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Solar Thermosiphon Water Heater Calculator

Enter household size and climate zone — get collector area, storage tank size, solar fraction (50-80% of hot water free), and payback period with 30% ITC.

Household & hot water use
people
gallons/day
$/kWh
System configuration
Solar thermosiphon water heater sizing
40 sqft collector — 80 gal storage tank
Solar fraction62% of hot water from solar
Annual energy offset1,823 kWh/yr
Annual savings (electric)$237/yr
Annual savings (if gas)$75/yr
System cost (before ITC)$4,040
30% ITC tax credit-$1,212
Net cost after ITC$2,828
Payback period11.9 yrs
vs. PV + heat pump water heater: An equivalent solar PV + HPWH system would cost ~$1,539 after ITC — typically less than the thermosiphon system. Solar thermal is simpler and cheaper for dedicated water heating; PV is more flexible but requires the additional HPWH investment.
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How to Use This Calculator

Enter your household size and hot water usage

Household size drives collector area requirements. The calculator defaults to 15 gallons per person per day — a reasonable average for mixed shower, dishes, and laundry use. If you have a high-efficiency showerhead or dishwasher, your usage may be lower. If you have a large soaking tub or teenagers, it may be higher. Check your water heater capacity (the tank label shows gallons) as a sanity check.

Select your climate zone and collector type

Climate is the single biggest variable in solar thermal performance. A tropical installation can reach 80% solar fraction (80% of hot water from solar), while a cold climate system may only achieve 50%. Evacuated tube collectors outperform flat plate in cold and cloudy conditions by 15-20%, justifying their 55% higher cost in cold climates. In tropical and subtropical areas, flat plate collectors are usually the better value.

Understand the freeze protection recommendation

True thermosiphon systems (roof-mounted tank, no pump) are only suitable for freeze-free climates. In climates with any freeze risk, you need either a drainback system (water drains from collectors when pump stops), a glycol antifreeze loop, or an indoor (split) system with pump. The calculator warns you if your climate + tank location combination poses freeze risk.

The Formula

Daily BTU = Gallons × 8.33 lb/gal × 60°F temp rise × 1 BTU/lb°F Annual kWh = Daily BTU × 365 ÷ 3,412 BTU/kWh Solar Fraction = Climate Zone Fraction × Orientation Factor Annual kWh Offset = Annual kWh × Solar Fraction Collector Area = Household Size × 10 sqft/person (adjusted for climate + collector type) Storage Tank = Daily Gallons × 1.25 (1 day buffer) System Cost = Collector Sqft × $/sqft + Tank Cost + Installation Net Cost = System Cost × (1 - 0.30 ITC) Payback = Net Cost ÷ Annual Savings

The 60°F temperature rise assumes cold water enters at ~60°F (groundwater temperature) and is heated to 120°F (standard hot water delivery temperature). In colder climates, groundwater temperature is lower (40-50°F), requiring more energy per gallon — one reason cold climates need larger collectors. The storage tank is sized at 1.25x daily usage to provide a buffer for cloudy days without over-sizing.

Example

The Jensen family — 4 people in a temperate climate

The Jensen family of 4 uses 55 gallons/day of hot water in their California home. They have a south-facing roof and are replacing their aging electric water heater. They're considering a flat plate thermosiphon system.

Household4 people, 55 gal/day
LocationTemperate (California)
CollectorFlat plate, south-facing
Electricity rate$0.22/kWh (CA)

Result

Collector area~40 sqft
Storage tank60 gallons
Solar fraction62% of hot water
Annual kWh offset~1,550 kWh/yr
Annual savings~$341/yr at $0.22/kWh
System cost~$3,200
After 30% ITC~$2,240
Payback~6.6 years

The Jensens' thermosiphon system covers 62% of their hot water for 6.6 years payback — and the system lasts 20+ years with minimal maintenance (flat plate collectors are simple with no moving parts). In California at $0.22/kWh, the savings are significant. They'll run a backup electric element on the tank for the remaining 38% of cloudy days.

FAQ

A thermosiphon system works on simple convection — no pump required. Cold water sinks to the bottom of the collectors, absorbs heat from the sun, and naturally rises to the storage tank positioned above the collectors. As hot water is drawn off the tank, cold water flows in from the bottom to replace it, creating a continuous natural circulation loop. This is the simplest possible solar water heating system — just collectors, a tank, some pipe, and gravity. The tank is typically mounted on the roof directly above or beside the collectors for this to work.
Solar fraction is the percentage of your hot water energy that comes from solar. Realistic ranges: Tropical climates (Hawaii, Florida): 70-85%; Subtropical (Arizona, Texas coast, Georgia): 65-75%; Temperate (California, NC, mild WA): 55-70%; Cold climates (Colorado, Wisconsin, Maine): 45-55%. These figures assume a properly sized system. Over-sizing collectors increases solar fraction but with diminishing returns — beyond a certain size you generate more heat than the tank can store and the system stagnates. A well-designed system targets 50-70% solar fraction to balance economics.
Flat plate collectors are simpler, more durable, lower cost (~$35/sqft), and perform well in sunny warm climates. They're the right choice for tropical, subtropical, and most temperate installations. Evacuated tube collectors cost ~$55/sqft but are 15-20% more efficient, especially in cold weather and diffuse light conditions. The vacuum insulation reduces heat loss in freezing temperatures. In cold climates (Colorado, Wisconsin, Maine), evacuated tubes typically pay for the premium in 3-4 years of improved performance. In tropical climates, flat plate is almost always the better value.
Three freeze protection options: (1) Drainback system: the pump shuts off when temperatures drop, and water drains from the collectors back into an indoor tank. Simple and reliable — no antifreeze to replace. The gold standard for reliability. (2) Glycol antifreeze loop: a propylene glycol solution circulates through the collectors via a pump. Requires a heat exchanger to transfer heat to potable water. Glycol degrades over time and must be replaced every 3-5 years. (3) Recirculation: pumps warm water from the tank through the collectors when near-freezing. Only suitable for mild freeze climates (occasional brief frosts). True roof-mounted thermosiphon systems without pumps should only be installed in freeze-free climates.
Both can dramatically reduce water heating costs, but they work differently. Heat pump water heaters (HPWH) use electricity but at 3-4x efficiency (COP 3-4) — a $1,200 unit can cut electric water heating costs by 65-75% with no roof penetrations or plumbing changes. They work year-round regardless of sun. Solar thermal provides "free" energy from the sun but requires roof collectors, a solar-compatible tank, and periodic maintenance. Solar thermal typically wins on lifecycle cost in sunny climates. HPWH wins on simplicity and upfront cost. If you already have solar PV panels, a heat pump water heater is often the better pairing since it's easy to install and PV generates the electricity to run it efficiently.

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