Solar MPPT charging circuit
MPPT algorithms work something like this.

If you don't take any current from the panel, the voltage is maximum, but the output power is zero.

If you take maximum current from the panel by short circuiting it, the output power is zero.

At any intermediate level of current, power is finite. 

At some specific intermediate level of current, the MPPT, power is maximum. At levels of current above and below the MPPT, the power is less. 

The actual level of current drawn to achieve MPPT depends on the present lighting level, which will change throughout the day and from second to second, so cannot be predetermined.

So the MPPT controller makes a guess of an initial current, and measures the output power. If the load is a relatively constant voltage like a battery being charged or an inverter into the grid, then it's simply a matter of measuring the current output into the load, no need to multiply Vin by Iin.

Now the MPPT increases Iin a little, and measures Iout. If it increases, it does it again. If it decreases, it drops Iin and tries again. Rinse and repeat all day.

To work like this, the MPPT controller must be able to dump any level of output current into its load. Batteries or the grid-tie connection must be sized to allow this given the size of the panels connected. If the panels are upgraded, then the load capability must be rechecked.