300W refers to the max load the PSU can handle. It will draw a little more from the inverter than your PC draws from the power supply.
Just pondering this as I know the PC won't use 300w, no-where near it but smaller PSU units seem to be more expensive (need a new one, brother burned out the old..) and I was wondering if you'd need a 300w rated inverter in order for the PSU to switch on or is this just the max load the PSU is designed to handle?
No, you need an inverter that will cover the computer itself plus a little extra for the inefficiency of the psu.
It will work if that's what you're asking...but if your PC use anything near the peak (meaning not at peak but come close like 260 ish) you PC might freeze, your HD might freeze,your PC might shutdown unexpectly, or your inverter would shut itself down.
I had lots of experience where my inverter shut down cause it can't handle the load my PSU wants. So best bet is to look for a 200-250 watt PSU. Micro-ATX cases had these small PSU and they're really not that expensive.
So I recommend to either get a 400 inverter or a 200-250 PSU. Either way is fairly cheap.
Good stuffs, was only thinking 300w because I could buy that for a fiver off ebuyer along with the remaining parts for my carPC hardware at the same time and save on shipping.
Thanks for clearing that up though, only running a Celeron 500, HDD and CD-Rom, doubt It'll go over even 150w!