Solar Hot Water [SHW], maybe?

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Junior Member
Jan 6, 2014
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Hawai'i has some of the highest electrical costs in the country.
The last time I checked, 4-1/2yrs ago, we were paying $0.42/kwh.

We had an 8kv PV system [16 panels/inverters] installed 4-1/2 years ago on our home.
We are grandfathered into the net-metering program, more payback for us than those in the newer system.

We use our roof as part of a catchment system feeding a 10k tank in the back yard.
We do not have a solar hot water installation, ... yet.

The 2018 Kilauea LERZ eruption missed our place.
We were close enough to roast marshmallows, though.
We did get damage from the SOx gasses evolved by the eruption.
Our insurance just paid out a very nice settlement for the damage to our property from the gasses.

We are having a new metal roof installed.
The roof removal and installation should begin in about 2 weeks.
The PV installation will need to be removed for the roof replacement, then replaced and inspected/tested afterwards.

I had bought a timer for the hot water tank a few years ago.
I finally installed it recently.
Powering the water heater only 8hrs per day has dropped our power consumption enormously.
The overage drawn from the grid has dropped 34kwh in the last 7 days.
In 3 or more weeks we will be producing more than we have used from the grid, if we continue to use the timer.
This has convinced me that we do need solar water heating.

After the roof/PV job is done, I will have a solar hot water installation done keeping the electrical as backup.
That should drop our power consumption even more, allowing higher water temps than the 120°F we're now set at.
I'd like to see at least 130° to 140°F.
I miss HOT showers and baths after getting myself chilled.

We used all our S facing roof for the PV installation.
All that is left is E, W and N facing roof sections.

All new residential construction is now required to have SHW on it.
In looking around, I see a lot of SHW installs that are facing E and/or W.
That may be because that is all that is available on those houses because of their orientation.
This has me thinking that being S facing is not as important for SHW as it is for PV, maybe one on an E facing and one on a W facing section might do the job.
Far from an expert here, and our sunlight is nothing compared to what you get I’m sure. My nephew a few years ago put in an above ground pool and made his own solar heater by spiraling out a few rolls of black PVC pipe on the ground and running a small pool pump to circulate water from the pool. I was over there last summer and his pool was almost hot tub hot and the water coming from the DIY unit was so hot you couldn’t hold your hand under it. So he was basically level mounted in northern PA summer sun with a DIY designed rig that I’m sure wasn’t optimally designed.

So I would say yes you should get some good solar exposure on other roof than south facing where you are located. Do some experiments with a black panel and a cheap non contact inferred device and see what the difference is when you change the angle and orientation/direction. Harbor Freight sells a cheap unit.
How hot does your attic get? In Flordia they heat pools by putting piping under the roof, inside the attic or by using a radiator of sorts in the gable vents. They then blow attic air across the radiator and heat the water. HI tends to be much milder than anywhere on the CONUS. Though much depends on which island, which side of the island and prevailing winds.
I've been thinking of doing something similar for our spa tub, with a coil on the roof of its pergola.
We only use it for cooling down on hot summer days, keeping water temps at ~92 to 94°F, warm enough to not get chilled sitting in it for an hour or more.
Unheated and with the cover on, water temps stay in the mid to high 70's.
Don't need to bring it up much to make it comfortable.

I would want a thermostat controlled 12v solar powered pump operating it.
The greatest problem I see with it is getting it plumbed without an unsightly minefield of piping or hoses going into and out of the tub.

We are on the windward side of the island.
We do get a lot of sun.
But, we also get about 150"/yr of precipitation, which means we also get a lot of cloudy, rainy weather with a jungle like landscape.
We've had a few days like this the last couple of months:

The Kona [leeward] side is more like SoCal, mostly clear sunny days with an arid landscape.

Once the new roof is on and the PV installation is back in service, I'll call a SHW installer to come out, take a look, and give us an opinion and a number.

