In electricians terms, consider a incandescant, vs, Flourescent, vs LED (not a good analogy, but it is the best I can do on short notice and no sleep for 2 days.
Incandescent gives instant light, great color, no flicker and almost no complaints or problems (except for the short bulb life) To do this, it is very inefficient and uses a massive amount of electricity since so much energy is pure waste. (softener set to 15 punds per cubic foot)
Flourescent, some flicker, so so color rendering, a little slow on the start, but much more efficient (softener set at 8 pounds per cubic foot). Considered efficient enough for almost any application, and it strikes a good balance of energy savings, light quality, total operational cost, etc.
LED, harsh white light, bad color rendering, hates electronic controls and systems, but insanely efficient. (softener set at 4 pounds per cubic foot) Most people like LED, some do not and want the Flourescents back.
In the simplest terms, running a softener with a lot of salt will make it very inneficient, 16 pounds will give you 30,000+ grain removal capacity. If you have 25 grains hard water, your system will be able to soften 1200 gallons
Cut the salt in half, you will now get 24,000 grains removal, you will now get 960 gallons of softened water, as you can see, this is a massive efficiency gain, half the salt, 80% of the capacity.
Cut the salt in half again, 4 pounds per cubic foot, you will now get 640 gallons of soft water, you now get over 50% of the original capacity for 1/4 the salt, simple math, huge gains in salt efficiency.
Nothing is free though, to gain this massive amount of salt efficiency, you lose some water efficiency. Water is usually cheap and plentiful, and the amount a modern softener uses is minimal and its water efficiency is rarely an issue. Many softeners can be adjusted for better water efficiency, and when done carefully, their is little if any detrmiental affect to the system.
You will also lose water quality at very low salt settings. Most people are happy with the water quality of a softener set at 4 pounds of salt, but some are not and a setting of 6-8 pounds will usually produce near perfect soft water that 99.9 of people are happy with.
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