Saving energy with solar hot water

Zero Energy House

Every year, water heating accounts for 30% of the typical Kiwi home's energy use, equating to 3,200 kWh or a cost of around $765. An additional 30% of the energy is used for space heating¹. For the Zero Energy House, these percentages are different. Because the house has been designed to remain comfortable without the use of mechanical systems, water heating makes up a higher percentage of its occupants’ energy needs, around 45%.

However, instead of relying on energy from the grid to heat their water, Shay Brazier and Jo Woods, owners of the Zero Energy House, have turned to the sun. By installing a solar water heating system, they have reduced the amount of electricity required for hot water to a small fraction.

Planning for the solar hot water system

The first step in the solar hot water design was to understand how much hot water would be required by the house's occupants in order to size the system. As part of this process, Shay and Jo looked for low-flow water fittings and appliances that would increase efficiency while keeping its size and cost down. The showerheads that were chosen, for example, cut the water use of every shower by 40%, which in turn reduces the amount of hot water that needs to be heated. With showers being one of the biggest consumers of hot water in a home, this had a significant impact on the size of the solar water heating system.

After understanding their hot water needs, Shay and Jo had to choose the right type of solar hot water system for the house. Solar panels used for water heating are commonly known as solar hot water (SHW) collectors and there are two main types used in residential buildings: evacuated tubes and flat plate collectors. Both types operate based on the same basic concept: collector plates are heated by the sun’s energy and are insulated to minimise the amount of heat being lost back to the air. The efficiency of the panels is influenced by their placement, as well as by their ability to absorb and transfer heat to the water. Each of these systems has its own advantages. Evacuated tubes perform better in colder climates, while flat plate collectors offer more flexibility in their configuration and are more discreet. Because their absorber plate is insulated around the sides and bottom with mineral wool and in the front with a sheet of glass, flat plate collectors have the appearance of a skylight.

Left: Evacuated tubes solar hot water collectors. Right: Flat plate solar hot water collectors

The solution for the Zero Energy House

The resulting solar water heating system installed in the Zero Energy House has eight flat plate collectors placed near the ridge of the north-facing roof. The Kiwi-made collectors are fitted flush below the roofline and are laid out next to one another to run the full length of the roof.

Like most houses, the Zero Energy House stores its hot water in a cylinder. The difference, however, is that the majority of the water heating doesn’t take place in it. Instead, the system works by pumping water to the roof, where it is heated using the solar energy captured by the collector plates. Water is then returned to the hot water cylinder on the first floor of the house to be stored. A backup heating element inside the cylinder tops up the hot water on very cloudy days or when there is an unexpected increase in demand.

To pump water onto the roof and through the panels, the system uses a solar circulator pump. On cloudy days, or when there is less solar energy available, the pump operates at a slower speed to keep water on the roof for a longer period of time. In bright sunshine, water is heated more rapidly, so the pump speeds up to shift water around the panels faster. A variable speed pump such as this one can use up to 80% less energy than a comparable constant speed pump. And this is important because it reduces the house’s overall energy use, and therefore the size - and cost - of the photovoltaic system required to achieve Zero Energy.

A solar hot water controller adjusts the flow rate by changing the pump speed. How fast the pump runs is determined by temperature measurements taken in the cylinder and at the outlet of the solar collectors. It is constantly adjusted to keep the temperature difference between those two points at around 10°C. 

The system also includes a relative pressure sensor, which monitors water pressure and sends an alert if there are any leaks in the system.

Eight flat plate collectors with protective glass covering were installed along the ridge-line of the Zero Energy House

Predicted system performance

Shay and Jo’s solar hot water system is larger than most installed in standalone houses as they expected the system to supply around 80% of their hot water needs, helping them to reduce their electricity use between 35% and 40%. The chart below was created during the Zero Energy House’s design stage and shows the expected energy usage for hot water over the course of a year.

  • The yellow area is the maximum expected to be generated by the solar water heating system.
  • The red area is the portion produced by the backup heating element.


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