How will your solar water heater work throughout the year

This is a question we get asked a lot. As long as you have a well set up system, its performance throughout the year is dependent upon the sunshine it will see. So how much sunlight does it see? Below is a solar radiation map showing average energy across the different regions of New Zealand over a 1 year period.

NZ Solar Irradiation Map

This is the data Mirak uses to size a solar water heater for where you live. So what do the numbers mean? The figures above are the amount of energy that falls on each square metre of ground, on average, each day in New Zealand. The units are Mega Joules per square metre per day. 15MJ/m2/day is equivalent to 4.17kW/hrs/m2/day. kW/hrs (kilo watt hours) are the units on your electricity bill. (1 unit = 1 kilo watt hour.)

Because these numbers are an annual average, they are very good for sizing systems, but they do not tell you what will happen in the middle of winter or in the middle of summer. In summer the solar energy will increase to around 6kW/hrs/m2/day. In Winter the solar energy will decrease to around 2 kW/hrs/m2/day. This means there will be differences in the temperature of the water in your hot water cylinder across the seasons. A well set up system, sized to provide you with the correct amount of hot water will heat your cylinder to the following temperatures on a clear sky sunny day:

During mid summer (December January February) – 70 to 80 degrees C

Autumn & Spring (March April May – September October November) – 65 degrees C

During mid winter (June July August) – 45 to 50 degrees C

These figures are entirely dependent on how much hot water you actually use, this is why correct system sizing is so critical.

These figures come from systems we have measured the performance of over time. Below is a temperature log from one of our installed systems. This measurement was made at the bottom of the cylinder. The cylinder was running on the top of two elements so there is no electric heating in this graph.

Cylinder temperature log

Logging the performance of your Mirak system is currently part of our standard installation procedure.