Transformer energy conservation

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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.
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In short no, the dcr of that winding will result in excessive heating if you attempt to draw the full rated power from a single secondary. You can place the two windings if identical in parallel.

Note also that there are several loss mechanisms in transformers including excitation current (the current required solely to magnetize the core), core and copper losses so not all of the energy put in will be available at the output.
to play safe that is true. in reality you can extract more from one secondary assuming the other is.unloaded.

theoretically up to 1,41*50VA becase then total.powerloss in secondary equals the power loss in 2 secondaries at 50VA.

in practice it is the temperature rise that limits this power.
If the two secondaries are essentially wound in the same place, so they share heat dissipation mechanisms, then you can almost get 41% extra current from one if the other is unused - the one you use generates as much heat as the two would normally do.

It is unlikely to be that good, and worst case would be the two secondaries being wound side by side so they share almost no heat dissipation. Then you can take no extra current.
well, you would have considerably less power loss in the primary if limitied to only one secondary at 100% load , so something more I believe you can pull.

Total temperature increase is proportional to the total heatloss. And primary is typically wound innermost, and thus suffer the most heat increase (winding cross area is proportional to current drawn so that power loss is shared fairly equal between primary and secondary, to save copper)

Secondaries of a toroid is almost always wound bifilar and around the whole circumference , I have never seen any other vareity.
(reduces Lleak and magnetic leakage ) therefore it should give roughly equal cooling capacity, but 41% (1,41*50 = 70 VA) is probably not realistic.

But again, that said to play it safe- one winding and limit power to halv VA of the transformer.
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