Transformer energy conservation

If I have a 100VA transformer with say 0-50AC and 0-50 AV (i.e. dual) secondaries and I use just one of the secondaries (keeping the other unconnected), is ALL of the 100VA available in the 1 secondary or is just 50% of the 100VA available? I would have expected that all of the 100VA would be available due to conservation of energy.