When going solar, the last thing you want to do is to under-size a system leading to situation that does not deliver according to one's expectations. The table below provides a guideline based on potable hot water consumption. A soft science approach is to start at the temperature that one washes or showers at being 40 ºC. A good 5-minute long, high pressure shower at 20 litres per minute amounts to 100 litres per person at 40 ºC . Two people will therefore use 200 litres. In order to get 200 litres of 40 ºC hot water, a solar water heater of 150 litres will provide this but it needs a Q performance of at least 17.5 to 20.5 at 20MJ /m2 per day.
|System & Household Size||Daily Requirement of Hot (40 ºC), Potable Water|
|Thrifty Usage||Average Usage|
|People:||140 litres||200 litres|
|SWH System Size:||150 litres||‘Q’ Factors Required:||17.5||20.5|
|People:||210 litres||300 litres|
|SWH System Size:||200 litres||‘Q’ Factors Required:||25||30|
|People:||280 litres||400 litres|
|SWH System Size:||200 or 300 litres||‘Q’ Factors Required:||34||40|
|People:||350 litres||500 litres|
|SWH System Size:||300 litres||‘Q’ Factors Required:||42||50|
|People:||420 litres||600 litres|
|SWH System Size:||300 litres||‘Q’ Factors Required:||50||60|
Check with your supplier both the size and the power output of the system at 20 MJ. Too small or not powerful enough and you will always be using the electrical back up, and will be disappointed by the electrical savings. This is one area where size does matter!