the simplest thing to do is get yourself a few 25l containers and a Di vessel, some resin and a tds meter. then coonect the resin vessel to a tap and hey presto you have the pure water. you are better off getting the above as those residential cartridges can work out expensive. How much water you get from each fill of resin depends on many factors but for what you want to do it should be ok and not cost to much. You will get to know how much water is being produced by checking the waters tds. ) 0.00 is best but you should be able to use the resin till the tds is about 4 without problems. The only thing I would say is that you can pick up an RO system quite cheaply (i think it was you I costed one out for?) and I think that once you have cleaned the few windows that you are planning to use it for then you will see the benefits of WFP and will want to use it all the time. Unless you are in a soft water area (with low tds) you will get through resin like there's no tomorrow and will suddenley find you want an RO. What I am saying is if you decide to get the di vessel get something that can be used in conjunction with an RO in the future. That way you wont lose anything on the original investment.