One of the beauties of IWI is that it can be done piecemeal as you do up each room, however the effect of what is already done is much less than the area covered, due to the heat that would have been lost through the treated bit of wall migrating to untreated areas. You also need to be extra careful about heating and ventilation near the edge of the insulation so as not to get condensation forming at the nearest points to the now colder section of outside wall. This is why an overlap is required with EWI, as the two insulation layers do not abut.
It is good that you are considering electrical work as that enables you to move all electrical fittings to internal walls.