Lead Acid battery trickle charger sizing
The simplest way to not waste power is to lower the voltage you start with to just about the &quot;float voltage&quot; of the lead acid battery.
If you can get an adjustable switcher, that would be a good place to start.
At the float voltage a LA will take forever and a day to charge back up so you want to raise the voltage when the battery has been discharged.  This is usually about up to 15V.