Amazon has Renogy, HQST, Newpowa and other 100 watt panels for $101 or less.
If your solar controller is a PWM type you need to wire the new panel in parallel with the existing one. This will add the current of the two panels together which is usually about 7 amps each. You need at least a 15 amp rated controller and as Bob says, 20 would be better.
If your solar controller is MPPT (doubtful) you can wire the new one in series with the old one and not increase the amperage. You will increase the voltage, but most MPPT controllers can handle the increased voltage which can be as much as 42 volts. Do not do this with a PWM controller.
David
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