To find out which array will charge the battery faster, you really need to know the current that the array is capable of supplying. If it is not listed on the back of the module, it can be found by dividing the wattage by the voltage.
Keep in mind the voltage may change while in operation.
There are many factors that can have an influence on PV performance. The same module might exhibit high voltage when cool and once its been sitting in the sun for a while, the voltage might drop to 18v. Also, the physical size of each cell may determine its current rating. It depends how the cells are series, and parallel connected inside each module, and how each module is series and paralleled in the array.
It also depends what type of charge controller you maybe using.
It also depends what type of battery, its size (Ah), how old the battery is, and the temperature of the battery.
Im assuming you are using HF panels? These are amorphous modules. They have slightly different temperature characteristics than most PV modules, but everything has its pros and cons.
The 18v module and the 27v could have the same wattage ratings (for all I know). The cells in the 18v module could be slightly larger (supplying a slightly higher current) than the cells in the 27v module. The 27v module then would just have smaller cells, but more of them. The 18v module might have 36 cells, and the 27v module would have 54 cells. So, since they could be the same wattage, its important to measure the current so that you can have a full understanding of whats going on. You could then use Ohms law to work it forwards and backwards. You just need two of the three variables.
Hope I at least half answered your un-answerable question. :-)
