It really depends on how quickly you want the charge to happen. If you live in a very sunny area, then you can hook up a more powerful solar charger for the day, come back and find that the battery is topped up. A lower powered one may take several days of trickle charging to get the battery to 100%. It really just comes down to speed and monitoring at the end of the day. If you want to plug it in and forget about it for a few days, go lower powered. If you want to blast the battery for a few hours whilst you work on the car, then we would suggest you go for more power.