Volt Amperes versus Wattage
The Quick Answer
If you are installing LED lights, use the manufacturer VA rating in the calculators for Load and choose a Transformer based on VA rating.
If you are installing non-LED lights, use the manufacturer Wattage rating in the calculators for the Load and choose a Transformer based on Wattage or VA rating.
The Long Answer
Non-LED systems seem less troublesome to install because in this case, VA is simply Volts times Amps which gives Watts or Power (also known as True Power).
But there is a bit of a twist in LED systems. LED lights are actually Direct Current (DC) and require what is known as a driver to convert household Alternating Current (AC) to DC the light can use. While drivers do not draw power, they do drop Voltage and draw Current. The loss is called Reactive Power and it must be taken into consideration to calculate the total load on the Transformer.
Reactive Power + True Power = VA or Volt Amperes (Total Power or Apparent Power). The Volt Amperes / Total Power / Apparent Power output of a Transformer is determined by the construction of the core and it does not change based on the load.
Some landscape lighting manufacturers publish both the Wattage and VA requirements for their lights, since the Wattage number is lower and therefore looks more efficient. However, designing an LED lighting system based on Wattages will result in under-calculating the Total Power necessary for the Transformer to supply. If an undersized Transformer is purchased, there will likely be many problems with the installation such as: flickering lights, dim lights, dead lights, tripped transformers etc.