Solar Thermal: A Simplified Guide to Sizing Your System.
Step 1: Determine water usage
A very simplified figure is 20 to 25 gallons per person. This would be adequate storage.
Step 2: Determine cold water temperature
This can vary widely depending on where you live.
A simplified rule: the further south, the warmer the intake water. The average temperature is about 52 degrees in New York State, and about 70 degrees in Florida.
The extremes: Anchorage, Alaska is 38 F, while Phoenix, Arizona is about 82 F.
Step 3: Determine the energy needed (in BTU) to heat water to 125 degrees.
You need 8.34 BTU to heat one gallon of water one degree.
So here's your simplified formula for the energy needed:
BTU needed = 8.34 x gallons (from step 1) x (125 - your cold water temp from step 2)
Step 4: Find the thermal performance rating for the collectors you are considering.
Panels are rated for BTUs per day. This figure should meet requirement of the SRCC.
Step 5: Calculate the number of panels needed.
The figure in step 4 is divided by the figure in step 3.
Keep in mind that a system that raises your water temperature by 50 to 70% will provide significant long term savings.
Note: the calculations above are all drastically simplified, to give you a very general idea how many solar panels you will need.
To get all the facts, contact a solar thermal dealer.