A starting point, not a set-and-forget: run it a week, test free chlorine, nudge the % up or down 10 at a time. Keep stabilizer in range — an SWG without CYA is pouring chlorine into the sun.
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The SWG output math
daily need (lbs) = loss ppm × gallons × 8.34 ÷ 1,000,000 output % = need ÷ (cell rating × pump hours ÷ 24) × 100
The trap in every SWG's rating: "1.4 lbs per day" assumes the pump runs 24 hours. At 8 pump hours the same cell makes a third of that, which is why the % that "should" work keeps coming up short. This calculator does the pump-hours correction that the spec sheet leaves as an exercise.
A worked example
A 15,000-gallon pool losing 3 ppm a day needs about 0.38 lbs of chlorine daily. A medium (1.0 lb/day) cell running 8 pump hours can make 0.33 lbs at 100% — it can't keep up. The fixes, cheapest first: stretch pump hours to 10 (now 0.42 lbs available → run at ~90%), cover the pool to cut the loss, or upsize the cell next replacement. The oversize-by-2× advice exists precisely so cells can cruise at 50% instead of redlining.
Keeping the cell happy
Cruising percentage beats redline: cells scale less and last longer under 80%. Check salt monthly (the salt calculator doses top-ups), keep CYA in the SWG's preferred 60–80 range (the CYA calculator explains why higher-than-normal is right for salt pools), and when output fades for good, the replacement calculator does the repair-vs-replace money math.
Frequently asked questions
What percentage should I set my salt water generator to?
There's no universal number — the right % depends on your pool's daily chlorine loss, your cell's size, and how long the pump runs. The math: your pool needs a certain amount of chlorine per day; the cell makes its rated amount only during pump hours. This calculator solves for the % — most pools land between 20% and 80%.
How much chlorine does my pool lose per day?
Summer sun typically burns 2–4 ppm of free chlorine daily (with stabilizer at proper levels — without it, far more). Measure yours: test in the evening, test the next evening before the SWG has run much, and the difference is your daily loss. That number makes this calculator exact instead of typical.
Should I run the SWG at a higher % for fewer hours, or lower % for more hours?
Total production is what matters — 50% for 8 hours equals 100% for 4. Longer pump runs at lower % win on water quality (more circulation, more filtering, gentler on the cell). If you're forced to 100% just to keep up, the fix is more pump hours, not more percent.
Why is my salt cell not keeping up even at 100%?
In rough order of likelihood: pump hours too short for the cell's size, stabilizer (CYA) too low so the sun eats chlorine faster than the cell makes it, a scaled or aging cell (inspect and clean per the manual), salt level below the cell's happy range, or water warm enough that demand simply outruns an undersized cell. Cells also lose capacity with age — 3–7 seasons is a typical life.