Matthew Nistico wrote: It also won't work on a flat roof, like the tar or gravel roofs of some commercial buildings, or on the flat roof of an RV.
I’m thinking it would work just fine on those roofs (better, really, since they get hotter than sloped roofs) but you’d need a different way to disperse the water, like a sprinkler or soaker hoses. Flat roofs are not truly flat, they all have very slight pitches toward drains. But even if they didn’t, you just need to control the water flow so it keeps the roof wet but doesn’t create standing water, which would be wasted. On an RV, the water drains off the sides or ends. Again, just a slight misting to keep the roof wet is adequate. Where this will not work particularly well is on a steep metal roof, since the water will simply run off too quickly.
I like the idea of a solar panel to run the pump. Similar to the roof vent fan units that incorporate a solar panel to run automatically when the sun shines. If you have rain barrels, the downspouts are refilling them with any excess runoff. Once you had a sense of water usage, you could have spigot water on a timer to top them off, to compensate for evaporation loss.
The 12v RV pumps typically run a gallon per minute at 30 psi, which would be adequate. They draw less than 5 amps, which would be a 60 watt solar panel- less than $50 and not very large. So, yes, this would be very efficient compared to running A/C!