You should note that this circuit has no kind of protection like low-battery protection, overload protection, short-circuit protection, etc.
With 500W power, you're going to need a HUGE heatsink to dissipate the power that the single MOSFETs will dissipate. Conduction loss in MOSFETs when driven completely on = (500/12)²*0.018 = 31.25W. It's better you use multiple MOSFETs in parallel. For that you need greater drive current that the PIC can't provide. So, you need a MOSFET driver, either based on one or more ICs or discrete driver made with transistors.
Hope this helps.
Tahmid.