What the rating on the box means
A salt chlorine generator is an electrolytic cell. Salted water passes between plates, current is applied, and chlorine is produced in the flow and returns to the pool. The cell is rated in pounds of chlorine equivalent produced per 24 hours at 100 percent output, and separately marketed as suitable for a pool of some size. Those are two different statements, and only the first is a measurement.
Four things stand between that rating and what the pool receives. The pump does not run 24 hours, and the cell produces nothing when there is no flow. The output setting is usually well below 100. Cold water reduces output, and many units cut production entirely below a set water temperature to protect the plates. And the cell wears: the coating on the plates is consumed over the life of the unit, so a cell in its fourth season produces less than the same cell in its first. This page multiplies all four together, which is the arithmetic most people do in their heads and get wrong.
Why cells are sold oversized on purpose
The common recommendation is to buy a cell rated for roughly twice the pool volume, and the reasoning is entirely about the four factors above. A cell rated for exactly your pool is rated for 24 hours of running at 100 percent in warm water when new, which is a condition your pool will never be in. Buying at 2x means ordinary summer operation sits near half output, which leaves headroom for a heat wave or a heavy weekend, and it means the cell spends its life working gently, which is the main thing that determines how long the plates last.
| Factor | Effect on production |
|---|---|
| Run hours | Direct and linear. Ten hours is 42 percent of the rating before anything else. |
| Output setting | Direct and linear on most units. |
| Water temperature | Falls as water cools; many cells stop entirely below a set point. |
| Cell age and scale | Gradual decline over the life of the plates, plus temporary loss when scale builds on them. |
| Salt level and flow | Below the manufacturer's window the unit derates or refuses to run at all. |
The interaction between run hours and output is the one worth understanding, because it is the only free lever. Doubling the run hours at half the output produces the same amount of chlorine while running the cell gentler and circulating the water more, and on a variable-speed pump the extra hours at low speed cost very little. The pump run time page works out what those hours are actually worth in energy.
Sun, load and the things a calculator cannot see
Demand appears on this page only as a number you enter, from your own records, and that is deliberate. What a pool consumes in a day depends on the sun it sits under, the bather load it carries, the debris that lands in it, the water temperature, and the stabiliser chemistry that determines how fast sunlight destroys free chlorine. None of that is visible to a web form, and a figure that looked right for one pool in one month would be wrong for the same pool in a different one.
If you want your own demand figure, it comes from your own testing: measure, note the reading, note it again after a known interval with the generator off or at a known steady output, and the difference across the volume is what the pool used. That is a real measurement of your pool. Anything else is a guess dressed up as a number.
What this page will not do
It will not tell you what level to hold, what to set the output to, or what to add. Every one of those decisions depends on tested water and on the specific equipment, and getting them wrong affects people who swim in the pool. The generator manufacturer's instructions govern the salt level, the flow requirement and the temperature cut-out for your particular unit, and the water balance side belongs to your own test results and, when it is not behaving, to someone who can look at the pool. The water replacement calculator covers the one case where the answer is water rather than product.
Bonding and grounding of pool equipment is licensed electrical work, and the reason is that water and people are in the circuit. No wiring procedure appears anywhere on this site. If a pump, heater, light or panel needs anything beyond a plug in a working receptacle, that is an electrician's job.
Questions people ask
What size salt cell do I need for a 20,000 gallon pool?
The usual practice is to buy a cell rated for around twice the pool volume, so a 40,000 gallon rated cell on a 20,000 gallon pool. The reason is that the rating is measured at 100 percent output over a full 24 hours with a new cell in warm water, and your pool will never present those conditions together. Running a 10 hour pump schedule already cuts the rating to 42 percent of the label before you touch the output dial. The oversized cell then runs at a modest output setting most of the time, which leaves headroom for hot weather and heavy use and puts less wear on the plates.
Why does my salt cell produce less in spring and autumn?
Cold water. Production from an electrolytic cell falls as water temperature drops, and most units include a low-temperature cut-out that stops production entirely below a set point, typically somewhere in the fifties Fahrenheit depending on the manufacturer. That is protective rather than a fault, because running the cell in cold water damages the plates. It is also why shoulder-season and winter pools are usually not run on the generator alone, and why turning the output to 100 in cold water accomplishes nothing except wear.
Does running the pump longer produce more chlorine?
Yes, directly and linearly, because the cell only produces while water is flowing through it. Ten hours produces twice what five hours does at the same output setting. This makes run hours and output setting substitutes for one another, and on a variable-speed pump the trade is very favourable: energy for a fixed volume of water goes with the square of pump speed, so twice the hours at half the speed costs a quarter of the energy while producing the same chlorine and circulating the pool more thoroughly. That combination is the main practical advantage of pairing a variable-speed pump with a generator.
How long does a salt cell last?
The plates carry a coating that is consumed in service, so a cell has a working life measured in hours of production rather than in calendar years. Manufacturers commonly quote several thousand hours, which becomes a number of seasons depending entirely on how hard you run it. A cell held at 80 percent output all summer wears far faster than an oversized cell sitting at 40. Scale on the plates reduces output temporarily and is a separate issue from wear, though scale that is left alone accelerates the wear. Follow the manufacturer instructions for cleaning intervals and method, because aggressive or over-frequent cleaning takes coating off.
Can this calculator tell me how much chlorine to add?
No, and it will not, because that is a dosing question and a web form cannot see what your water needs. What to add depends on your current tested balance, on the specific product and its concentration, on the water temperature and on what has already been added recently. Getting it wrong affects people in the water and can damage equipment. The safe procedure is always the same: test, make one adjustment, let it circulate fully, then test again before doing anything else. Never combine two pool products, and take the quantity from the label of the product in front of you or from a pool professional who can see your test results.