Question regarding solar hot water heating

new guy here. I've got a solar water panel 4x10 and a solar 80 gallon tank. Live in Hawaii in sunny Kihei Maui. Still, we need more than just solar heating of water. About five years ago I bought a Craigslist propane water heater and plumbed it downstream of the solar tank and left the electric element off in the solar tank. It worked good. My wife was happy to have full time hot water. But I bought a lot of propane. Anyway the propane tank has rusted out finally. I often felt that propane heaters with their annular doughnut shape, aren't the best storage tanks of hot water, cooling it at night. I wonder what to do. So far, I've suggested to my wife to turn on the element at the breaker just when needed. But, that's unreliable. Likely the element will get left on all day. I've also thought of buying a propane tankless heater to top up the water from the solar tank. Does that work well for anyone? Can propane instant heaters handle warm or hot water input? I also think about getting a second panel. Maybe also a larger solar tank. But I want a practical solution. Any thoughts about the best type of water heater after solar preheating?
Comments
I do have a Caleffi mixing valve at the inlet of the tankless so as to not confuse it's brain, The less work the tankless has to do, the less propane is used, Look for a multi-stage burner.
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Where you live should really be pretty easy for solar hot water.
I would use a 50 gallon tank for 2 people with one collector. There are better ways to heat the water on bad solar days but again where you live should not need anything but a gas heater tank. All the other things in a good solar installation like a short run, well insulated pipe, and a decent controller. The gas heater pilot should only be on a few months where you live.
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They have a minimum fire rate that makes them cycle (very irritating) with low flow rates that need minimal amount of temperature rise.
Burning too much propane - consider adding insulation to the tanks and increasing the solar heaters storage and/or capacity.
It will help some to put a timer on the 80 gallon tank's electric element. Say 6pm to 6am. And install a 1 GPM shower head.
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In Hawaii where I have some clients, solar hot water is so easy to do with just a 50 gallon tank, a large collector, and gas back-up burner or electric if grid tied. Why make it complicated. I do the same thing up here in snow country for 10 months so I know it is done easily!
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I kept the temperature setpoint on the gas water heater at 105 deg F on a bottom mounted sensor. Our 5 five daughters were younger and living with us at the time, and the gas bill was near the minimum connection charge 9-10 months of the year. I was more than happy with the performance.
I eventually added a little 1/4" circulation line to furthest sink to preheat the HW pipes during the hours that the kids were home. I saw a big difference in water usage when they quickly realized that they didn't have to wait for hot water upstairs.