Here is how solar panels and batteries work with driveway gate openers (for example: MIGHTY MULE MM260 ) in regards to determining if you need more of one or the other or both:
The gate opener’s battery is the storage capacity for the power captured by the solar panel. The solar panel wattage is the speed in which the battery will be recharged. The gate opener runs off the battery and the solar panel is meant to bring the battery back up to full charge each day. So you need to make sure that you have enough battery to handle your gate opener’s power requirements. If the gate opener seems to run fine off the battery but after a month or so the battery goes dead that would mean you have a charging problem. That would mean that the battery is large enough to hold more power for the gate opener but the solar panel is refilling it too slowly. You can add another solar panel to make sure the battery becomes filled by the end of the day.
If your panel always brings the battery to full charge before the end of the day but at night or a single cloudy day makes the battery go dead this means your battery is too small. You would need to first get a larger battery and then monitor the charging. If the battery if becoming fully charged before the end of the day this means your solar panel would be able to further supply a battery more power if the battery was large enough to store it. However the solar panel’s power is being wasted because the battery is full. In this case you would need a bigger battery still if you wanted to capture 100% of the power the solar panel is providing.
So to sum up. The Wattage of the gate opener solar panel is the rate at which the battery will charge. The size of the battery is the storage capacity for powering the gate opener and capturing all of the solar panel’s output.