The manuals for some or all of the First Watt models, which are available at the website, indicate power consumption in the list of specs. For example the SIT-2 (rated at 10 watts output per channel into 8 ohms) is indicated as consuming 200 watts. Since it is Class A, that power draw will be essentially constant regardless of the dynamics of the music.
So that amp will generate approximately as much heat as two 100 watt incandescent light bulbs.
Regards,
-- Al
