One other omission from the analysis is there may be a fair amount of waste when you buy large bottles of soda water from the store. Economically it would make sense to buy 2L bottle, but I may not be drinking it fast enough to warrant that. You lose a large amount of fizziness after day one, and by day 3, you just dump out anything that is left. With a Soda making device, you always have fresh fizzy water on demand.