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A little PV unit would not hurt and might get it topped up, which I also doubt happens in 30 mins unless starting is the only thing it is asked to do.
As for size, well lets say the bat is losing 1% a day (that’s high, it is likely less). One percent is 7.3amps over 24hr or about 0.3amp/hrs. In other words if a panel was to supply about 0.3 amps continuously it would overcome the self discharge. Of course a panel only works 12hrs a day (max) so you would want a panel that can supply at least 0.6 amps. It needs to supply that at a higher than fully charged battery voltage, say 17volts so a 17x0.6= 10watts, a 10 watt panel would be needed.
But there is no way a 10watt panel will actually give you a regular 10watts for 12hrs a day, day after day. It is very safe to double that to 20 watts and not have to worry about a regulator. Just check water regularly.
In other words and fewer words, yeah IMO that 20watt panel would do the trick, and on sunny weeks might even fully charge the battery.
Have faith that the oceans are going to rise and flood the world, that plague and pestilence brought on by Climate Change is going to punish us for not believing. Please do as they say it is our only hope. :P