Idle power consumption

Most Class-D modules in the "hundreds of watts" power range draw similar power with no signal. If you climb in output power (hence supply voltage) this usually gets significant, specially when you want several channels in the same enclosure (hot, hot!)
Our sonora modules have low power consumption. For the S250 (250w/4ohm) it is 5W approx, good but nothing too special. But the S750 (750W/4ohm) also draws around 5W, three times less than some of our competitors.
Note that we are talking about no signal power consumtion, no "stand-by" power consumption (we go to half of that numbers in this case)
 
So for a given module is it correct that idle consumption increases poroportionally to the square of the rail voltage?

No. It would entirely depend on the construction of the module. Many modules have no input sensing that completely shuts down the output when no input is detected.

Many low power modules have idle consumption in the sub 1W domain even without such shutdown.
 
7W seems about normal.

I have measured a few I have using a steplight power meter:

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(and previously a less accurate unit).

hifimediy T2 measures 7.5W idle
connexelectronics TA3020v3c measures about 13W

YuanJing TDA8920 measured 5W idle, but the Connexelctronics TDA8920 measured 0W idle on the older less accurate unit I had... which is probably actually 2 or 3W.

All of these using linear power supplies.