What needs to be considered in terms of size and heating capacity?
You need to consider: Hot water demand, how much hot water is required on site for maximum demand, number of outlets and occupancy. When considering a solar hot water system, the demand for hot water will dictate how big your collector areas are.
You need to consider: The volume of the tank, and the capacity of water it can deliver when you need it most.
A tank volume does not need to be the same as the amount it can deliver all in one go. For example, if you have a 50 litre (L) tank and use 75L per day, you might assume that your tank would be too small. If that 75L consists of using 25L of water 3 times throughout the day, the tank is actually in excess of what is required. Larger tanks take more energy to heat, so enhancements like thicker tank insulation help to ensure that the burner or element is not working too hard.
Flow rate is the most important thing to consider. This will determine how many hot water outlets the system can reliably supply with hot water at once.
To calculate the flow rate, consider how many outlets might possibly be used at once. Calculate the rough flow rate through each outlet for the total flow. The result is roughly what you will use in litres per minute (L/min) at peak demand times in your facility.
In researching continuous flow systems you will likely see that the options for gas systems outstrip those of electric systems. Gas systems simply take less time to get the water to the right temperature, and produce fewer greenhouse emissions (CO2) while doing so.
IThey are capable of supporting a higher number of outlets, and some of the highest capacity industrial hot water systems are gas continuous flow systems.
In short, the greater demand there is for hot water, the greater the need is to heat water. As a rule of thumb, properties with a higher demand for hot water will need to install more solar collectors.