Depending on what the SWS-200 solar still is compared against, it will have different payback times. Payback time is defined as the period of time that it takes to spend on some alternative source of water the price of the SWS-200 still. The significance of the payback time is that if you purchase a SWS-200, then once this time period has passed, water obtained from the still from that point forward is effectively “free”. All calculations assume a drinking water intake of 1.5 quarts/day, and a round price of $500 for the SWS-200 still.
Bottled water at the grocery store is quite pricey, about $0.50/quart. At a drinking rate of 1.5 quarts/day ($0.75/day), it would take 1.8 years to spend $500, the approximate cost of the model SWS-200 solar still. 1.8 years, therefore, is the payback time of the still in this situation. After 1.8 years, the cost of the still will have been re-couped, and any water generated thereafter by the still is effectively free.
On the other hand, if you buy distilled water at a water store outlet, the cost is about $0.85/gal. If we assume that the water store is a 15 mile roundtrip drive, then gas cost would be $1.80 (30 mpg, gas $3.60/gal). If each trip is for 15 gal of water, then that increases the water cost to $0.97/gal. At this price for purchased water, it would take about 3.8 years to spend $500. This is the payback time of the SWS-200 still if compared against water store carboy purchases. Note, though, that this payback time does not include the time cost of driving to the store and back. If the trip takes 1/2 hour, and your time is valued at $20/hour, then that adds $10 to the cost of the 15 gal water, reducing the payback time to 2.2 years.
A counter-top electric water distiller typically costs about $400, and is able to produce 6 gal/day running at 800 Watts. This translates to 1.2 kW-hr/day for 1.5 quarts/day of water. If electricity costs $0.15/kW-hr, then an electric distiller costs $400 plus $66/year in electricity. Thus it would take about 1.5 years to spend $500 for water if you purchased an electric distiller. If you purchase the SWS-200 instead, then after 1.5 years you will have recouped this cost, and of course will have no electricity costs. To be fair, the initial cost of the electric distiller is a one-time cost which should perhaps be amortized over the life of the unit, say 25 years. In that case, the yearly cost would be $400/25 + $66 = $82/year. But, the SWS-200, viewed in this manner, would have a yearly cost of only $500/25 = $20/year.
Reverse osmosis units typically cost about $350. The drinking water that they generate is not as pure as distilled water, but is of course an improvement over straight tap water. R.O. units are rather wasteful of water, though, requiring about 4-5 times as much source water than what is generated. Since these units are located inside, this waste water typically goes down the drain rather than being re-purposed (to water trees, for example). Water is not expensive (yet), but nevertheless this is an undesirable feature of these systems. R.O. units also require several different kinds of filters that need to be regularly replaced, which gets expensive. If we assume such filter costs are $45/year, then it would take a little over 3 years to spend $500. To be fair, the wick of the SWS-200 will occasionally need to be replaced, but much less frequently than the filters of an R.O. unit.