A 15 GPM-class system is a strong mainstream target for many four-bathroom homes; very large fixtures or frequent simultaneous use can justify 22 GPM.
15 GPM is the common middle tier
VIQUA Arros 15 is rated 15 GPM at 30 mJ/cm² and 12 GPM at 40 mJ/cm², giving useful residential flow headroom.
22 GPM for high-demand homes
Arros 22 raises the 30 mJ/cm² rating to 22 GPM and the 40 mJ/cm² rating to 16 GPM.
Bathroom count is only a shortcut
Four bathrooms with low-flow fixtures can need less than a luxury three-bathroom home with rain heads and body sprays.
Size to the dose you care about
Do not compare UV units by a single headline GPM without checking the validated dose condition attached to that rating.
How to size and install What Size UV Water System for 4 Bathrooms?
Equipment performance depends on matching capacity to the load rather than choosing the largest model. For pumps and heaters, that means required flow, pressure or temperature rise. For dehumidifiers and pool equipment, it means the environmental or hydraulic load. For UV treatment, it means validated flow at the required dose with appropriate upstream water quality.
Check electrical service, drainage, plumbing connections, clearance and ambient operating limits before purchase. A system can look ideal on a specification table but become expensive when the installation requires a new circuit, condensate pump, larger plumbing, structural changes or professional labor that was not included in the hardware price.
Operating cost, maintenance and failure points
Compare energy use and consumables over several years. Pumps, heaters, dehumidifiers and UV systems add electricity cost; pool and spa equipment may add filter or chemical maintenance; leak-detection devices may require batteries, valves or hub hardware. A lower purchase price can lose its advantage when operating cost or replacement parts are materially higher.
Look for serviceable components, warranty terms, replacement-part availability and the maintenance tasks the owner will actually perform. Equipment that depends on inaccessible proprietary parts or frequent neglected maintenance carries more long-term risk than a slightly more expensive system with clear service support.