We use a 100 watt panel and it keeps our batteries charged most of the time, obviously dependent on sun availability. Point being that 90 watts will work, but be careful with power usage.
One thing that people tend to forget is that batteries don't have to be recharged everyday to 100%. For example, if you draw down to 70% overnight and can recharge up to 90% the next day, then down again to 65% and recharge the next day to 85%, next day is sunny and you go back to 100%, etc, etc. You can still go many days and have a usable capacity over 50%(for 6 volt golf cart batteries) or the minimum that you feel comfortable with.
Solar is not fast, but it is almost always available in sufficient supply to keep you going, even if not 100%.
Vann