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.

What??? Is the inlet 1/2” or 3/4”? Or is the outlet 1/2” or 3/4”? Either way, you have some major replumbing to do to make any of that work. Including the gas line needed.

I like tankless heaters, I installed them in the last two houses. But there are things to understand about them: 1. Tankless heaters are very expensive to install when done as a retrofit. 2. They take an extraordinary amount of gas at one time - so you have to have the gas supply. Having the supply to run two of them would be unlikely. 3. They do take longer to supply hot water. 4. they do need regular maintenance So again, let me suggest a hybrid electric heater. These don't require venting at all. They cost less to use than even a tankless heater. They will cost much less than a tankless to install and there are often large rebates from the power company. In addition they will dehumidify your basement. They will give you hot water faster. There is an 80 gal available rather than installing two.

Thanks much everyone ! Another reason that got me thinking along these lines is to do space heating using the hot water. What I was thinking of was to use a heat exchanger on the pipe coming back to the tank and use that to heat the home (though forced air). I have to admit I haven't done any calculations on the effectiveness of this, but it is just a thought. Space heating does cost a good bit (we run the heater about 7 months of the year). It is clear/sunny most of winter. Besides, my dad prefers to keep the house at a toasty 78 resulting in high energy bills. What I imagined is that I would get most of my space heating bill (about $1000/year) reduced this way.