A pressurized commercial tank is constructed similar to a residential solar water tank but has a larger volume. As such they cost more to manufacture and more to ship. Most tanks have internal heat exchangers but sometimes a pressurized tank will utilize an external heat exchanger and a pump to circulate the tank water through the heat exchanger. This save in cost as internal heat exchangers are expensive and maintenance means replacing the whole tank. A good commercial tank also needs to have good insulation value to be effective and this can add to the manufacturing cost.
Certification of a pressurized tank is a must as it could possibly explode under improper use. As such a pressurized tank must have a NSF/ANSI certification number. Because of the relative low production volume it is costly for a manufacture to obtain this certification which adds to the cost.
Multiple pressurized tanks can be used as a cost savings alternative by daisy chaining the tanks together to form a large volume. This has its advantages as the availability of tank under 120 gallons are plentiful and the costs are relatively affordable. It is certainly less expensive to use 2 x 120 gallon tanks than purchase a 240 gallon single tank.