Author: Brian Ashworth April 27, 2015

Do you need a separate solar hot water system?

Could you just heat the water by electricity generated by your Photo Voltaic (PV) solar cells?

I have carried out a desktop exercise to help you review the options.

Solar Water Heaters

There are now hundreds of thousands of solar hot water heaters installed around Australia so the technology is well understood.

One of the main reasons for their choice is they are fairly simple.

Also they are very efficient, as around 60% of the sunlight that falls on the panel is converted to usable heat.

This efficiency has meant that the panel size can be kept relatively small.

 

Direct Heating with PV Panels

Efficiency of Solar Photo Voltaic (PV) is much lower than solar hot water at around 20%.

To provide comparable direct heating using PV panels and an immersion type heater will need an area of panels larger compared with a solar hot water heater.

How Many Panels Will Be Needed

The equation for calculating the power to heat water is:

kW hours = litres  x temperature rise in degrees centigrade / 853

Assuming the incoming water will be around 16ºC you are going to need to raise the temperature to around 60ºC

for 200 litres of water the equation becomes:

kw hours = 200  x  44 /853  =  10.3 kwhr.

1,000 watts of panel on average generates around 3.5 kw hours per day .

So to provide hot water by PV panels you will need around 3,000 watts, a considerable area, and expense.

 

Using A Heat Pump With PV Panels

One way of reducing the area of PV panels needed for water heating is to install a heat pump.

The most common hot water heat pump systems extract heat from the air.

Efficiency of heat pumps does vary between winter and summer  but an overall Coefficient Of Performance (COP) is typically around 3.

That is for every 1 kW of electricity 3kW of heat are produced. (300% efficiency)