

The
average daily hot water usage is 70 liters per person per day (this
can be scaled back somewhat for children). Therefore if five adults
live in a house it is possible that 350 liters of hot water will be
required per day.
However as the sun will not shine 100% every day even in summer, we design for additional storage to counteract this. Usually in the order of up to 50 %.
It is important not to oversize the storage as the mild disinfection chemical in the water supply can deplete in time and could cause illness.
To size a hot water tank for our five friends then is 350 liters x 150% = 520 liters (say 500 liters).


What Area Of Solar Panels Is Required? 


Now we know how much hot water storage and usage we require. We can calculate the required surface area of solar water heating panels.
The figures used here are particular to the Evacuative tube model which we supply.
By calculating the specific heat capacity of water, the volume of water and the delta T (temperature difference) we can derive how much energy is required in kilo joules.
4.187 * 500* 60 = 125610 Kj.( make any sense yet?)
In order to obtain a power quantity we have to consider time. Dividing this figure by 3600 will result in the number of kilowatt hours. 125610/ 3600 = 34.9 KWH.
On a good day the suns irradiance can reach over 10 KWH per square meter area.
However evacuative tube solar panels are amongst the most efficient
solar panels which are only 60% efficient. In short that means that
of the 10 KWH energy from the sun on one square meter only 6 KWH can
be transferred to heat the water.
Therefore by dividing our heating load KWH by our yield per square
meter KWH will give us our area of solar panels in square meters.
34.9/6= 5.82 sq meters of solar panel will be required 



