I want to plug in a 300 watt inverter into the vans Hab system. If I then use it to charge a MacbookPro laptop which uses a 60 watt charger plug. Allowing for losses, what am loading the system with ?? The socket runs from a 10 amp fused circuit.. I will be most grateful if one of you techno electrical wizards could help ....PS. I think the laptop charger works at between 18 and 20 volts.