General information on solar water heating

Solar water heating is the use of solar energy to heat the water supplied to your house to provide your home with the majority of its sufficient hot water for all your living needs.

Water heating typically makes up 30% of your household energy bill. The good news is that many homes in Nelson are in reliably sunny areas – well-placed to harness the free energy of the sun for solar water heating. Using the sun's energy to heat your home can make a good long term investment and help reduce energy consumption.

Solar water heating can:

  • Lower energy bills
  • Lessen your impact on the environment
  • Add value to your home

Lower energy bills

A well designed and installed solar water heating system will heat 50-75% of your hot water every year. In summer you may be able to get all your hot water from your solar water heating system. In winter, or on less sunny days, you’ll need some extra heating from your ‘booster’ gas, electricity or wetback supply.

How much money you save from solar water heating will depend on a number of factors. These include how much hot water you use, how well the system is designed to meet your needs, and the quality of the installation.

For an average household an effective solar water heating system will:

  • Cut between 1,800 kWh (kilowatt hours) and 2,700 kWh from your annual electricity use
  • Save between $350 and $500 a year, depending on the cost of your electricity or gas supply

Good for the environment

With a solar water heating system you’ll also be helping to reduce New Zealand’s dependence on non-renewable energy sources.

Each household solar water heating system is estimated to save an average of 1.26 tonnes of greenhouse gas emissions per installation a year, compared to using a standard electric hot water tank. (Assuming an emission factor for water heating of 0.56 t CO2-e/MWh, - sourced from EECA)

Add value to your home

As energy efficiency becomes an increasingly hot topic in New Zealand, installing solar water heating may add to your resale value. It also protects you against potential energy price rises in years to come.

How solar water heating systems work

Components of a solar water heating system

The main components of a typical solar water heating system include:

  • The solar collector
  • The storage tank (cylinder)
  • The circulation system
  • Controller or timer

In a packaged system all the components are chosen for you to ensure your solar water system is as efficient as possible. It's important to look at how a whole packaged system performs rather than just the individual components.

How the components work together

The collector absorbs the sun's energy and this heat is transferred to the water in the storage tank (or cylinder) by the circulation system. How efficiently this is carried out depends on the performance of each component and how well they are matched together as a system.

The size of the hot water tank in your system needs to be matched to your hot water usage and the size of solar collectors on your roof. If your tank is too small or too big you will be paying extra to boost the temperature of the water.

A properly insulated system (pipes and tank) is also very important. This helps to reduce the amount of heat lost while the hot water is being circulated around your house.

Timers, thermostats and controllers are other important components that contribute to the efficiency of your system. These, for example, will ensure that the backup heating is not heating water that would otherwise be heated by the solar heating system. While doing this, they'll also ensure there's enough hot water available when it's needed.

Solar collectors

There are two main types of solar collectors for solar water heating systems: flat plate panels and evacuated tubes.

A flat plate panel looks similar to a skylight. It absorbs sunlight and transfers the heat into the water (or heat transfer fluid) flowing through the collector panel. A typical house would have around 1m2 of collector per person in the house.

An evacuated tube system is made of a series of glass tubes between 1.5m and 2m long that slope lengthwise up and down the roof.

Usually solar collectors are positioned on your roof. However, an in-roof installation option can be available. In-roof solar collectors are put into the roof in a similar way to a skylight. It is also possible to mount solar collectors at ground level, which can reduce the installation costs.

Storage tanks

In solar water heating systems, the storage tank can be part of the system on the roof or set up in another area of the house.

You can use a conventional hot water tank or a specialist solar water heating tank. Specialist tanks have extra water and thermostat connections. This type of tank is specially designed to maximise the use of solar energy. Your system is likely to perform better with one of these.

Historically, domestic hot water tanks were around 180 litre capacity. This is generally too small for a solar water heater to achieve optimal performance for a household of three or more people.

Open or closed loop

Open loop systems circulate water through the collector panels on your roof, heat it and return it to your hot water tank.

With closed loop systems, a heat transfer fluid (usually a mixture of water and glycol) circulates through the collector panels. This fluid passes through a heat exchanger in your hot water tank, heating up the water. The heat transfer fluid and the water in your tank don't mix.

A closed loop system could be a good option if you have hard water. Check your kettle - if it has a build up of minerals on the element, you have hard water. In an open loop system these mineral deposits can build up in the collector over time and reduce the performance of the system.

Talk to your supplier or installer about which system is best for you.

Circulation system

Water or fluid can be circulated around a solar water heating system using a pump. Or it can be circulated naturally, in what's called a 'thermosiphon' or 'passive' system.

In these systems, the storage tank is located above the collector panels, usually on the roof. Cold water or fluid moves down from the tank into the bottom of the collector panels. Once it's heated by the sun it naturally rises to the top of the collector panel and back up into the tank.

Systems that use pumps to circulate the water or fluid are often called 'active systems'.

Thermosiphon circulation doesn't require a pump and isn't dependent on electricity. A pump system does require electricity to work.

With a pump system, the hot water tank can be positioned below the collector panels. This can be helpful if you want to use an existing hot water tank, or if you don't want to see the tank on the roof.

Controllers must be used to turn the pump on when there is enough solar energy available and off when there is not.

Controllers

A controller manages the use of supplementary or 'booster' gas or electricity in solar water heating systems. If there's a pump, it controls this also. The control of the supplementary heating can greatly affect overall system performance, so it's important to have the controller set up right.

A controller will ensure that you'll never be without hot water, regardless of the time of day. Talk to your installer or supplier about the type of controller you need.

Frost protection

In frost-prone areas you'll need to ensure your system has frost protection. Talk to your supplier or installer about what method is appropriate for the system you choose.

Choosing the right system for you

To get the most out of your solar water heating system, it's important that you choose a size that's right for you, that it's well-installed, and that the components work well together. The Energywise programme out of the Energy Efficiency and Conservation Authority (EECA) has great tips on comparing product quality, installation, and maintaining your new solar water heating system.

Visit the Energywise website for more information

Thanks to EECA and Energywise for some of the content on this page.