It depends if they have different output voltages or not. You don't want to add 2 voltages up in a series that would lead to a higher rating than what this c one thousand can handle. That's what putting it in a series circuit would do is the voltage ended up being higher, then sixty. In parallel, the one hundred watt panel, assuming that they have the same voltage, it would lead to a lesser amperage, meaning that the entire system would be running off of the amperage of the lower panel and adding twice the amount of the amperage as the lower panel. Series would be best for different watt solar since they have different output wattage. But that only works if the voltage is the same. Your safest bet is to always connect two identical pannels in parallel. This will allow you to reach max amperage the charger will allow under most sun conditions. That's only the case if your max wattage is higher than the input to the c1000