MPPT are certainly a valuable addition when trying to achieve maximum solar output from the panels.
But no matter how smart the MPPT algorithm is, there are some certain limits to the achievable gains.
The problem starts with the lowish cell
temperature in the standard
test conditions (STC).
Most 12V panels have their max power voltage at around 17V, as measured under STC, meaning the cells are only 25 degree warm during the
test.
So while this is fine for chilly winter day conditions, on hot summer days, when the cell
temperature can reach 70 degrees, the max power point voltage drops with the cell
temperature coefficient of around -2.2mV/deg.
Your typical 12V panel consists of 36 cells, thus the max power point voltage would drop from 17 down to a not-so-exciting 13V.
Because MPPTs convert the over-voltage (panel max power voltage, minus
battery voltage) into useable solar amps, you can see that in summer, there isn't much to gain. Maybe 5 to 10% in average.
Double these gains are to be had on cool/cold winter days, when the cell
temperature is lower, and the max power voltage higher.
Take the 35% claim with a grain of salt though.
That's because if the
battery is sitting on 13V while being charged by the MPPT, the max power voltage is only 4V higher (on a cold winter day that is), so there is only a theoretical gain of 4/13, or 30%. Multiply this by the MPPT conversion efficiency of around 0.9, and the result is even less spectacular.
For a real world (warm to hot Australian conditions) I estimate the average gains by the MPPT are probably in the order of 15~20%.
That's like paying 15~20% less for your installed solar wattage, but forking over a bit more for the MPPT at the same time.
Best regards, Peter