Thermostat setting for electric preheat with PV diversion

[RSInouye]

I plan to get an LP on-demand water heater for my off-grid house, and would like to add an electric tank water heater to pre-heat well water. I asked two on-demand brands about incoming water temps. Rinnai said their units require a minimum delta temp of 35 degrees, so an output temp of 130 would require an input temp of no more than 95 (to avoid excess cycling). Rheem said their units require a delta T of 30 degrees, so a 130 output would require an input temp of no more than100 degrees. I've looked at a few electric tank brands, all of which listed a minimum thermostat setting of 120 degrees. Any suggestions about a simple way to address this issue? Thanks in advance-

 

[efficientPV]

This is the water heater I use in my off grid house that I power from 60V solar array. It is 50L (13 gal) and it is sufficient for my needs. Used to have a 10 and a 6 gallon in series, but I can run my dishwasher on this which is kept at 62C. I kicked my propane heater out of the house. This 1500W 120V horizontal tank was only $150 shipped but it has funky 1/2 inch connections. They are end compression with a washer. Plastic seems to thread on but thread buildup is need to prevent leaks. I modified the display to work on 12V. Even with minimal solar your propane shouldn't have to turn on. And one neat thing is this thermostat works down to ambient temps. It does need to have added insulation. I boxed it in a little and stuffed it in. It is still a pretty neat tank for a camp. The other tank is an ECOsmart 6 gal which is a much better tank. I havent felt the need to add it into the system yet. The ECOsmart thermostat also goes below ambient.

A 35F differential seems really crappy for a water heater. What model is this?

 

[RSInouye]

A 35F differential seems really crappy for a water heater. What model is this?


Didn't specify a (Rinnai) model, but note that this is the minimum temperature differential (degrees F), not maximum.