Triggered by the discussion of DC-DC converters I decided to trough together a test setup and see what really happens.
The regulator module shown is a cheap chinese made unit using a LM2596 (possibly real, probably a clone).
These are cheap, readily available and very convenient.
I’ve use 6x 3R3 resistors in a 2x3 config to give 2.2Ohm load
I’ve set the output voltage to 5.3V to allow for plenty of headroom through a Pi.
Bench PSU providing 15.0V, 1.15A (17.2W)
Load: 2.2 Ohms
Result: Heatsink Temp: 91C
I then repeated with a more realistic load (2x2 3R3 resistors)
Bench PSU providing 15.0V, 0.75A (11.2W)
Load: 3.3 Ohms
Result: Heatsink Temp: 65C
Something to remember is that this reg isn’t in a sealed box in the full sun.
The conclusion is: don’t put you finger on the reg.