efficiency of high power toroids at low load?

This seems to be my question asking day...

High power rating on toroids is often recommended from a sonic point of view (higher stored energy, ...). One hard fact is that the higher the power, the higher the efficiency. This is usually stated at the nominal power.
Now if I use a high power toroid at 5-10% of its rated power, will I still draw less power than with a 3x smaller transformer that gets used at 15-30% of its rated power?

What if the power company does to measure the real power consumption if the inductive load gets too high?

Your question is very easy to answer:

You have no-load losses and you have losses at full power. It's wise to have a "lagom" as we say in Sweden, big transformer. Small toroids have never high efficiency because of effects of the insulation. Transformers above 100 VA are efficient but 15 VA aren't.

I recommend you to have a max load not less than half of the rating. There is no advantage to have a too big transformer!
Heavy smoothing

I forgot one important thing: If you have lots of smoothing caps you can only take out half the power rating. 150 W DC power needs a 300 VA tranformer! The caps generates lots of reactive power wich generates heat in the tranformer.

What is heavy smoothing? Surf into http://www.toroid.se. I can't find anything useful there but I have somewhere papers from them. You can always send them an email and ask.
Generally speaking, bigger transformers are better because they have better regulation, more stored energy and less internal resistance. Capacitor input power supplies have a power factor of about 0.62 (from memory i think that's right :D) so power factor is pretty lowsy so having a big transformer will help minimise further losses in the power supply. But be warned, if the transformer is a toroidal type then you must consider the high magnetising current that is drawn @ power on.
well inductance in a transformer is not always a bad thing.... usually is but not always... you just gotta exploit it 2 it's fullest advantage.... it has particular importance when designing 3 phase transformer power supplies.

Geoff posted the link 2 a really good pdf file about rectifiers and power supplies in a thread many months ago.... if you can find the link, i suggest you read the article ... BE WARNED the pdf is about 26MB from memory so it takes ages to download it

the magnetising current doesnt increase too much as you change to a larger transformer either.
My $.02

Magnetizing current depends on several things, but one of them certainly is what flux is set up. High flux means more VA out of a smaller core. Generalizing, low flux means less physical noise, heat at idle etc.

Audio amps are usually set up with relatively low flux and relatively inefficient core material. They will thus be physically larger than regular use transformers.

Efficiency at zero load is zero (think engine running, car standing still)
Efficiency at very high load will not be good
Efficiency will be at a maximum between the two extremes

Not all that useful I guess, but still possibly worth $.02?