Solar Modules carry approx 0.5v each so a 36 cell produces 18V which is perfect for charging 12v batteries using a solar regulator (either PWM for cost efficiency or MPPT for yield efficiency).
A 60 cell panel was originally designed for use in residential installs where higher voltages are used to string series arrays together. They are typically connected to the grid and use MPPT to convert the varying energy throughout the day into a usable power on the grid. When used in portable applications an MPPT controller can convert the 30-40 volts output by a 60 cell panel into a 12 or 24 volt battery bank.
More recently 72 cell solar panels were used when solar was used to charge 24 volt systems like in off-grid. It was previously typically done by connecting two 36 cell 12v panels in series. Later panel manufacturers series connected 72 cells to produce the same voltage output which is actually closer to 38 volt open circuit than it is the 24 volt often referred to within the mobile industry. These panels are best used in high voltage strings in commercial applications up to 1000VDC because of their size and weight. But when space is tight on a roof and there's only room for a few panels in an off-grid system you can utilise the large frame size and high power output using MPPT controllers to bring the voltage down to 12, 24 or 48 volts. These panels have been up-sized from their humble beginnings of 200W right up to 365-400W today with larger wafer sizes, they are almost 2 metres by 1 metre in size. (Sept 2019).
Cells numbers when connected in series gain higher DC output voltages and those voltages are best matched to the solar controller's maximum DC input. Do not exceed the max input voltages or input max current.
More recently half cells have been produced to help negate shading reduction. A 120 cell is very similar size and power output to a 60 cell but has cells half the size of the 60 (thus twice as many).
