So I'm assuming since your only power is the running water, that it flows well year round? I'm also guessing it's pretty cold. Might look into a water cooled unit, and use a loop off the stream. The real advantage here is that it would be condensed with colder water (may have to have a tempering valve on it to recirc warm return back to the condenser to keep it from getting TOO low). The colder condenser would equate to a more efficient refrigerator, meaning less power draw. I do agree with the above statement about dual smaller units. The redundancy alone would be worth it, especially with multiple households depending on the storage. If you are calculating 6500-7000 BTU/hr, I'm thinking 2 5,000-6,000 ish units would work well, not have excessive starting surge individually, and pretty much keep up if one was down plenty long enough to get a tech out to repair.