You need probably 14V to have some over voltage. So based on your specs look at having at least 14 V but not too much over that (tells you how many panels you need in series)

I would have blocking diodes (probably the solar cell has them built in) to insure the battery does not discharge thru the panel.

Here is a way to experimentally measure the output of a solar panel using several resistors: http://www.reuk.co.uk/Measuring-the-Power-of-A-Solar-Panel.htm By experimentally determining the power curve we can assign a wattage to the panel.

you can get a reference make solar panels here http://makesolarpanel.jhocom.com/

Assuming no losses in the system or components, if your meter does not have the capacity to measure 10 Amps, you will need to get one that does. Analogs are better than digitial for this.

Most often the 10 Amp circuit of the meter has a discrete positive plug location for the positive test lead. Most have a common ground connection with the rest of the meter functions. If you are measuring current at the battery in the charge mode- Negative lead of meter goes to positive terminal of battery and positive lead of meter goes to the cable that was connected to the positive terminal of the battery.

Since the Wattage is a nominal measurement, if you measure between the panel and controller, take measurement on one leg of the panel, as you want to measure the current with the loaded active circuit. Meter is oriented the same way as the measurement at the battery would be made above only this time the meter is between the panel and controller.

At peak solar conditions, expect current to be measured at 2.5 to3 Amps or more per panel. Under optimal conditions your panel will have higher output than their ratings indicate. Open circuit can be close to 20 volts. Never short circuit the output of the solar panels.

It is volts multiplied by amps that gives you Watts.