Mine in my last home was shared in cost by the power company.
Completely allwed me to turn off the power to the water heater all summer long up until the no-ac needed season.

I've used them for 30+ years. I really feel that they should be a code requirement in hotter climates. As mikie said, during the summer, you'll get all your hot water from it AND increase the AC SEER rating 3-2 points. I've found an average of $50 a month savings using one. Search around and you can find a dual-coil unit that will work with 2 ACs. Much better than solar hot water since most hot water use is early morning or the evening when the solar panel can't heat the make-up water and will require the back-up element to switch on. In hot climates the ACs and HRU will continue to heat water at night when they come on. The real kicker is, the more hot water you use, the lower your power bill goes. Clothes wash much better in hot water and a HRU makes it cheaper to wash in hot than cold or warm since the more heat you can shed from the water heater tank, the lower your AC costs go. The reason behind this is when the hot gas from the AC compressor is re-routed to the HRU, the gas is cooled by the HRU, reducing it's pressure, easing the compressor's work and lowering it's current draw. By putting a switch on the HRU's circulator pump, you can effectively demonstrate the savings. My AC's current draw will drop 2-3 amps when the HRU is circulating water and go back up if the pump is switched off. I've even considered installing a mixing valve on my sprinklers to water the grass with warm water to help shed heat.