IBM 5160 - 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 5160 at the wall socket.
Wattage - Breakdown
The IBM 5160's power supply is rated at approximately 130 Watts. Expressed that way, the wattage is misleading. For example, it does not mean that 130 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 5155/5160 Technical Reference: "When the supply is switched off for a minimum of 1.0 second, and then switched on, the 'power good' signal will be regenerated."
So, after turning off the power supply, wait at least 1 second 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 5160'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 5160 motherboard alone is normally enough of a load to start/operate the IBM 5160's power supply.
The Tandon full-height floppy drive alone is normally enough of a load to start/operate the IBM 5160's power supply.
Click here for one example.