hi smhn72
your battery is rated at 12v 40ah, so if u use a constant current charger to charge your battery , u need to give battery (AH/10)amp i.e. 40/10= 4ampere, now u also have to apply 14.1 volts to battery terminal so your power required to charge the battery is 14.1*4 = 56.4watts. and also u hve to run a fan simultaneously of 12v 2amp so power consumed by the fan is 12*2= 24watts, so total power is 24+56.4=80.4 watts , so u can buy a pannel greater than 80.4watts .