The purchase price is the number everyone compares, but for many treatment systems the running costs over the following years decide whether the equipment was a good buy. Before you sign, ask each provider to split the ongoing costs into separate lines so you can compare them on the same basis.
Recurring costs to name
Ask about every item that consumes material or keeps the equipment working. For a softener, that usually means salt, the water used during regeneration, and the electricity that runs the control head. For a sediment or carbon system, it means cartridges and the schedule for changing them. For a UV system, it means the lamp and the quartz sleeve, along with the cleaning or replacement cycle. For a reverse osmosis unit, it means prefilters, the membrane, and the post filter. A service visit or maintenance plan may sit on top of any of these.
Ask how the softener estimate was built
A salt or regeneration estimate is only as good as its inputs. Ask which hardness figure the provider used, where that figure came from, and what household water use they assumed. Then ask how regeneration is triggered: on a fixed timer, or by measured use. A timer-driven setting can run its cycle on schedule whether or not you used much water, which changes both the salt bill and the water bill. Ask whether the unit can be set to regenerate by measured use, and whether the provider will set it that way at installation.
Filters, lamps and membranes
For cartridges and membranes, ask for the replacement interval the manufacturer lists and whether the provider's estimate matches it. If a provider quotes a shorter or longer interval, they should explain why in writing. Ask who buys the replacements, whether you can buy them elsewhere, and whether using a different brand affects the warranty. For UV lamps, ask whether the sleeve is cleaned or replaced on a schedule and who does that work.
Turn the list into a written number
Ask for a one-year running-cost estimate with each line item listed separately, including parts, labor, and any visit fees. Compare two or three providers on that page rather than on the headline price. If one provider's total is lower only because a line is missing, the gap will show up later as a surprise invoice.
What moves the number most
Your water test results, your household's daily water use, the type of equipment, and how the system is set up all shape the running cost. A system set for your actual use can cost less to run than one left on factory defaults. Ask the installer to explain the settings they chose and why.