A Guide to Solar Water Heating

While a solar water heating system can help to drive down your power bills, you’ve got to keep in mind that it’s going to be a long-term investment. Canstar explores all you need to know about solar water heating.

For most households, water heating makes up a significant share of electricity costs. One option to lower your power bill is to switch to a solar water heating system. However, the initial investment can be substantial. So it’s important to weigh up the short-term costs and long-term benefits when deciding what’s right for your household.

According to the Energy Efficiency and Conservation Authority’s Gen Less website, water heating accounts for around 30% of typical household electricity usage. So if your power bills are climbing, or your current system needs replacing, it’s certainly worthwhile considering solar.

In the following guide, we look at how solar water heating systems work, the types of homes they’re suitable for, what to keep in mind when shopping for a system and the costs involved.


A Guide to Solar Water Heating. In this article we cover:


Solar water heating fundamentals

At a fundamental level, solar water heating harnesses the sun’s energy to heat water. Solar water heaters are available in a variety of designs and sizes, which are suitable for different types of homes and climates.

According to the Solar Association of New Zealand, most solar water heating systems available in New Zealand have the same basic components:

  • Collector – this absorbs sunlight and collects heat. Two types of collectors are commonly available: evacuated tube collectors, which comprise long thin glass tubes with a vacuum layer between the solar absorber and the outside air. And flat panel collectors, which usually comprise a selective surface absorber for water heating.
  • Hot water storage tank – commonly referred to as a cylinder. In most systems, a controller measures the temperature of the water in the cylinder to ensure it reaches its optimum operating temperature.
  • Means of moving water around – most systems in New Zealand are pumped (active) systems, which use a small circulating pump to transfer heated fluid from the collector to the cylinder.
  • Supplementary heating controller – this acts as back-up in the event solar alone cannot cater to a household’s hot-water requirements. It’s either electric, gas or uses a wetback booster.

Collectors typically use two types of fluid. Either water, which is used in a direct system that heats the water that flows out of your taps. Or a diluted glycol fluid, in an indirect system. This fluid is then pumped between the collector and a heat-exchanger in the cylinder to heat your tap water.

The cylinder may be incorporated as part of a system on the rooftop, or located separately (often referred to as a split system) at ground level.

tap dispensing hot water with steam

Is a solar water heater right for your household?

Gen Less advises that solar water heaters can potentially meet 50% to 75% of a household’s hot water needs. As an initial step, it’s worthwhile running the numbers on the different water heating options for your household.

There are a range of online calculators available to help break down the costs. The water heating systems tool  on the Gen Less website is a good starting point.

In assessing whether a solar water heater is right for your household, the following are among a range of factors that will determine if the numbers add up:

  • Hot water usage – how much hot water does your household use, and when it is being used. It’s important to choose a system suitable for your household’s needs.
  • House build – ideally panels should be positioned in a north-facing direction. Also, it may be necessary to strengthen your rooftop (particularly for systems that incorporate the tank on the rooftop).
  • Rooftop positioning – it’s important to have clear access to the sun. Obstacles such as neighbouring buildings or trees have the potential to impact a system’s performance.
  • Weather – areas with greater sunlight will naturally provide an increased opportunity to gain benefits from solar technologies.

With regard to water usage, it’s also important to consider how different water efficiency measures can complement the operation of a solar water heating system and reduce a household’s hot water demand.

For instance, households should also look to the benefits of installing water-efficient taps and showerheads, along with water-efficient appliances, which can help to drive down water usage.

How to choose a solar water heating system

There are a range of solar water heating options available. So shop around and consult with experts to determine which system is best for your household.

The Smarter Homes website advises that when choosing a system you keep in mind:

Type of collector

Either an evacuated tube or flat panel collector (experts will be able to advise on the efficiency of each and the best option for your household).

System size

Determined by your household’s demand for hot water. The Solar Association recommends the cylinder hold approx 75l of hot water for every 1m² of collector area. Typical installations use 1m² of collectors for every person in the house.

Booster

To keep the water hot when the sun isn’t shining. Electric, gas and wetback boosters are the most common types.

Pump or no pump

Active pump systems (reliant on electricity) allow for the cylinder to be located below the collector panels, with the pump acting in conjunction with a controller. Passive thermosiphon systems (no reliance on electricity) use natural circulation. For these the cylinder is located above the collector panels.

Controller

To manage the use of gas or electric booster heating, along with the pump, in pump systems. Two types of controller are available: a time trigger or minimum temperature controller.

Open-loop or closed-loop

Open-loop systems heat the water directly, while closed-loop systems heat the water indirectly via a fluid (usually a mixture of water and glycol).

Cylinder

Unlike conventional hot water cylinders, specialist cylinders are designed to maximise the use of solar energy.

While it may be possible to use a household’s existing cylinder as part of a solar hot water system, it’s important to ensure it’s up to the task. The Solar Association advises that packaged systems are properly designed to deliver optimal energy performance.

While solar water heating systems typically require little maintenance, some regular maintenance is required. So ask the experts about the specific maintenance requirements of different systems and the potential costs involved.

When it comes to the legal requirements of installing a system, a building consent is required. Further information is available at the Building Performance website.


If you’ve an electric hot water cylinder, the amount you’re paying for your electricity will have a big impact on its running costs. And one of the simplest ways to slash your water heating bill is to switch to a cheaper provider. Canstar Blue’s latest review of NZ power companies compares them on customer satisfaction and value for money. The table below is an abridged version of our full results. For more details click on the big button at the bottom of the story!

See Our Ratings Methodology


What’s the cost?

The total costs of solar water heating systems can vary dramatically, according to the type of technology used and the size of the system. Plus, there will be other costs specific to your installation requirements (such as strengthening a rooftop).

While the costs of a solar water heating system (which, including installation, might range from around $4000 to $10,000) are likely to be higher than that of an electric system, it’s important to keep in mind the lifetime running costs. Try using tools such as this water heating systems tool.

In this respect, a solar water heating system is very much a long-term investment. So it’s important to weigh up the short-term costs and potential long-term savings, and to consult with experts in determining what will be the best path forward for your household.


If you’re thinking about changing or updating your water heating system, it pays to double check if you’re on the best electricity plan for your family. And if you’ve received a big electricity bill recently, read up on what to do about it with our story: Have You Received A Shock Electricity Bill? Here’s What to Do!

Compare electricity providers

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