If you got pure DC with only a negligible small AC part you may use Watt only. If you got AC or a mix of AC and DC, you should use both Watt for real power and volt-ampere for reactive and apparent power.
For a clean sine wave and a pure resistive load reactive power will be zero and apparent power will equal to real power.
For a load build with resistors, capacitors and inductors, the reactive power will be bigger than zero and apparent power will be bigger than real power. If the load is a water heater, only the real power will heat the water, the reactive power will heat only the cables from the heater to the grid. Of course heating the cables only is not for free, a power plant will burn more fuel to generate the wasted reactive power. A hydroelectric power plant will use more water.