I have a script that will charge it during the daylight, only in exterior locations, up to 6 shots per game hour, to a maximum charge of 100. It must be in your inventory. It will automatically 'load' the 6 charges per hour to the weapon.
If you drop the weapon, the solar energy will be removed from your inventory and will be stored with the weapon. So, if you pick up the weapon again, it will retain the amount of charges it had when you dropped it and add it to your inventory.
In the meantime, I am using the alien blaster. It sure would be neat to go with the 'real' solar scorcher from Fallout 2. Any takers?
http://fallout.wikia.com/wiki/Solar_Scorcher