Thanks for the thread, Xringer.
I do enjoy a novel idea
I calculated some time ago that my shower consumes about 20 kW while running, so ~ 1.2 kWh of DC PV a day is good for about 3.6 minutes of shower time a day.
Not bad, but I wonder if you have exhausted conservation ideas that are more efficient and less expensive ?
1. Low flow shower head. I switched out our old 2.5 gallon/min head with a 1.8 gallon/min. My wife *likes* the new one more, so I was a hero for a day, and cut my shower energy consumption by 28%. I paid about $25 if IIRC. Lower flow devices also are on the market but I have read complaints that the spray stings. ymmv.
2. If you have a shower that is often used on the second floor and some plumbing skills, check out drain heat exchangers. I think they cost under $400 and conserve about 50% of the hot water energy.
Drain Water Heat Recovery | Department of Energy
Here is a DIY
. I am not sure if the installation can be horizontal, or what the heat recovery is. Seems like the tubes should be sized to match the shower flow rate.
3. I am a wimp in the winter, but in the summer I turn off the water stream while I am soaping and shampooing. I think I keep the flow time to under 5 minutes.
4. If not done, well insulate your tank and pipes. I find that setting the hot water thermostat to about 120F is more than enough.
5. Wash clothes in cold water.
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Lastly ...
For your situation, comparing alternatives to oil heating is completely reasonable since that is what you have, but also consider whether spending your money on completely unrelated energy savings projects would give you a better return -- pollution and/or money wise. E.g., in our home I decided to replace our electric DWH heater. I considered ASHP, natural gas, condensing NG (high efficiency), and solar heating. In the end, I chose cheap tank natural gas and used the money saved to self-install windows. The windows cut my winter home heating bill by over half -- much more important than my DHW after conservation. In short, take a wide view of your home's energy use.
Good luck, and thanks again for the interesting thread.