current solar panels are based upon silicon cell technologies which combine the output of a number of cells
these are usually rated at around 80-350W when tested at Standard Test Conditions of 1000W/sqm incident sun, 25degC and AM1.5 air mass spectral conditions.
the larger the power rating, the larger the surface area needs to be to create that power output for a given panel efficiency
sellers (especially those on Ebay) may over-estimate the output of panels, test actual outputs or do the maths:
The standard test of efficiency and output is done with sunlight falling on the panel with an intensity of 1000 watts per square metre and 25degC. The efficiency of the panel will be in the specs, and should be around 17%. So the panel should be producing 1000 x 17 / 100 = 170 watts per square metre of its area. So to get the actual output of the panel just use the dimensions of the panel, calculate its area in square metres, and multiply that by 170.
traditional panels have glass fronted panels mounted in an aluminium frame making them bulky and heavy
more modern “soft fold” panels are much lighter, more compact and more convenient but considerably more expensive than the older aluminium framed ones
for max. efficiency place at correct angle to the sun (at midday, this angle should be your latitude, whereas in the morning or afternoon, the angle will need to be more than this), this may increase output of a 200W panel from 7A laying flat to 9A placed at a more optimum angle (11A/18V in perfect conditions), plus this allows better ventilation to keep them cooler and they wont kill the lawn as may occur if you lay them flat
older or cheaper panels may not have blocking diodes in each section and the output may become zero if even one section falls under shade of a tree
a portable panel is generally more useful when camping instead of a fixed roof panel
it allows you to park your vehicle in the shade and place your panels in the sun (you may need an Anderson plug extension cord for this)
it allows you to optimise the angle of the panel to the sun throughout the day
you can combine the output with a fixed roof panel output for added versatility
however, they are at higher risk of theft and damage and obviously you do need to set them up
you can combine two panels into one controller which should give the combined current
you also need a solar regulator to charge a battery and ability to choose the correct charge mode for certain batteries such as LiFePO4