Phase measurement and correction in IoTaWatt

I think that’s only true when there are no harmonics present. Have a look at: Measuring reactive power to build NILM system - #7 by dBC

Your situation won’t be as bad as the one in that diagram because you’re using a purely resistive load, but you will have harmonics in your V (and I) due to the non-sinusoidal nature of Vgrid (flat-tops etc).