IBM 5150 - Power Supply Unit (PSU)
Turning off the power supply's big red switch does not remove voltage from all components within the power supply.
With that information, some people may decide to also power off the 5150 at the wall socket.
Wattage - Breakdown
The IBM 5150's power supply is rated at approximately 63 Watts. Expressed that way, the wattage is misleading. For example, it does not mean that 63 Watts of power can be drawn from the +5V line if no power is drawn from the other lines. Each of the output voltage lines has its own power rating, that must not be exceeded. Click here to see those ratings.
Minimum Off Time
From the IBM 5150 Technical Reference: "When the power supply is turned on after it has been off for a minimum of 5 seconds, it generates a 'power good' signal that indicates there is adequate power for processing."
So, after turning off the power supply, wait at least 5 seconds if you are planning to turn it back on. Any shorter; the motherboard may not start up (due to lack of a POWER GOOD signal).
The IBM 5150's power supply requires a sufficient load in order to start/operate.
In some cases of insufficient loading, the power supply starts (you see the fan turn) but the power supply stops after a second or so. In other cases, the fan won't even turn. Note however that both of those symptoms can also be produced by the power supply being overloaded by something.
The IBM 5150 motherboard alone is normally enough of a load to start/operate the IBM 5150's power supply.
The Tandon full-height floppy drive alone is normally enough of a load to start/operate the IBM 5150's power supply.
63.5W versus 63W
From what I have seen, the 110V units are labelled "63.5W" and the 220V units are labelled "63W".
Click here for one example.
Modifying a 120V AC unit to 230V AC
Paul of the VCF has produced such a procedure.
Use it at your risk.
Note that IBM may have periodically used different 'guts' within the PSUs, and that the prodedure may be applicable to only some variations of 'guts'.
The procedure can be downloaded from here.