Can you run a hot tub on solar power? Solar Hot Tub Design

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Can solar power a hot tub?

I’ll break down this question and answer it in parts. You need to answer the questions below to work out what kind of solar energy you need and how much of it:

  1. Are you going to use electric PV solar panels, solar thermal collectors or a mixture of both?
  2. Assuming your hot tub is electric powered, how much energy in kWh does it consume per hour?
  3. How strong is the sun’s energy in your location?
  4. How many electric PV solar panels would you need based on point 4?
  5. Do you need batteries? How many?
  6. Solar savings and payback time
  7. Conclusion – what would I use to heat my hot tub?

You can run a hot tub on solar power, either electric PV or solar thermal, but solar thermal is by far the most cost effective. The average hot tub consumes on average of 300kWh/month, which could be delivered with 2000 watts of solar panels and a 24 volt 250Ah battery.

1. What Kind of Solar Panel Is Best for a Hot Tub?

Electric PV or Hot Water?

A great question and the answer isn’t evident. On the one hand, solar electric is quick, simple and easy to install. On the other hand, it isn’t that efficient – just 20% of the sun’s energy is converted to usable power, on average.

If you have an electric heater heating up the water, it would seem to make sense to use electric solar panels to power up the hot tub, but there’s also the possibility of solar hot water as well.

Solar thermal is a bit more complicated to install but they are definitely more efficient – up to 75% of the sun’s energy is converted into heat energy.

The big question is – Can solar thermal panels heat the hot tub water to a high enough temperature?

Video – Can you run a hot tub on solar power?

2. How much energy does your hot-tub consume per hour?

I don’t know how much energy your hot tub consumes in kilowatts per hour (kWh) and so I’ll assume an average of 300kWh per month. (It varies between 100kWh and 600kWh.)

We can use this figure for sizing both electric PV and solar thermal panels. Polls show that most people run their hot tub twice a day, which makes sense.

Once the pool is up to temperature (about 100 degrees F) running the electric heater regularly will keep down costs, particularly if you have a very efficient cover to hold the heat in.

How much solar power do you need to run a hot tub?

Based on the average I guessed at, 300kWh/month, you’re going to need a system capable of sustaining that energy output and we’ll size the panels accordingly.

Because solar systems have losses, like any electrical or mechanical systems, I normally add up to 50% extra capacity so I’m not caught out. It also accounts for the possibility of low-sun days or passing cloud cover.

3. How strong is the sun’s energy in your location?

The sun’s strength is measured in kilowatt-hours per square meter per day or year. Another way of expressing that figure is by calling it peak-sun-hours.

It’s really convenient for working out the energy you can can get from solar electric panels – you just multiply the wattage rating of a solar panel by the peak-sun-hours and you have it!

It’s also useful for working out the energy transfer in solar hot water panels. We know the approximate efficiency of such panels, so given the mass of water, we can work out how many panels it would take to raise the temperature to where we want it.

Fortunately, you don’t need to measure the sun’s energy wherever you live, but simply enter your location into one of the websites set up to deliver historical solar data based on your city geographical coordinates – Global Solar Map.

The image below shows how it’s done. Enter your location and save the data – you’ll use it later on in this post.

Houston Texas irradiance for hot tub solar panel sizing

Enter your hot-tub’s location and read off the irradiance in peak-sun-hours – Houston, Tx shown above.

Can solar power a hot tub all year round?

In the last section you found the peak-sun-hour value for your city but this is an average value over the year. It’s going to be more in the summer and much less in the winter.

As hot-tubs get more use in the winter, it makes sense to base your solar panel sizing on the lowest energy time, which is of course in the winter months.

The table below shows the difference in the sun’s energy throughout the year in Houston, Texas:

Insolation by month for Houston, Texas by month (kWh/m2/day - also are known as Peak-Sun-Hours)

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

2.70

3.23

4.25

4.98

5.61

5.79

5.94

5.44

4.88

4

3.03

2.56

As you can see, in the winter it’s more than 50% less in January than in July. You’re gonna need a bigger panel …

Can solar run a hot tub in the dead of winter?

This could make or break you project. It might make all the difference between deciding to use solar electric or solar hot water panels, for example.

Of course, you could still use solar the rest of the year and back it up with grid electricity in the deepest winter months. You’d still enjoy significant savings.

4. How many solar panels would you need to power your hot tub?

How many solar panels to power a hot tub? (Electric PV panels)

I’m going to assume a couple of things for this example calculation:

Hot tub location: Houston, Texas

Hot tub electric heater size: 2 kilowatt (kW)

Run up time from ambient to 100 degrees F: 10 hours

Energy used for run-up: 10 hrs x 2 kW = 20kWhours

Note: It takes a lot of power to raise the water temperature from cold, and a lot less to maintain the temperature.

I would use the mains for the temperature run-up, which may only be a couple of times a year, and solar to maintain the temperature.

Energy used/day to maintain temperature: 2kW x 3 hours = 6kWh or 6000 watt-hours

Dividing the energy required by the peak-sun-hours/day, we get: 6000/4.253 = 1410 watts of solar

or 5 x 300 watt solar panels.

How many solar panels to heat a hot tub?

Electric PV or thermal collectors to power your hot-tub?

Do you need batteries?

Yes, batteries will be required – I would recommend 250Ah 24 volts. The beauty of using batteries is that energy generated throughout the whole day is stored and can be used in a short period .

If relying on the instantaneous power generated at any time, twice as many panels would be required and even then, it isn’t sure they would be in full sunshine the thermostat calls for heat.

Lithium phosphate batteries are preferred, as they are inherently deep-cycle. This means they can be discharged up to 95% of their capacity without damage. They have a very long life.

5. Solar savings and payback time

The real test if using solar panels is worth it or not is how much is saved in electricity costs. How much is saved and how many years would it take to payback the initial capital cost of the installation?

Using Houston, Tx as an example, we have a power cost of 11.83 cents/kWh on average, and 6kWh is used per day just maintaining the temperature:

  • Unit electricity cost: $o.1183
  • Energy consumption/day: 6kWh
  • Cost per day (grid use): 6 x 0.1183 = $0.70/day
  • Cost per year: 0.70 x 365 = $255/year

Using solar should save $255 per year.

What is the cost of solar in Houston, Texas?

A small system costs in the region of $2.90/kW solar installed, making the proposed 1500 watt hot tub solar system cost around $4350.

I’m going to divide that installation cost by the annual savings to see how many years it will take for the solar power system to pay for itself:

4350/255 = 17 years

Keep in mind that electricity costs do increase annually, so that figure will reduce progressively as time goes by. The payback time will also be adjusted according top your irradiance value and electricity costs.

Conclusion – what would I use to heat my hot tub?

I think a sensible approach would be to consider all the options available and combine them according to the circumstances.

For example, for running up to temperature I would use solar thermal collectors, perhaps evacuated tube type. They reach very good water temperatures and would probably maintain the temperature quite well once it was reached.

After most of the water mass is heated to 90% of the temperature required, I would finish off with grid power, then using the solar hot water collectors to maintain temperature.

If the lengthy solar payback time didn’t bother me, then I’d simply install solar electric PV and run the hot tub with it exclusively. It’s clean and hardly needs any maintenance.


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