And doing the math a bit.  i think that a standard bottle can do around 50 liters of water.  Which for some of us vanishes rather quickly.

And instead of just waving my hands in the air, here is a longer version ( and not trying to sell amazon products, its just for picture reference instead of trying to describe them ) 

• Old way - 2 pound paintball tank to stream adapter -> https://www.amazon.com/Conversion-Adapter-SodaStream-Paintball-Canister/dp/B0798DTRJ
• The "gauge" set i have for the 10 pound tank -> https://www.amazon.com/gp/product/B008EKR0UG        ( bought mine at a local home brew shop, way cheaper )
• Gage to plugin adapter -> https://www.amazon.com/PERA-assembly-liquid-disconnect-brewing/dp/B079LX9575
• Then plugin to PET bottles ( i use a 1 liter to make it easier to shake.. )  https://www.amazon.com/MRbrew-Stainless-Carbonation-Coupling-Pressure/dp/B0CSXWY571
• I have both a 5 and 10 pound tank, mostly just use the 10 now and the 5 is for backup when i cant get up to the store. Refill it at the local gas supply place in town for not too much. Can also get larger tanks, but that was plenty for me.  typical tank ->  https://www.amazon.com/Aluminum-Cylinder-Siphon-CGA320-Valve/dp/B00A7X4S44   ( tank style, but NOT that one.. did not pay anywhere near that at the local store )  

I started with the real thing, it was on sale ilke 2/3 off.  "sure why not, get back into experimenting with fizz stuff, its been a while". But after the first bottle of gas ran out in a blink of an eye, "Nope".  Got the paintball adapter, lasted 3x as long. and 4 bucks to refill and fit inside my stream so no extra space.  Then the local sporing goods place was getting to be hit-miss on stock, and then eventually stopped selling it totally ( no more guns! evil guns! ), so just tossed it in the garage and instead of going with a gauge -> stream adapter just went with the gauge -> PET bottle.  Originally was going to get a "filter stone", like you do with beer.  But, that meant taking up a lot of space in the fridge ( or a new one ) and turns out it takes forever.. so went with the soda jerk idea.. open the valve and just shake it for a few seconds, done.

Ages ago i had a large bottle thing, same sort of connectors and what a lot of beer people use today, was about 3' tall and had a trap door on top. Fill it with stuff, add dry ice, wait.. I had quickly outgrown using little co2 cartridges. Sort of lost interest over the years, but doing it the above way, with my home made flavors, to me is worth the effort. 

And, i didnt mean to imply above that sodastream should not make money, but not a fan of gouging customers like they do with the gas.