A home soda maker is the most useful piece of kit in an alcohol-free bar. Cold sparkling water is the base of a spritz, a shrub, a lime-and-mint cooler or a tall glass of nothing at all, and having it on tap means you never run out an hour before guests arrive. But is it cheaper than carrying crates of bottles home? Sometimes, and it depends on three numbers you can find in a minute: what you pay for bottled sparkling water now, what a gas cylinder costs you, and how much you drink.
The idea is simple. You fill a bottle with cold tap water, press a button and a small steel cylinder of carbon dioxide (CO2) pushes gas into it. The machine costs a one-off amount, the cylinder is the running cost, and the water is close to free. So the question is how many litres it takes before the machine has paid for itself.
The most common cylinder holds enough gas for "up to 60 litres" of sparkling water, according to SodaStream's own figures. "Up to" matters. Each press of the button releases gas, and the harder you carbonate, the sooner it runs out. Short, firm presses give a gentler fizz and stretch the cylinder; three long presses for every bottle will empty it faster. For a sensible guess, use around 50 litres per cylinder if you like it very fizzy and 60 if you like it light.
Cylinders are usually exchanged, not refilled by you: you hand back an empty one and pay for the gas in a full one, at a shop or by post, depending on the brand and the country. Some brands use a screw-in cylinder and some use a push-in "Quick Connect" type, and they are not interchangeable, so check which kind a machine takes and who will swap it near you before you buy.
The machine's price matters once. The cylinder price matters every week.
Try the sliders. Drinking 5 litres a week, even a cheap machine takes the best part of a year or more to catch up with discounted supermarket bottles. At 20 litres a week, a household of sparkling-water drinkers, it takes a few months. The gas cost per litre barely changes, so heavy users gain most. The break-even only works if you really would have bought the bottles: if you now drink tap water, a soda maker is a treat, not a saving.
There are costs the calculator leaves out. Machines need a place on the counter, bottles wear out and need replacing after a couple of years, and the cylinder swap sometimes costs extra to post. Against that, you carry nothing, recycle fewer bottles, and can make exactly the fizz you want. A light fizz is lovely in tea and fruit juice, a hard one suits a lime spritz.
Sparkling water at home is not only for drinking plain. It is the lengthener in most low and no drinks: two parts water to one part shrub, cordial or cold-brew tea. It also keeps a drink light when you want something long and unhurried instead of another round. Because you control the fizz, you can make a drink more or less lively than any bottle on the shelf. Chill the water first, because cold water holds gas much better than warm, a point covered in why cold water makes better fizz.
Two safety notes. Only carbonate plain water in the bottle the maker came with: syrups, juice and tea foam up and can spray, so add them to the glass afterwards. And keep cylinders upright and away from heat, as the maker's manual says.
If you drink or mix with a lot of sparkling water, a soda maker is a sound buy. If you use a bottle a week, it is a nice gadget, and that is fine too, as long as you know which one you are buying.