Being a man who learned physics in college, I have tried to calculate that in the simplest way. As for example, if we need to know the power consumption to compress steam from 50C saturated pressure level to 1 bar i.e. 100C saturated pressure level, the best way is to know the gross enthalpy of both the levels first and then subtract the lesser from the bigger. As for example, enthalpy of of water at 50C is 209.0 J/g and the latent heat of vaporisation is 2381.4 J/g. Therefore, gross enthalpy of steam is 2590.4 J/g. While that of water at 100C is 419 J/g and 2256.3 J/g and that means 2675.3 J/g. Therefore, in ideal case the power necessary is (2675.3 - 2590.4) J/g or 84.9 J/g. With a 70% efficient compressor, it will go to around 121 J/g. But, recently a steam expert shows me how to calculate the power. It's the same as gas. But, where I have doubt is here. We all know that gas and vapour are basically different. We can only call a gaseous fluid a "gas" when its temperature is above the "critical temperature". Below the critical temperature, it's vapour. Therefore, how the laws of gas compression can be applied to steam (having critical temperature at 375C) at far below its critical temperature. But, as the opinion comes from an expert, I haven't gone into debate with him. Just want to put my doubts before the readers here.