electric + propane water heaters

We switched from an electric to propane water heater 5 years ago mostly for comfort during power outages. Propane cost has gone up so much - over $2/gal while electricity is still $.06/kwh. Now we could save 20% going back to electric. Even if it would never pay back, I still like the idea of not emptying the propane tank so quickly.
I am thinking of installing a medium size tankless electric water heater in-line ahead of the propane water heater. It would shift about half the energy use to electric while preserving the 40 gallon reservoir. Maybe a 40 amp 240V unit, something like 1 gpm at 70F rise or 1.5 gpm at 45F rise.
What's good or bad about doing this? Is it OK to preheat the water into any old propane water heater? Do I need to look hard for a tankless unit that has no flow restriction, since I'd still want the full flow through the regular tank?
thanks
Bob
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Why not keep the best of both?
If you have a power outage and you're counting on electricity to heat your water, you're out of luck.
If it is going to cost less money to run a water heater on electric, why not install the electric water heater. Then pipe the hot from your electric water heater and dump that hot water into the cold side on your existing propane water heater. That way, the water in your propane tank is already hot. You will only need propane to keep the water hot (which is not a helluva lot if you're using the propane water heater as a storage tank. What you're really doing is increasing your capacity, lowering your propane bills, and safe-guarding against power outages all at the same go.
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Does anyone know how to advise when to go propane over electric as to their comparative cost?....it seems that electric maintains a more constant rate, while propane fluctuates wildly.........

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