How big a solar hot water system do you need?
Obviously this will depend on how much hot water your family uses in a typical day.
If you use more water than your solar panels are designed to heat - then your electric or gas booster will be working overtime and your bills and CO2 emissions will be higher than they need to be.
If you use less water than the system is designed for - then your up front cost will be higher than it needs to be.
As with all Renewable Energy projects, you should always invest in efficiency before investing in generation. For every dollar you spend on reducing water usage, you will typically save at least $5 in solar hot water hardware.
So make sure that you install good quality low flow shower heads. The good ones feel as good as high flow ones by injecting air into the stream of water. Follow this link to see the kind of (Australian Made) thing I'm talking about.
Also invest in the most water (and energy) efficient dishwasher and washing machine you can afford. Also make sure you get a machine with a hot water inlet. Otherwise, you will just be heating cold water inside the machine with dirty old electricity!
Our sister site sokitt.com can help you find energy efficient appliances with hot water connections that are affordable... Give it a try!
Here's a table that breaks down the hot water use for a typical Aussie household (just click on the bars underneath to change household size):
As you can see, by investing in water efficient shower heads, taps and appliances, you can quite easily cut your hot water use by almost half. Which means you will only need a solar hot water system half the size.
As a rule of thumb, a relatively water efficient household is looking at a system size of 75 litres per person. A typical solar hot water panel is 2 square metres and will supply 150 litres. So a typical 4 person family will need a system sized at 300 litres which has 2 panels.
Having said that, many 4 person households who are very water efficient get by just fine with a 150 litre system with 1 panel. At the end of the day a good installer will look at your usage and advise you exactly what size to go for.