To charge a battery we can't really expect to do it even with a step battery charger in less than 4 hours so 20 ÷ 4 = 5A so really 5A would be maximum size to recharge however at 5A too small to use a MPPT or PWM regulator so really looking at 8 hours i.e. the whole day to recharge so 3A is around the right rate so 3 x 12 = 36W allow a little extra so around 40W solar panel to recharge the battery over the day.
Without a MPPT or PWM regulator and step charger the charge rate will be low at about 10A (120W) you start to get PWM regulators however as to step charging there is a problem if you use and charge battery at the same time.
The idea with step charging is stage one you charge with the highest current that you can until the battery reaches around 14.8 volt at which point you change over to voltage regulation but monitor the amps. When the amps drop to a pre-set level for a 20AH around 0.5A then the third stage kicks in and it float charges as around 13.2 volt. Drawing on the battery unless done through the PWM unit will mean the battery will not drop down to 0.5A and will be over charged. There is normally a timer which kicks in should this happen but you do need to be able to top the battery up with water as over charging will get rid of water in the battery.
If you look at this PWM charger you will see how there is an output for lighting although I have tried to find out more details I have not as yet found out what the lighting output is. It could switch on only in dark or it could allow compensated use of the battery while charging likely the former. But unlikely to take the wattage you require.
Using a float charge of around 13.6 volt rather than a step charger it would not fox the charger but recharge time would be longer. Also the output from the solar panel would be reduced.
As well as the step charging the MPPT or PWM regulator changes the voltage at what the power is taken from the solar panel. In the morning and evening it steps up the voltage and at mid day it steps down the voltage keeping the voltage at the panel at a level where it can give maximum output. So without a MPPT or PWM regulator the panel will start and finish producing power later and earlier and will also not produce as much at midday.
I use a very small solar panel in my caravan to counter self discharge without any regulator but my battery is not sealed.
Using sealed batteries with step chargers is OK if you don't want to use power at same time as charging. There are systems with pulse charging that better monitor the batteries requirements Sterling make some very expensive systems for boats.
Oddly solar does work better than wind on the Falklands to keep electric fence batteries charged solar panels could be 1/5 of the size of wind chargers and only failed in snow.
There are loads of MPPT or PWM regulator chargers at 5, 10, 15, 20A and clearly panels and regulator must match or at least panel needs to be smaller than the regulator so 200W is about the largest panel you can use with cheap MPPT or PWM regulators after that the regulators get expensive. You can't use two regulators to charge the same battery and using batteries with diodes one does get a volt drop of 0.6 volt.
Best place to ask is on a boating forum as this is there bread and butter but unless like the boat user you have no real option of using another way to charge the battery then solar is not really the answer.