Not sure of the rebates, tax incentives, etc. right now.
We did very well for the PV install with state and federal incentives.
I understand the situation has changed recently for SHW.
Financial help might not be as readily available now.
Plus, we're both retired and are not generating taxable income anymore, which might figure in.

The attic does heat up quite a bit on sunny days.
That may change with the new roof, and still leaves the plumbing problems even more complicated.
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Heating water takes a lot of power, so makes sense to use solar. From what I have read, however, with the price of solar cells dropping, its getting pretty close to where its cost effective to simply put up more solar cells to power the water heater. Do the math.
Just about anywhere, code says hot water needs to be limited to 120 degrees F. I suppose to save you from scalding yourself. Your hot water tank can be set to a higher temperature (it helps to limit the potential growth of bacteria in the tank), but there is supposed to be a mixing valve over the tank so that outlet temp is not higher than 120 F.
The answer is not as simple as you've made it sound.
Florida's NextEra had made a bid to purchase Hawaiian Electric a while back.
At the time, NerxtEra was notoriously anti solar in its Florida operations.
To sweeten the pot for them, HE made some changes.
It changed its PV installation financial return system, in particular ending the Net Metering program replacing it with one that gives less compensation for having a PV array.
We are grandfathered into the Net Metering program.
If we make any additions to our array, we'll lose our Net Metering standing.
We'll end up having to pay much more to be grid connected even when we produce more than we use.
Then, we've also maxed out our S facing roof top, no efficient areas left for installing more.

In the end, the people of Hawaii rejected NextEra's bit to purchase our electrical utility.
The existing utility and state government were moving towards making it so.
Over whelming public opinion squashed it.

There is nothing that I know of that makes legal requirements on what temperature you set your hot water at.
120°F is a recommended energy conservation temperature.
It is also touted as a safety consideration.
Because of using catchment water, 120°F, especially on a a timer, is not hot enough to sanitize the water.
I find that after showering, I need to wipe areas of my skin with alcohol to stop minor irritating fungal growth.
Higher water temps without using a timer would more than likely eliminate that problem.
I no longer take baths.
The water is not hot enough for bathing.

I grew up in the PacNW where electrical power was the cheapest form of residential power.
We had Grand Coulee and all the other Columbia dams all to ourselves at the time.
Because of the low electrical rates, aluminum smelting became a major industry in Eastern Washington.
All home energy needs were supplied by electricity.
Home hot water was always set to 180°F.
Homes were heated with electricity.
Everything was electrically powered.
We had the lowest electrical rates in the country.
Things like washing dishes, clothing and other things were sanitized by the water temps being used.
You can still do it if you want to pay the price.

Adding SHW should allow us to run temps at 140°F or higher while not using more power than we produce, eliminating my fungal problem.
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Just reminds me of how grateful I am we have natural gas ! :)

I have never seen how wind or solar would ever pay off for us .

God bless
I saw a study recently which concluded you're better off using solar-generated electricity to heat water than direct solar, although some of their argument wouldn't apply in Hawaii.:

When we went through 3 hurricanes in Florida a while back, I just looped a 100' black garden hose on the roof and plumbed it in in series with the water heater. Worked like a champ - hot water for showers, at least, but didn't taste very good for cooking, etc. That prompted me to install a solar water heater - one frame, 10' x 4' as I remember, and we had 180° water. Circulation pump kept failing until I changed to a bearingless motor - also powered by a small solar array:
I keep my electric water heater set at ≈ 180°. I like hot water for dishes, washing whites and the ability to make my shower as hot as I want. I have electric heat and the WH is in heated space so the heat loss from the WH just helps heat the house. In the summer it's not directly in AC space but I know I am paying for heat loss.

A timer might pay for itself just used in summer months. I have thought about connecting an old electric clock between my thermostat and ground to see how many hours it is on for selected periods but just haven't gotten beyond the thinking stage. My cost is about 11¢ per kwh so it would take 454 hrs of off time from total picture to recover timer cost of $50..
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