If connected in series, then shading one panel will bring the output of both down as you said, if connected in parallel, shading one will not affect the output of the other.
Yes, I just looked and I could replace with 2x35w panels in the same space.
For what it's worth, I have one 50W panel rigged at the moment and we've not run out of power for our portable compressor fridge.
If you have a solar controller that prevents discharge at night (nearly all models do this) installing blocking diodes, especially on lower voltage panels, is likely to be counterproductive. In other words it will result in less total output. This applies even in situations with multiple panels in shaded conditions.
Blocking diode merely reduce received voltage at battery by 0.7V. As output on my 100w panel is 40v open circuit it wouldnt make much difference as at battery its about 13.5 in sun ie loses 25v under load anyway
This 25 V should not be "lost". If you have a MPPT controller (and you should have this type of controller with a Voc of 40 V) the higher input voltage from the solar panel will be converted to more current at the lower battery voltage.
That's exactly how mine are set up with an mppt controller in parallel, I also messed about moving the boom but I think with just 20w each never made much difference.. So glad I never upgraded to the planned single panel underneath the boom, as only just realised by luck I had it right by having 2 panels there instead of one big one.
The controllers contain blocking diodes - else the battery and any other panels would try and send current into the shady panel. Its simple electronics. There is voltage loss and heat dissipation across all electronic devices. One could use germanium diodes not silicon to reduce the voltdrop but as anyone who ever built amplifiers knows thermal overload kills germanium
MPPT solar controllers typically do not use simple blocking diodes. They are too inefficient. The controller electronically detects when the solar panel array current has dropped below the self consumption of the controller, or when the solar panel voltage is close to the battery voltage. The popular Victron controllers disconnect the solar panels when the solar panel voltage drops to less than 1 V above the battery voltage.
Yes with 40v panels and 12v system you do need an MPPT controller. Though 40v panel will work with direct connection or PWM controller but with huge losses of power. Typical 40v domestic panel will produce around 5 amps into a short circuit or a 12v battery. However an MPPT controller can get this to near double current by switch mode transforming.
