From a long term economics point of view, it is important to generate the maximum volume of hot water from each unit, but it is also important to achieve this, at the highest possible solar input, in order to minimise the amount of traditional electric back-up energy needed to operate the system effectively.

A typical good quality 300 Litre solar hot water system with two flat plate collector panels operating in a moderate to tropical climate, may produce (on average over a year) approximately 250 Litres of hot water every day from each 300 Litre system at 60 degrees Celsius, where the solar contribution (or solar input) of the system is around 80%.   This means that approximately 80% of all of the energy needed (over the year) to constantly produce 250 Litres at 60 degrees Celsius from each 300 litre system every day, would come free from the sun, and the other 20% would come from the electric backup element.    In terms of actual kWh of energy, this can be calculated mathematically as follows, assuming that the incoming temperature of the water is approximately 25°C and we want to increase the temperature to 60°C

kWh    = Lt. of water x Temp. Rise x Specific Heat of Water ÷ 3600

= 250 x 35 x 4.186 ÷ 3600

Total    = 10.17 kWh of energy for each 300 Lt. system per day.

Therefore, to produce 250 Litres of hot water per day (by electricity only) it would take 10.17 kWh.    If the numbers of Solar hot water systems are calculated correctly, by dividing the total litres of hot water requirement by 250 Lt. then you could expect to save 80% of that energy annually.    This would also give you sufficient storage capacity, and booster capacity to ensure constant hot water, even during any periods of prolonged inclement weather.

As a general rule of thumb, a Solar Contribution Factor (SCF) of between 80% to 85% is ideal for gaining the best economic benefits whilst ensuring that the system, or systems, do not “over perform” during periods of low usage and high radiation availability.

Over performance of the systems naturally causes overheating of the water in the storage tank.  This can trigger a release of built-up over temperature hot water from the pressure and temperature relief valve, therefore leaving the systems with very little hot water to use, generally at a time when it is most needed.   If this does occur, it usually happens at around the end of the day, and the storage tank can be left with very little (Solar generated) hot water for the evening period, and you are then back to using electric back-up.   Therefore, correct calculation and sizing is critical for the most economical installation.

Mel Peatey