I always like the lazy method and using this calculator was a tool I have found useful - how much power through the inverter. So I am looking at adding a Master Chef 1380 watts convection oven to my boat so what is my power usage.
The one negative of the calculator is it only deals in amps. So if you divide watts by 120 (north america) with the above scenario you get 11.5 amps, input that into the calculator and the answer I get is roughly 120 (actually slightly more but to0 lazy to get the exact answer), So I know with this convection oven through the inverters I'm using up 20 amps every ten minutes.
I am also looking at a cheap crappy toaster, you know those ones you can buy for $14. I like these cheapies because they are the lowest watts user. One that I will be purchasing is 750 watts, so again divide 750 by 120 = 6.5
Now using the calculator input 120 in the VAC box, then 6.5 in the "Amps AC" box, tab on 12 volts at the bottom of the box and you get an answer of = roughly 72 amps. So if I use this toaster for roughly 15 minutes in the moring, I will be consuming (12 amps per every ten minutes) 18 amps.