I put one in last year, and according to my kill-a-watt meter, we're paying less than $10 a month for hot water even at our summer electricity pricing, which is 22 cents/kw. That said, the heart of the technology is basically the same as a $99 window air-conditioner, and the new units are made in China - so who knows what the durability will be. I paid $699 shipped, but apparently, these units were selling for $499 before the tax credit was announced, and it looks like the price was recently jacked to $799... The quality seems decent, but it does have a "buzzy" sound when it's running that would be really annoying if you had it near a sleeping or living area. The install was super-easy for me - I used a new AOSmith 50-gallon electric water heater as the tank (I hooked up the power supply to the standard heater, but turned off the breaker, so I can use the standard elements if the Airtap fails) Would I do it again? - MEH..I dunno - playing around with the pricing to steal the tax credit seems kind of sleazy, the warranty is really ambiguous - to qualify for the tax credit they had to offer a five-year warranty I believe, but their website now says two years - the new integrated units from GE and Rheem have ten years of coverage, and they have the benefit of being able to use both the heat pump and standard electric elements to give you super-fast recovery if you have guests over, or a huge garden tub to fill. A quick Ebay search shows the competitors are around $1500~ish, but if you haven't taken the tax credit yet, you could snag $500 in government cheese and nullify the cost difference with a better warranty. The technology is an excellent idea, and when the competition drives pricing to where it really belongs - (Until then... meh... Certainly it can pencil out, but the energy-saving payoff exceeds the warranty which makes me nervous. Solar really isn't any better, and tankless seems to be snake-oil when you look for real savings data, but to each his own...

Be careful with the first hour ratings. On a 100 gal tank you will get about 70-75 gallons of hot on that first load. You then get the balance of the first hr rating over that first hr. For the kind of lay-out you suggest I think you should split the house up into two systems and try to centralize each tank to its load. That minimized pipe losses with the recirc, and yours will be substantial. The very best recirc system is the Metlund Design system. It complies best with all the new Title 24 regs. You will need 3/4" insulation on your hot piping. Again T24. Be very careful with the pex sizing. I re-piped with Uponor and the brass fitting here in the Foothills of N CA. There is a bushing affect on the fittings. The ID of 1/2" is 3/8, 3/4=9/16 and 1" is 13/16. It does make a difference. I have a couple faucets I was surprised how poor the pressure is on my current well 40-60 psi set-up. I had intended to go to a constant pressure well pump but now will definitely do so. Everything is good about the 50 psi range. Not so at 40# I am a fan of tankless. I represented a major manuf of tankless for 20 yrs. As far as your suggested products I will only suggest that you avoid Bosch. They, imho, do not have the right technology. Your first step regardless of what you do for equipment is a thorough water test. Manuf of tankless have to list annual cleanings for maintenance, but I had one unit for 12 years and never touched it. It was a pre-production model and I had to replace it prior to selling the house. I cut it in half and it was as clean as a whistle. Many of your plumbing questions and answers begin with the compete water test. Tankless (2) could handle your loads until you bring the tubs into play. They represent a massive load, even for your Vertex's. To handle your tub flows you should have two units on each side of the house. The thing about the tankless is they will each make your 7 or so gpm 24 hrs per day, but 135/7=about a 18-20 min fill. Will you wait that long for your bath. I have a bath fill controller on my tub. I set it at 104 and 35 gal (the vol of my tub) and it will deliver 35 gal of 104* water and shut off the hot and beep to tell me my bath is ready. Pretty cool actually. Your shower should be okay with one tankless, but your shower head(s) must fall within the flow capacity of the water heater. I can go on and on with this topic. Get your water tested and report back. That should be step one regardless of what your are doing.

If you're really sure you want tankless (and you kinda seem to be), I'm not going to argue with you. I'll just say a couple of things and then I'll bow out. Disclaimer: I'm not an expert on these matters. They just interest me and I hope to be more energy independent in the future, so I read up on them. I always welcome corrections from knowledgeable folk. First point: IMO electric resistive heating of ANY kind basically just doesn't play nice with PV, off grid or on. The only time it really makes sense is when you're off grid and your system produces more energy during the day than you can store in your battery. This can also apply to some extent if you're intertied and your utility pays you wholesale or less when you sell your power, essentially penalizing you for conserving. Either way, dumping your excess PV energy into water heaters is one way to store it for later use. However, it should be obvious that to store that energy, you have to use traditional storage water heaters, not tankless. Second point: Peak demand matters. It matters to your electric utility, and it matters to your bill. You probably know more about CA rates than I do, but it appears to me that right now California is more into time of use (TOU) metering than peak demand metering. However, more and more utilities are adopting peak demand metering for residential customers (they've had it for business for ages). So even if you're not dealing with it now, it may well be in your future. Demand based metering watches your power (not just energy) usage. The higher it is, the more you pay for all your electricity. In one demand scheme, you pay a surcharge based on your peak power usage (in kilowatts), In another, the price they charge you for every kilowatt hour you use depends on your peak usage -- the higher your peak, even if it's only once in the month, the more you pay per kWh that month. Let me give you an example of the second scheme. Let's say your house's big energy hog right now is an electric range.. Most of the time, you just use one or two surface units at a time. So your peak demand is relatively low, and you pay (say) 14 cents per kWh most months. But when you cook Thanksgiving dinner, you have all the surface units going, plus the oven, and a microwave. On that one day, you have a much higher peak demand. Your utility takes note. And that month, you pay 20 cents per kWh for EVERY kWh you use all month. So even though that month's usage is only a little higher than usual, the final bill for the month is MUCH higher. Now imagine what could happen to your bill if you add an appliance that increases your peak demand every day of every month. Guess what: when it comes to peak demand, tankless electric water heaters are MONSTERS. Think about it. With its mild climate, California is a great place for heat pump storage water heaters. That's what I'd get if I were you. Not tankless, and definitely not electric tankless. And that's all I have to say about that.

100' of 3/4" PEX pipe holds about 1.8 gallons of water. You would have to run a bathroom sink almost 2 minutes before it would start to get warm. A small electric tank heater VERY near (In) the bathroom would provide nearly instant hot water to the sink for washing your hands or shaving. The link is to a 6 gal unit. These small units are slow to recover, so you want enough storage to take care of hand washing, etc. At 6 gallons it will also have enough storage to buffer the initial almost 2 gallons of cold water in the pipe when you are taking a shower until the hot water from the main unit takes over. Electric resistance is the most expensive way to heat water. However, a small point of use heater in series with a gas tankless or electric hybrid would save both energy and water when the main heater is far from the master bath. Copper pipe has a slightly larger inside diameter than PEX. That same 100' of 3/4" pipe in copper would store about 2.5 gallons of water instead of 1.8 for PEX. In addition to the water in the pipe, the thermal mass of pipe has to warm up, and then if it's not insulated and in a slab (not code) the copper is going to conduct the heat more readily to the concrete than PEX. For those reasons I'd say copper will take longer to get hot water than PEX. I want to again say that the best solution is not to have a water heater 100 feet from where the water is being used. Be sure to insulate the hot water line in the slab to R5 - not the R3 pipe insulation you find at the hardware store.