Selecting solar hot water size



Reducing electric boost


When you look at data for the CO2 emissions for solar hot water it is surprisingly high. Shouldn't it be zero?

The reason for the emissions is that the systems are supplied with a gas or electric boost. That means that if the water drops below 60oC because the sun doesn't shine, the boost will kick in. If you have a show at night, the boost will kick in overnight ans leave noting to be heated by the sun. So you have bought a solar hot water system, and you still have power bills!

If you install the correct size for your climate, you can turn the boost off.

If you buy the recommended sized solar hot water and leave the electric boost on, your solar hot water system will use a lot of electricity. If you buy an oversized system, turn the boost off and accept a temperature of 40oC, you will use none or very little.

Until now the law insisted on water being kept at a minimum of 60oC. This has recently changed to 40oC with periods at higher temperatures to kill Legionella. This has changed everything and will save a lot of energy.

The following are the areas / person of the solar collectors:

Hot inland : 0.5 sq m

Warm wet : 1 sq m

Average to cold : 1.5 sq m

Miserable most times : 2 sq m





Reduce the boost

When I had our solar hot water system installed the plumber left with the words "I've turned on the electric boost so it will keep the water hot during cloudy weather. Seemed reasonable.

But that night while I was showering I realised that my shower would leave the water cold, and the electric boost would heat the water overnight, leaving nothing for the sunlight to do next day.

So I turned it off, cut down an overhanging tree, and have only turned the electric boost on for 6 nights in a year.

The reasons for this working are: 

1)The system was over sized. We are empty nesters, so a 4 person system serves only 2 of us. We have twice the collector area recommended, nearly 1.8 sq M per person. The normal recommendation is 1 sq M per person.

2) The tank is large enough for 3 days of hot water, allowing the system to iron out fluctuations. (It needs to be better lagged and insulated.)

3) It does not bother us if the temperature drops to 40oC now and then. It spends enough time at 60oC to kill any Legionnaire's disease. The old systems were designed never to let the temperature to drop below 60oC. This required frequent boosts, and about 30% heat loss.

However the new regulations allow this and have changed the design requirement entirely. (See right hand column)







Legal temperature of HW

In Australia, prior to 2010, standards required a HWS to have a fixed set point of not less than 60C for the sake of control of Legionella. 

Since 2010 the new standards require that at least once in seven days the tank be heated to:

70oC for 1sec  or

66oC for 2 min;  or

60oC for 32 min;  or

55oC for 6 hr

Calculating collector size

If we want a shower at 40oC, then we need the water in the tank to be at least 45oC. Let us assume we need 50 Litres/Person/Day of hot water.

The Bureau of Meteorology, BOM, has wonderful data giving the sunlight energy (insolation) for every day, for every year, for every weather station. You can easily find what sort of fluctuations your system will have to deal with. There is no point in doing averages and calculations here. You can just look at the charts and rule your line to see where the insolation (sunlight) is too low. Then choose a collector plate and tank size that covers the fluctuations.

If you don't want to do all this, just put in a largish system. 1.5 sq m per person will be plenty for most places.

In the calculations below it appears we can generate hot water at 45oC with only 4.3 MJ/m2  of sunlight per day, if we have 2 sq M of collector per person..  

Looking at the Bureau of Met.'s readings of sunlight (insolation) for 2011 in Australian cities, we can see that the sunlight very rarely gets below that figure.  Our tank is 300L which is three times the capacity recommended for 2 people. It evens out any fluctuations.

So, the solar boost can be left off.


Sunlight needed to heat water

We are going to calculate how much heat is needed to heat water to 40 and 60oC. Then see how much sunlight is required to do this.

HW each person 50 L/day
People 1 Person
Cold water temp = Soil temp 15 oC
Hot water temp 45 Minimum
Area of collector 2 m2
Efficiency of HW collector 60%  
Latitude 34  
Sunlight required 45oC 4.3 MJ/m2/day BOM reading
Sunlight required 60oC 6.5 MJ/m2/day BOM reading


BOM readings are for a horizontal square metre. Assume the HW collector is angled at right angles to sun at solstice.

There is no allowance for heat loss. About 15%.

60% efficiency.

No allowance for excess heat stored from previous day. 
Cloudy days have diffused sunlight, but this can heat water. May give higher reading on BOM flat plate than collector.

Sunlight ​required - MJ/m2 / day

People Collector m2 Water at 45oC Water at 60oC
1 1 8.7 13
1 2 4.3 6.5

This table shows the dramatic effects of allowing the hot water to occasionally drop to 45oC, and doubling the collector plate area to 2 m2 per person.

When choosing a solar HW system you need to choose how much electric or gas boost you wish to provide and pay for.

In this case the lines have been drawn for water at a minimum of 45oC.  A system with 1.5 m2 per person would probably supply hot water without the need for a boost. 2 would allow a guest to stay occasionally. Alice Springs could use only 0.5 sq Metre.

A larger system will also have a larger tank which will help iron out fluctuations.


Sunlight minimum for some cities 2011.

Months with least sunlight chosen in each city in 2011.

To find data and charts for other locations and years, see BOM

To change location 

The horizontal lines represent the sunilght energy required for each collector plate size/person. E.g. in the first graph (Sydney), a 1 sq M collector will produce cold water on 5 days. But a 3 day tank will iron out these fluctuations.

Data -

Efficiency of solar hot water systems

Collector Flat plate Flat plate Flat plate Evac tube Evac tube
System Close coupled Split system Split system - working fluid Split system Split system - working fluid
sunlight -  3.2 kWh/M2/day 62% 68% 60% 42% 53%
sunlight -  6.5 kWh/M2/day 68% 78% 64% 44% 53%


Close coupled = tank on roof as part of collector.

Calculation of heat requir​ed for data in table above:  

Sunlight required in MJ/day/m2 =

(Litres/day)*(Number of people)*(Temp rise)*(J/kG/Deg=4,180)/((Effic. of syst = 0.6)*(Area of collector)*Cosine(Latitude)*1,000,000)