Hot Water Heater for a Hot Tub

[voyager]

We have a hot tub/spa with a capacity of about 250 gallons.
It was already installed when we bought our home.
Late 2014 we has a roof top 8 KW PV system installed on the house.
Feeling kinda smug, we began using our hot tub.
Electrical usage skyrocketed, so we turned it back off.

Last year we had Solar Hot Water Heating installed for the house.
So, we tried the spa again.
Electrical costs again went up, not as bad, but still too high.
We want a minimal electrical bill, $25/mo or less with production credits.

Since last turning off the spa, we've accumulated about 200 KWH of excessive production, down from about the same in over usage.
We now want to do an inexpensive DIY solar hot water heating add on for the spa.

We're in Hawaii at about 20° N Latitude, on the cloudy windward side of the island.
Our spa will hold at about 80° F when left fallow with a cover on it. maybe a degree or two less during our winter months.
We find that 94° F to be a comfortable daytime water temp to cool us down with.
So, we're only looking at raising and holding the spa's temp upward about 15°.

I'm thinking a coil of 3/8 inch ID black pvc food grade tube in a 4' square box, enclosed by a sheet of acrylic. My main concerns are proper tubing diameter and length, plus sizing solar panels to a suitable 12 volt circulation pump for it.

With power provided by solar radiation, I think over heating of the tubing's water in the panel is of little concern.
Fancy controls should not be needed.
It should turn on and off about the same as our roof top installations.
Advice and suggestions are welcome.

[J.P.M.]

See: "builditsolar.com" for lots of ideas about solar water heating including spas/hot tubs. The more control you want of the spa water temp. and its availability, the more system control you'll need.
Otherwise, it's a pretty simple process. Size the collectors' area to make up for the system losses at operating temp. during the seasonally coldest time of operation and cover some of the collectors during the hottest/best solar time of the year.

[Mike90250]

Plastic tubing is great to heat up a 3 gallon batch of water in the tube, but the plastic is a very poor heat conductor, and once the pump has been on for a minute, you have used up all the hot water.
Look for better solar thermal water collectors that don't use plastic as the heat collector..

[bob-n]

Although it varies considerably, an average medium-size hot tub will use roughly 100 kWh per month. These people have an calculator and show the equations to calculation yourself:

https://www.hotspring.com/hot-tub-installation-planning-tools/energy-efficient-hot-tubs-energy-costs

At your electric rate, would 100kWh/mo explain your added electric cost? If not, perhaps your hot tub is somehow wasting energy. It might have a pump that is stalled, defective, or running too often. Water temperature also matters. The amount of time you use jets also matters.

Have you thought about adding insulation to the hot tub? The bottom is tough to access, but insulating the sides and top should be feasible. If you can double the insulation thickness, you could reduce your heating expense by a factor of 2.

Unfortunately, insulation, lowering water temperature, and a solar hot water heater won't change the pump energy usage.

You can buy or perhaps borrow a whole-house energy monitor, and connect it to your hot tub lines to see how much energy it is really using. Then you can try experiments, like turning off the heaters and just running the pumps. In many hot tubs, it is bad practice to run the heater without running the pump, but pump without heater is safe. Here's one very capable power monitor. There are others:

https://www.amazon.com/dp/B001Q1G4WK

Even a budget clamp-on ammeter will tell you a lot. This meter measures up to 40A, which should be enough for the hot tub.

https://www.amazon.com/dp/B0721MKXBC

To use either, you will have to work in your circuit breaker panel (load center) with your fingers near 240VAC. Not everyone will feel safe doing that.

[Stephen123]

Most electric heaters need a 240-volt dedicated circuit; This circuit includes a 30-amp two-pole breaker with a 10-2 non-metallic cable or metal-clad cable. A perfect breaker size should be proportionate to the electrical specifications of your water heater.

[LucMan]

Check out "Alternate energy technologies" solar pool panels.