I believe there is not an electric tankless water heater on the market that will, with one unit, accomplish your goal. A 45-degree incoming water temperature in Anaheim seems strange, however. You may want to double-check that.
It is possible to set up two tankless water heaters in parallel or possibly in series to accomplish your goal. Another possible option is to use two water heaters in different parts of the home, depending upon the the distribution and location of your hot water needs, as well as how easily you can get power to those locations. The manufacturer we often look to recommends a parallel configuration.
In any case, you are looking at a lot of power needs. I like to focus on preventive medicine rather than treatment of symptoms, so I would also look more broadly at the root situation and see if there are other alternatives that may create a lower upfront cost or lower operating costs. The factors that need to be taken into consideration include:
- Flow rates of fixtures.
- Whether the 8-9 gpm is a sustained flow rate need or very temporary.
- The available recovery time between high-use periods.
- Flexibility to time-shift certain usage patterns.
- Available space for equipment.
- Plumbing layout.
- Solar profile of your home.
Depending on the above, some additional solution tools that might be effective for you or others are:
- Reducing the flow rates of fixtures. With very effective 1.5-gpm showerheads, you could have three showers going simultaneously for 4.5gpm. That will take care of a house of teenagers.
- Reducing the number of fixtures. If you have a shower with multiple heads, consider the tradeoffs of using one head at a time versus the costs of a two-heater system.
- High-efficiency appliances. Front-loading clothes washers are tremendous water misers.
- Solar preheating using an 85-, 100- or 120-gallon tank. Solar water heating in many parts of the country is a no brainer. Anaheim is a great location for solar, if that is where the home is located. I just saw a solar factor profile for San Diego (similar to yours) and, depending upon usage patterns, solar heat alone could take care of most of your hot water needs most of the year. California and federal incentives are plentiful, allowing solar hot water to pay for itself in about three years or less.
- Employing a heat pump water heater, which extracts heat out of ambient air and uses an exchanger to heat the water in the tank. Recovery time may not be fast enough for your high-volume needs, depending upon how sustained they are and the size of your tank.
- Heat recovery. This is more applicable in areas where homes have basements. It involves wrapping shower or other drainpipes with a heat exchanging pipe or an incoming supply pipe to adsorb the waste heat from water going down the drain in order to pre-heat incoming water.