Hi et al,
yep justin maths is good so far;) but unfortunately the items you are doing the maths on won't give you the answers you need
I e wattage on the inverter is irrelevant as it s just it's max rating. you may be only drawing 25% off that for a particular appliance.
Orange mentioned about the "power" the actual inverter uses and yes it does however, these days even the cheaper one don't tend use more than 10%, better quality ones are below 5%. The type of inverter is also important as there are many and it can seriously affect what you can safely run on them. This is due to the "type" as I will call it, (frequency and waveform) of power they produce.
If there is a "want" I can do an online" tutorial in the tutorials section, starting with the very basic s and building up over a period of time. Adding in Q & A s
just need someone with the knowledge to teach all of us who have just gone what??? 
