also considered. Bias statistics are presented that show the effectiveness of the previously proposed method including the effects of P- for both the standard and the non-standard designs. The effect of P- on the actual hysteretic energy demand on a structure is also studied. The MPA-based method including P- effects remains less demanding on computation and also suitable for adopting in design guidelines.