I’ve never worked with major enterprise or government systems where there’s aging mainframes — the type that get parodied for running COBOL. So, I’m completely ignorant, although fascinated. Are they power hogs? Are they wildly cheap to run? Are they even run as they were back in the day?
Mainframes are basically large rack mounted computers, and typically require many kW of power to run.
They’re still selling mainframes. A new IBM z16 takes 3-phase power and can use up to 30kW, or about 1,000 times a typical laptop.
I think that refers to present day mainframes, while OP is asking about the behemoths of the 1970s. Those were in tall bays (not sure what they were called), they used high voltage 400 cycle(?) power provided by on site motor generators, and they were water cooled. Planning an installation involved arranging the facilities for all of this. You didn’t just wheel them into an office and turn them on.
I just spent some minutes with web search and it is surprisingly hard to find power consumption figures. Two that I found were:
IBM 360/91, a high end scientific mainframe from 1965, used 74KW for 1.9 mips of cpu performance (https://arxiv.org/pdf/1601.07133). This was a big increase in power efficency compared to older machines that used vacuum tubes.
Cray-1 supercomputer from 1976 used 250KW, https://www.cpushack.com/2018/05/27/mainframes-and-supercomputers-from-the-beginning-till-today/
Beyond the CPU itself, you generally had a room full of periperhals such as disk drives (they looked like washing machines and a row of them looked like a laundromat), tape drives (old movies often depicted big computers as tape drives spinning back and forth), etc.
The 360 series had an “emergency stop” knob just in case. The one I saw had a sign next to it saying not to pull the knob unless the machine was literally on fire. It seemed that there was some kind of knife blade behind the knob, so when you pulled it, the blade would physically cut through a bundle of wires to make sure that power was disconnected from the machine. You couldn’t simply reset the emergency stop after someone pulled it. You had to call an IBM technician to replace the cable bundle that had been cut through.
The story about IBM technicians was that they always wore tie clips to hold down their neckties. That was to prevent the ties from getting caught in rotating machinery.
One thing I miss about the old IBM mainframes is the console keyboard had a little LED “Wait” light. If it was on you weren’t using much CPU. But if you got it to turn off or mostly off you were really working out the system.