Your probably making this more complicated than necessary for estimating battery usage. Instead of trying to deal with watts convert everything to amps and then add up the various uses of amps for your total draw. For 12v devices just divide your watts by 12 to get amps and for 110 devices running off an inverter just divide watts by 10 (accounts for inverter efficiency)
You can use half of the 20 AH rating to get to 50%. So as an example using a 100 AH battery to power a 60 Watt LCD TV with an inverter. TV uses 6 amps/hour at 12volt DC (60/10) so you can run that TV approximately 8 hours (48 AHs)
