In practical terms, choosing the right size for your solar thermal hot water storage tank and collector array is one of the most important aspects of system planning. Get the wrong sizes and you could be in trouble - too small and your grid-tied bills will be unnecessarily expensive and the system risks overheating; too large and your installation costs will be too high.

Sizing the Storage Tank

This is the easy part. For every gallon of hot water you use every day, you want a gallon of storage. Since the sun rises every morning, the system can replenish hot water every day, so an average family of three will want a 60-gallon tank to allow for the "20 gallons a day" usage rule. Be warned that storage tanks come in standard sizes, so you may need to pick the closest one for your needs. In this case, always size bigger – it's better to have a little more than you need than not enough.

Sizing the Collector Array

Once you know how big your storage tank is, you can work out how big the collector array needs to be to heat it. This is a little more complex as the calculation varies depending on your location. Basically, the more sun your area gets, the more water you can heat per square foot of collector, so hot areas will need smaller arrays than milder climates.

The diagram shows a rough guide for the United States, with four separate areas defined by their available solar radiation. Each area can heat a given volume of water per square foot of collector array:

• Sunbelt: The sunniest areas – between 1.5 and 2 gallons per square foot of collector.
• Mountain & South: Between 1.25 and 1.75 gallons per square foot of collector.
• Midwest & Atlantic: Between 1 and 1.25 gallons per square foot of collector.
• North West & New England: The least solar radiation – between 0.75 and 1 gallon per square foot of collector.

Let's take an example storage tank that holds 80 gallons, for a family of four living in the US. If that family lives in New England, they will need:

Minimum 80 gallons ÷ 0.75 = 107 sq. ft. of collector
Maximum 80 gallons ÷ 1.00 = 80 sq. ft. of collector

If the family lives in Miami, at the other end of the scale, they will need:

Minimum 80 gallons ÷ 1.50 = 54 sq. ft. of collector
Maximum 80 gallons ÷ 2.00 = 40 sq. ft. of collector

In all cases, the surface area of the collector array depends on the specific location – as a rough guide, the further north you go in a region, the more collector area you need. To make things a bit simpler, the table on this page shows the calculations for different family sizes (assuming 20 gallons a day per person, with economies of scale included for larger households) living in different regions. You will notice that the smallest tank size listed is 40 gallons. This is because it is not recommended to install