Ideal switch is a switch that not consume or dissipate any power from its sources. The ideal switch is represent in Figure 1.
Figure 1. Ideal switch
When switch is closed, the current flow trough the switch is equal to the current absorb by the load. But, the voltage across the switch terminal is equal to zero. From here, we can write:
Isw (closed) = Iload
Vsw(closed) = 0
So that, the power absorb by the switch while its closed:
Pclosed = Vsw(closed) . Isw (closed) = 0
Then what happen if the switch is open? While switch is open, there is no current flow to the load, so the voltage across the switch is equal to the voltage source. Thus, we can write:
Isw (open) = 0
Vsw(open) = Vsource
So that, the power absorb by the switch while its closed:
Popen = Vsw(open) . Isw (open) = 0
From all equations above, we get:
Plosses = Pclosed + Popen = 0 + 0 = 0
So, as mention above, the ideal switches don't have a power losses.