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Sizing a solar hot water heater

Solar powered design and system sizing

Before installing a solar hot water heater, the solar powered design must be sized properly. The solar domestic water heaters' efficiency and performance will heavily depend on panel (or collector) and storage tank sizing.

Most professional solar installers agree that solar domestic water heaters should adequately supply about 90% to 100% of a site's total hot water use during the warmest and sunniest months of the year.

Keep in mind

With residential applications, in order to qualify for the federal solar tax credit and potentially receive thousands of dollars in reimbursement for all of your out of pocket cost, the SRCC certified solar hot water heater that is installed has to provide at least 50% of the residences' annual hot water load.

Daily hot water use

Saving energy is important, and harnessing the sun's free and environmentally harmless thermal heat to do so is undoubtedly attractive. With that said, utilizing renewable energy sources shouldn't require a compromise in overall comfort.

In order to be effective, a solar domestic hot water heater should provide an adequate supply to all major consuming fixtures. Your site's daily hot water use is the first important consideration to make when sizing any solar powered design.

Existing Homes or Sites

If your existing home or business currently has a water heating system, but the supply is inadequate, this is the most cost effective time to address it with a larger storage capacity. Just keep in mind, going with a larger storage volume can effect how other major system components within the solar design are sized.

If your current daily hot water supply is adequate, simply use the existing storage volume (which will serve as the solar back up) as a solar storage sizing reference point. It's important to note that there are other considerations to make when sizing solar hot water storage tanks.

Your existing conventional storage tank will likely serve as the solar hot water heaters back up source, so consider improving efficiency and reducing standby heat loss with a water heater blanket and a water heater timer.

New Constructions

In new constructions one must consider the amount of major hot water consuming fixtures and the flow rates of these fixtures. Washing machines, dishwashers and bathroom shower heads are the most important fixtures to consider because they typically consume the most hot water on a daily basis.

The number of adults expected to reside or work in the site should be considered as well. An average adult can be expected to use about 20 gallons of hot water on a daily basis. So a household with four to five adults should be expected to use anywhere between 80 and 100 gallons of hot water daily.

For new constructions it's always a good idea to consult with a general contractor or a local solar professional.

In both new constructions and existing homes (or businesses) making small and very inexpensive up front investments, such as water saving shower heads and faucet aerators, can go a long way to reducing the site's daily hot water use.

Larger and more expensive investments, such as energy star washing machines and energy star dishwashers, will further reduce the site's daily hot water loads.

These investments will save on the cost of the solar hot water heater, because a smaller and less expensive system will likely be suitable for installation.

Sizing solar hot water panels

Solar hot water panels, or collectors, are the heat gathering component of any solar hot water heater. These panels, or collectors, should always maximize as much of a site's solar resource as possible. This can be achieved by using the correct collector mount and collector tilt.  

How much thermal energy a particular hot water panel produces using a site's solar resource is referred to as the panel's or collector's thermal output. A panel's thermal output is measured in thousands of BTU.

Determining Solar Panel Size

It's important to note that some solar installers may use different methods to determine the size of the solar collector. For the purpose of outlining how one could determine the panel size in a solar hot water heater, suppose the site to be evaluated is in suburban Atlanta, Georgia.

Step One: Estimate the Solar Thermal Output that Will be Required

  daily usagetemp risewater density multiplied by it's actual heat = output
  • estimated daily hot water usage = 80 gallons (three to four adults)
  • temperature rise (120° - regional incoming water temp or 62°) = 58°
  • density of water x it's actual heat = 8.33  (this figure remains constant)

 80 (gallons) x 58 (degrees) x 8.33 = 38,651 (estimated thermal output in BTU)

Step Two: Determine Your Site's Sky Type

The SRCC rates water panels and classifies there thermal performance under various solar conditions. This allows consumers and installers to estimate how much thermal output, in BTU, to expect under several different sky types.

  • Clear Day = 2,000 BTU/square foot/day
  • Mildly Cloudy = 1,500 BTU/square foot/day
  • Cloudy Day = 1,000 BTU/square foot/day

For our example site in suburban Atlanta Georgia, we suppose that the site's solar resource provides an annual average of 4.7 hours of direct sunlight daily.

A local solar installer can accurately determine your site's solar resource, and what collector mount and tilt best maximizes your site's solar resource.

Solar energy is measured in kilowatt hours per day, but a solar panels' thermal output is measured in British thermal units per day or BTU/day.

Formula To Convert kWh/day To BTU/day

1 kWh/day x 317.1 (constant) = BTU/day

4.7 kWh/day (site's solar resource) x 317.1 = 1,490.40 BTU/day

This site's sky type as classified by the SRCC best fits the Mildly Cloudy category, which is estimated to produce about 1,500 BTU/square foot/day.

