I think about it this way:
In MPPT your system losses are mostly around the distance your solar controller input voltage is away from the instantaneous Vmpp of the panel, the efficiency of the DC-DC voltage conversion, and the voltage drop over the wires coming into the controller (P=IE).
In PWM you sacrifice voltage as part of the design, and the power you drop is the difference between Ppwm = ~Isc * Ebatt and Pmpp = Impp * Vmpp. The big voltage drop due to direct connecting to the battery usually subsumes any drop on the trip from the panels over the wires (except for near sunrise, near sunset, and possibly in lower light conditions). In PWM you are using the solar panels as a current source, and that current does not vary much with voltage drop most of the time (e.g. to the left of Vmpp on the solar current vs. voltage curve).
A typical example might be Vmpp of a panel is 18V, you have .5v drop over the wires. in MPPT that is lost power. In PWM since you are going to pull down the voltage on the panels below 17.5v to something between 10.5 and say 14.8v, the loss does not hurt you much at all. You are into the part of the IV solar panel curve where the amps produced are essentially Isc whether at 14.8v at the panel, or at 15.3 because of the .5v drop due to wiring. There *is* a small drop since the curve is not flat, but it does not usually add up to much unless Vmpp under current conditions gets close to Vbattery.
Jim
