The purchase price is the number most people compare first, but it is rarely the number that decides what a system costs you. Once it is installed, the running costs keep arriving, and over several years they can add up to more than the equipment did. Before you compare two quotes, ask each provider to put the ongoing costs in writing next to the install total.
Start with the parts that get used up
Every system has consumables. A softener uses salt and may eventually need resin service. A carbon or sediment filter uses cartridges or media on a schedule set by the design and by your water. A UV system needs its lamp and sleeve replaced on their own cycle. Ask each provider to name every consumable, the interval it runs on, and the cost of one replacement. Ask whether the interval is based on gallons, months, or a sensor, because that changes how often you will actually pay.
Check the water and power the system uses
Softeners and some other units regenerate on a cycle, and each cycle uses water and sends it down the drain. Ask for the water used per regeneration from the spec sheet, and ask how often the unit will regenerate given your household's use and your water's hardness. Then ask about electricity. Many units draw little power, but controls, pumps, and lamps add up over a year. Compare the figures on paper, not the sales description.
Count the service visits the warranty expects
Some warranties stay valid only if the system is serviced on a set schedule by the provider or a certified technician. Find out whether those visits are included, what each one costs when they are not, and whether you pay for the trip as well as the labor. A low equipment price paired with paid annual visits can cost more over the years than a higher price with visits included.
Put a running total next to the install total
Write the install price, the first-year consumables, the yearly service cost, and an estimate of salt, filter, and power use on separate lines. Then add a total for the first three years and a total for the period you expect to keep the system. Use the same assumptions for every provider so the numbers compare fairly. If a provider will not give you a running estimate, that tells you something too.
Questions to ask before you sign
Ask which consumables you must buy from the provider and whether you can buy the same parts elsewhere. Ask what happens to the warranty if you do. Ask who sets the replacement schedule and whether you can change it. Ask what the estimate assumes about household size and water use, and ask the provider to rerun it with your numbers.
For a side-by-side view of how we line up these figures, see our methodology page.