The oldest machine I own which theoretically works (I've never plugged it in, because I don't want to start a fire) is my Altair 8800.
It has a linear power supply with huge caps (one is about the size of a small soda can) - which is why I've never powered it up, because a fire could be the literal result (the whole system needs a complete refurbishment to get it into operating condition).
Even so, I doubt it consumes more than a 100 or so watts, though a chunk of that would be dumped as heat of course. The entire system is probably far under the general TDP of today's desktop systems (maybe just the GPU alone).