Step Three
: Consider the Type of Application

Solar collectors and panels are not just used for solar hot water heaters, there also used in solar pool heaters and space heating systems as well. The SRCC tests, rates and certifies each solar panel for every possible application.

The application category is displayed on the collector thermal performance rating label, usually in the far left column.

  • Category A: pool heating in a warm climate.
  • Category B: pool heating in a cool climate.
  • Category C: water heating in a warm climate.
  • Category D: water heating in a cool climate.

For most climate regions in the U.S, Category C should be used for sizing panels for solar hot water heaters. For colder climate regions, such as the North east, the Category D application will suffice for solar powered water heating.

When in doubt always consult a local solar panel installer.

Step Four
: Observe the Collector Thermal Performance Rating Label

Observe the SRCC provided collector thermal performance rating labels, and compare the thermal performance of several solar hot water heater panels.

SRCC logo

SUPPLIER: ACME Solar International Group

     1234 Easy Street San Diego, CA USA 
     MODEL: 20-01 
     COLLECTOR TYPE: Glazed Flat Plate Collector    
     CERTIFICATION#: 1234567
     SRCC OG-100 Collector Certification Program
Thousands of BTU Per Panel Per Day
Application Category   Clear Day    Mildly Cloudy  Cloudy Day 
Category A 52.0 39.9 27.1
Category B 46.0 34.5 22.1
Category C 36.3 27.1 12.0
Category D 19.2 9.7 0.0
 Collector Specifications

 Surface Area: 3.764 m2 40.51 ft2                         Dry Weight: 76.6 kg 169. lb
 Net Aperture Area: 3.43 m2 36.90 ft2              Fluid Capacity: 6.3 liter 1.7 gal
 Y - Intercept: 0.655

 Collector Materials
 Panel Frame: Anodized Aluminum              Absorber Plate Material: Copper Fin
 Panel Glazing: Low Iron Temp Glass          Absorber Coating: Selective Surface
 Panel Insulation: Polyisocyanurate

Step Five: Estimate the Percentage of Hot Water Output that Will be Provided

This particular solar water panel on average will produce 27,100 BTU a day with "mildly cloudy" solar conditions.

More importantly, if this panel is used in the solar hot water heaters' design, the system can produce about 70% of this site's daily hot water on an annual basis.

27,100 BTU/day (panels' daily solar thermal output)

38,651 BTU/day (desired daily solar thermal output)
 =  70.11% of the output

If our example were a residential application, the homeowner would be eligible for the federal solar tax credit because more than 50% of the annual indoor hot water load will be provided by the solar hot water heater, assuming the storage volume is sized properly.

Panel performance per square foot comparison

Estimating the daily output and verifying that a substantial amount of your daily hot water output will be provided is important, but it doesn't tell the entire story.

It's important to consider that integrated collector storage or ICS units, as well as flat plate collectors and evacuated tube collectors are available in many sizes. So when comparing thermal output, the only size specification that really matters is the panels' net aperture area.

To get a true thermal output comparison, use the water panels' net aperture area and estimate it's thermal performance per square foot, and it would be wise to do this for several models.

To get the thermal performance per square foot simply divide the panels' thermal performance rating (for the appropriate sky type and application) by the models provided net aperture area.

Panel Price Per Square Foot Comparison

Once you've estimated the performance per square foot for several solar panels, then compare the estimated price per square foot. This can be done by simply dividing the price of the panel by the estimated performance per square foot.

A solar hot water heater should be both energy efficient and cost effective, and paying for more collector than you need will compromise cost effectiveness.

Get the entire story by selecting a solar hot water heater with a water panel that produces the most of your daily needs for the lowest cost per square foot.

Sizing storage tank volume

Regardless of the particular design, properly sizing the storage capacity of a solar hot water heater is important as well. In passive solar water heater plans, sizing the storage can be as simple as referencing your existing system.

In active solar water heater designs, more times than not, a greater volume of storage is required. The increase in storage is required in active solar hot water heater designs mainly to prevent the system from overheating during peak hours.

Overheating can cause serious safety concerns, and it can also compromise the entire designs overall efficiency. A larger storage volume is required to also ensure that there's an adequate back up supply when the sun's radiation is unaccessible.

In active designs, the solar hot water heaters' storage volume should always be greater than the site's average daily use.

Storage Sizing Recommendations

Most professional solar installers strongly recommend increasing an active solar hot water heaters' storage volume by at least 1 1/2 gallons for every square foot of panel, or collector, net aperture area.

In regions that have considerably more annual sunlight, such as the Sunbelt, a 2 gallon increase in storage volume for every square foot of net aperture area is strongly recommended.

Much like with conventional tanks, solar hot water tanks are available in a wide range of sizes. When in doubt always consult a local solar professional.

More on solar hot water heaters

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