Rule of thumb power consumption of Class A amplifiers?

Hi,

I have a pair of Sheng-Ya (Vincent) monoblocks that the manufacturer claims are 100W Class A thence switching to Class A/B to ~350W at 8Ω.

I hooked up an el-cheapo power meter this morning, and they measure 600W at idle, dropping somewhat as they warmed up. Is there a rule of thumb for power consumption vs Class A output? I am just curious whether the 100W claim is accurate. FWIW the heatsink area is large, 4 heatsinks each about 500mm by 100mm by 160mm, and the amps do run hot.

Regards,

Phil