Solar Water Heaters

[mella l]

On Oahu I see that much of the Naval housing we drove by had solar water heaters. Impressive.

Which solar water heater do you use?

What solar water heater do you recommend for new builders?

thanks for your thoughts on this.

mella l
Art and Science
bytheSEA

[MarkP]

If you are putting in a solar PV system panels are now cheap enough to add a few more and run an electric resistance water heater. Depending on the wattage and voltage used the rate of heating can be pretty anemic unless you have installed an extra 4kw of panels (enough to run the typical electric water heater). At about $1 per watt that would bee too expensive just for hot water but here in Puna our sun is so variable that I for example am planning on increasing my panels from 3kw up to 5 or 6kw, just to have adequate electrical power during those cloudy periods. Most of the time my batteries get full by 10 or 11 am and I could potentially send a couple of kw to a water heater for 3 to 4 hours. That's what I am doing now with the 3kw of panels I've got. With the stock heater elements I've got I heat at 700 to 800 watts for 3 to 6 hours and put maybe 4kwh of heat into a big 120 gallon electric water heater on a good day. O
n a bad day and even on every other good day I heat with propane. If I add more panels I can get all my hot water from solar PVusing a standard hot water heater and simultaneously have halfway decent electric power output on cloudy days. Of course on those days electric power generation takes precedence and you must use a backup propane system for hot water.

If you are not already committed to solar PV for electricity the economics are not so favorable but for me if I don't make enough electricity I end up burning gas in a generator which costs money and is inconvenient. If I invest in more panels I get smoother power generation and lower generator costs. During summer all my hot water needs would be covered for "free". I don't think it would pay to install enough panels to cover all power and hot water needs all the time. Just hunting for that elusive sweet spot which is very subjective. Ease of use is part of it and going with PV instead of solar thermal collectors makes things lots easier.

Is this for Eden Roc or somewhere on the mainland?

[mella l]

Thanks Mark. Looking to learn about solar hot water for either, but the mainland primarily right now. I appreciate you sharing your experience. Is there a brand on island that is popular with home owners?

Good luck with your array and reaching your sweet spot.

mella l
Art and Science
bytheSEA

[Obie]

What works on island is not the same as what will work on the mainland unless you are in an area that doesn't freeze.

My system I had in Kapoho was sold and installed by Drainpipe plumbing and solar.It was made up of SunEarth components.

From SunEarth :
SunEarth offers four of the six leading solar thermal system technologies, including forced-circulation glycol, drainback, integral collector storage (ICS), and forced-circulation open loop. Our customers want and deserve products and systems that are climate appropriate. One type of collector or packaged system cannot adequately fit all residential and commercial market applications found in North America – what works in Puerto Rico and Hawaii will not work in Wisconsin.

https://sunearthinc.com/residential-systems/

[bluesboy]

I am off grid, and have the sun earth solar water system. It works well about 75% of the time. The remainder I have a paloma plumbed inline. I wouldn’t want to put the load or wear and tear on my solar pv system to heat water.

[tgalarneau]

I have been using a solar collector hot water system for going on 15 years. It works fine for my family of 4.

It was designed for simplicity. I use a 70 gallon hot water tank to store the hot water and a single large Sun Earth collector on our 2nd story roof. We use a single El Cid 10 volt solar pump and dedicated 10v solar panel instead of the electronic controls (expensive and prone to controls failure) to pump water to the roof collector. It pumps fast when sunny and slow when cloudy and at night off. Just what is needed to produce solar hot water.

Hot water is available around 90% of the time and the highest temp it reaches is around 125 degrees but with the cold water mixer valve I control the tap water at 110 degrees or less. I tried a cheap propane on demand heater but the inconsistent temps made it worse than cold water. And our catchment cold water is around 80 degrees.

The only thing I would change would be for a larger 100 gallon hot water storage tank. This, I believe would give me hot water 99% of the time. I would probable use 2 of the small Earth Sun collectors instead of 1 large as they would be easier to handle when need to replace or move.

Of course needs are different dependent of number of people and types of showers. And guests can destroy the best laid plans.

[Crybaby]

I have been using a solar collector hot water system for going on 15 years. It works fine for my family of 4.

It was designed for simplicity. I use a 70 gallon hot water tank to store the hot water and a single large Sun Earth collector on our 2nd story roof. We use a single El Cid 10 volt solar pump and dedicated 10v solar panel instead of the electronic controls (expensive and prone to controls failure) to pump water to the roof collector. It pumps fast when sunny and slow when cloudy and at night off. Just what is needed to produce solar hot water.

Hot water is available around 90% of the time and the highest temp it reaches is around 125 degrees but with the cold water mixer valve I control the tap water at 110 degrees or less. I tried a cheap propane on demand heater but the inconsistent temps made it worse than cold water. And our catchment cold water is around 80 degrees.

The only thing I would change would be for a larger 100 gallon hot water storage tank. This, I believe would give me hot water 99% of the time. I would probable use 2 of the small Earth Sun collectors instead of 1 large as they would be easier to handle when need to replace or move.

Of course needs are different dependent of number of people and types of showers. And guests can destroy the best laid plans.

---

Thanks for the clever solar pump suggestion. Sounds perfect for circulation.

[terracore]

I have the regular circulating pump, not the DC variety, however it is powered by solar via an inverter. IMHO 125 degrees is not hot enough for a solar hot water system on the East side where we have to go several days without any sun and you'll run out of hot water unless you either get a crazy large expensive tank or store water at a higher temp. As far as I know there is no limit to how hot our water can get up to the point it would set off the pressure relief valve. I usually set it at about 140 though I have turned it up to as high as 160 when we need to scald turkeys in the utility sink. The higher the temperature the shorter the life of the tank. Ideally when we have sun every day I should set it at about 130 and crank it up when we have weather patterns of little sun but I'm more of a set-it-and-forget-it guy.

[ScottEF]

I just discovered that the house I am considering buying in HPP has a two year old electric water heater, and am trying to understand why anyone would buy this in Hawaii (as opposed to having a relic they haven't yet replaced). I see suggestions here of coupling it with a PV roof system, but the house does not have this. How competitive would this be with simply having a solar water heating system on the roof? Using PV electricity to heat the water sounds less efficient. Or is it more efficient because it can use the PV power just when other electric demands are low but the sun is still shining?

[kalakoa]

An all-electric house means not having to deal with propane; some people prefer this.

With PV, an electric water heater becomes "additional energy storage" but most people don't see it that way because it's not in the form of electricity.

Trade-offs are in the eye of the bill payer.