When deciding how many solar thermal collectors to install for residential water heating or space heating, there are a few key factors to consider. The main considerations are the size of your household’s hot water demand, the climate and solar resource in your area, thecollector efficiency rating, and the total collector area needed to meet your energy needs.
How much hot water does your household use?
The first step is calculating your home’s daily hot water usage in gallons per day. On average, a family of 4 uses 80-100 gallons of hot water per day for showers, washing dishes, laundry, etc. Larger families or those who take longer showers will be on the higher end. You can check your water bill to get your average daily usage.
Next, you need to determine what percentage of your hot water heating load you want the solar thermal system to cover. Having solar provide 60-80% of your needs is typical.
For example: if your average daily hot water usage is 80 gallons, and you want solar to cover 70%, then your solar thermal system needs to produce around 56 gallons of hot water per day (80 * 0.70).
How much solar energy is available?
The amount of sun your location receives annually is key. Areas with hot, sunny climates like the Southwest will need fewer collectors than cooler, cloudier regions. The optimal solar thermal systems are installed facing south and tilted at an angle equal to your latitude.
Check the solar insolation maps for your area. The average peak sun hours per day indicates the intensity of solar energy available. Peak sun hours range from 3 in cooler climates to 6+ in the sunniest deserts.
What is the collector’s efficiency rating?
Solar water heating collectors have a rated optical efficiency indicating how much of the sun’s energy they convert to heat. Look for the SRCC (Solar Rating & Certification Corporation) or CEC (California Energy Commission) ratings.
Typical collector efficiencies are:
- Unglazed plastic collectors: 25-45% efficient
- Flat plate copper collectors: 65-80% efficient
- Evacuated tube collectors: 60-70% efficient
Higher efficiency ratings mean you can achieve the same water heating with fewer square feet of collectors.
Sizing your solar thermal system
Once you know your hot water demand, solar resource, and collector efficiency, you can determine the total collector area needed using this formula:
Collector area (sq ft) = Hot water usage (gallons/day) x (1 ft2 collector area / efficiency rating) / Peak sun hours
Using the example above of 56 gallons per day average usage, 5 peak sun hours, and an 80% efficient collector, the math would be:
56 x (1 / 0.8) / 5 = 14 square feet of collector area
This means for this household’s needs in that climate, a system with 14 square feet of flat plate collectors would suffice. If less efficient collectors are used, the area would need to increase proportionally.
Collector area guidelines
As a general rule of thumb, these collector area guidelines can assist your solar thermal sizing as well:
- 1-2 collectors (20-40 sq ft) for each 2-4 people in mild to warm climates
- 2-3 collectors (40-60 sq ft) for each 2-4 people in cooler climates
- Increase collector area up to 20-30% for households with high hot water use
- Increase collector area for systems designed to also provide space heating
These are good starting estimates but doing the full hot water demand calculation using the formula above is best for accurate sizing.
Other sizing considerations
When determining how many solar collectors you need, also factor in:
- Climate conditions – Colder regions need increased collector area to make up for lower winter sun levels and solar intensity.
- Shading – Nearby trees or buildings that will shade collectors must be accounted for to prevent oversizing.
- Collector tilt and orientation – Collectors facing true south tilted at latitude have optimal efficiency. Other orientations require 5-20% more area.
- Back-up heating source – Most systems have an auxiliary gas or electric water heater for days with inadequate solar gain.
- Future expansion – Consider potential household size increases when deciding collector area.
Solar Collector Types
There are a few main types of solar collectors used in residential systems:
Flat Plate Collectors
- Glazed flat plates are insulated, weatherproof boxes containing dark absorber plates
- Most common and moderately efficient (60-80%)
- Lower cost but higher heat loss than evacuated tubes
Evacuated Tube Collectors
- Parallel rows of glass tubes containing absorber fins
- Vacuum design minimizes heat loss
- Higher efficiency than flat plates but more expensive
Unglazed Collectors
- Thin plastic absorbers without protective glazing
- Lower efficiency (25-45%) but very affordable
- Best in warmer, sunny climates
Installation Configurations
Solar water heating systems come in two main configurations:
Active Systems
- Use pumps to circulate water or antifreeze fluid between collectors and storage.
- Allow collectors to be located away from tank.
- Higher performance and good for colder climates.
Passive Systems
- Relies on natural thermosyphon convection, no pumps.
- Tank must be installed above collectors.
- Simpler, lower cost, works well in warmer climates.
Consult with an experienced solar contractor on which system type makes sense for your climate and site.
Solar Water Heating Costs
Here are rough costs for a residential solar water heating system:
- Flat plate collector system: $3,500 – $5,500 installed
- Evacuated tube collector system: $5,000 – $8,000 installed
- Unglazed system: $2,000 – $3,500 installed
Solar pool heating typically costs $3,000 – $4,000.
Rebates, tax credits, and other incentives can offset 30% or more of the system costs. Buying a kit for DIY installation can also save on labor costs.
Getting Professional Sizing Help
While the tips here provide a good starting point, it is wise to consult with solar heating professionals to size your system properly. Companies that offer free solar site assessments can help provide accurate collector sizing and account for all the variables in your situation.
They will evaluate your hot water usage, roof layout, collector mounting options, and recommend the ideal system design for your home and family.
Proper sizing is crucial for solar water heating systems to perform efficiently and provide maximum energy savings over their lifespan. Undersizing can mean inadequate hot water, while oversized systems increase costs without much added benefit.
Conclusion
Determining the right number of solar collectors for residential use depends on hot water demand, climate conditions, collector efficiency, peak sun hours, and the portion of total load the solar system will provide. On average, 1-2 collectors per 2-4 people is sufficient in many climates, but doing a full system sizing calculation is recommended.
Consulting solar heating professionals is the best way to precisely size your solar thermal system. With proper sizing and installation, you can enjoy 60-80% of your hot water free from solar energy for decades to come.
