If you scrounge most of the parts for near free and do the labor yourself, you're out just a few bucks. Maybe you also had some fun and learned a few things?

An off-grid home that heats water with propane and has lots of kids who take many hot baths should jump all over this right away!

 So it your incoming water is 54F and your pre-heat system raised that to 95F, then you would be saving 41 btu's of energy per gallon used.

 Now to figure how much that would save you, you would have to determine the cost of your fuel used and it's end efficiency.

 You are not going to see a savings of heat loss in the tank, just a gain per gallon of incoming source water.

 So the pre-heating of the water is NOT useless, you've saved the energy it would use to initially heat it.

 Now it would depend on the cost of the system you use, vs the cost of the energy you would save. Some schemes have a much higher cost to payback ratio. For instance in your case, recovering the waste heat going down the drain while you take a shower would probably be worthwhile.

With the extremely low system load created by just you, a single tank SDHW solution would be most effective. It is very possible that your standing BTU losses exceed your initial water tampering loads because of your very small usage. In a single tank solution, the bottom limit in tank temperature is set by the backup energy thermostat. Solar energy starts heating from that point, lets say 110DegF for example. You install an anti-scald mix valve to control the output to the fixtures, but the tank can get very hot, I.E.180 DegF. As long as the sun keeps you tank hotter than 110DegF, your standing losses are no longer supplied by conventional energy but by solar energy. Additionally, when the tank is hotter than 110DegF, the incoming cold water is heated as well and may or may not cause the conventional heating source to turn on, depending on the tank temp and the amount of new water drawn in. This also has a very low front end cost since no additional tank is needed. It does not allow the solar to run in the most efficient mode but that is not the goal. The TRUE goal of any RE system is not efficiency but the most reduction in backup energy. Despite common since perhaps, those two paths do not always wind up in the same place....

Good luck...

I built and installed a Preheat tank system.
last fall, mid of August,On roof.
I have kept a diary through the winter as we Live in Montana. My
preheat tank is Electric 50 gal.(our main Indirect 40 gal tank is heated from a hot water boiler for home)  weather outside was -20 degrees below F
we had A lot of reflection from snow but,with clear sky's. the system raised the 50 gal.temp in tank 9 degrees.

I spent around $1400.00 on my system that includes 12 v pump and PV panel cost 2 flat plate heat exchangers also. Copper prices were high when I built it. my glass was free.I couldn't justify twin Poly as I considered it,till I saw the price.