Dim bulb tester bulb wattage

It depends on the load, the purpose of an incandescent bulb is to limit the current flowing to the load and not to be higher than the operating current of the bulb.
To startup an amplifier, I use a bulb that is about 1.5 times higher than the idle current draw of the amp.
For example, the Accuphase P-4500 class AB power amplifier consumes 62W of idle power, so a 100W incandescent bulb would be sufficient.
For the Accuphase A-48 class A amplifier, its idle power consumption is 180W and requires a 250W lamp.