Hi,
They should specify "max. output current". This is what determines the heating of the winding.
In best case they have a diagram "I_out_RMS vs V_out" or "VA vs V_out"
kVA and VA is the same, but with the multiplier "k" which means "1000". This is the typical specification for a fixed output voltage transformer.
W makes no sense at all.
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Variacs come with true isolated output or not isolated (autotransformer wiring)
For safety reasons I recommend to choose the isolating one.
Klaus