I pay for water, so no save.
I am an HVAC technician and from my experience of few years, I can say that Mistbox is one of the few developments that is set to change the way air conditioners operate. Many a times we see customers complaining about the efficiency of air conditioners getting reduced when it is needed the most. Another benefit of mistbox is that it can be controlled through an app which makes it all the more user-friendly.
$500, RTOI??
The biggest issue would be all of the scale, mineral deposits, etc. that build up when the water evaporates. This build up reduces the coil efficiency over time and may make the system overall worse.
Best thing to do is use your house's hot water system to pre-warm the cold water coming into your house with a liquid heat exchanger to your AC condenser.
The water usage would be negligible in comparison to the energy you could save with this. However, there is at least one additional maintenance item that they failed to mention here and that is what to do with the thing in the winter. You definitely don't want your water system self destructing due to freezing. Additionally, for me where I have 2 systems on the roof of my townhouse, getting water up there and maintaining it would be a bit of a pain, but I guess they didn't create this for me, but I could really benefit from it more than a person with it protected from the sun on the ground...
Comment from my AC guy after I sent this to him....
"In my opinion, a useless waste of money"
I've seen a similar product that simply attached to your hose. It used a small pressure actuated valve that would start the spray when the AC fan turned on. No electronics necessary, and far cheaper.
I did some research on this a while back as there are some very simple systems where a mechanical valve is operated directly by the air flow from the AC fan - only one moving part - no electronics! However there is the issue of the scale buildup, and needing to change the filter cartridge regularly.
There were also some very informative discussions from HVAC engineers explaining how this doesn't really work in most cases because of limits to the ability of the compressors themselves. Another idea that is good in theory but not in practice, it seems.
For less than $100 there is a misting system that uses a "flapper" to turn mister on/off and just turn it off if it's needed. Dollar-for-dollar the water & filter cost is less than the electricity used, unless you have water restrictions in your area. It doesn't need to be overly complicated.
I created an account specifically to comment on this. First, the article is wrong:
"A pump compresses a volatile gas into a liquid and circulates it through a coil. A fan blows warm air from the house over the coil, which expands the liquid back into a gas and cools the air. The gas is then circulated back to a second coil outside the house, where outside air carries away the heat "
That's not how the cycle works. It's close but not correct. The compressor pulls in gas from the low pressure side (cold) and compresses it. The hot, high-pressure gas is routed through the condenser coil (outside) where some of the heat is removed with the fan. The hot gas condenses into a liquid which is still under high pressure. That liquid travels inside the house, still hot, and then passes through a restriction to the low pressure side. The conversion of high pressure liquid to low pressure gas make the material (Freon) cold. The now cold gas is routed through the evaporator coil in the furnace where inside air is blown across it to remove heat.
Why the big explanation? For starters, the compressor and coils are balanced and sized specifically to work together to provide both cooling and humidity removal for the size of your home. They are designed to work within a range of temperatures. When it's hotter outside, that doesn't necessarily mean the system's efficiency is less. It's still rejecting the heat from inside and the air coming out of your ducts is cold. However, there is a limit to the amount of heat the system can move. As a rule, a home air conditioner can cool the inside no more than 20 degrees less than the outside temperature. If it's 95 outside then you will be lucky to achieve 75 degrees inside. That's a design day. Introducing a mister to pull more heat from the coils will work but it will only save energy if your thermostat is set to a temperature greater than 20 degrees lower than the outside temperature. On the other hand, if you are pulling too much heat from the Freon because of the mist, you risk freezing the coil inside or comfort will be sacrificed because the system is not pulling enough humidity from the inside air because the system is short cycling.