Energy

Designing homes to conserve energy and use it efficiently, from sources that cause least environmental harm.

Photovoltaic (PV) systems

Photovoltaic systems (PV systems) absorb sunlight and convert it into electricity. Most PV installations are 2-4kW sized grid-tied systems that have no batteries and sell their surplus electricity to the retailer.

On this page

  • Advantages and disadvantages
  • Configuration
  • Capacity
  • Maximising sunlight absorption
  • Types of solar cell
  • Efficiency
  • Array frames
  • Electrical connections
  • Consents and permits
  • Insurance
  • Costs
  • Warranties

Advantages and disadvantages

Advantages of photovoltaic systems are that they:

  • can provide a sizeable amount of electricity
  • are quiet
  • are non-polluting
  • have low operating and maintenance costs
  • contribute to reduction of greenhouse gas emissions.

Disadvantages of photovoltaic systems are that they:

  • have high initial capital costs
  • electricity production is dependent on availability of sun
  • electricity production may not match when household requires electricity requiring either batteries to store electricity (especially if there is no grid connection and/or an agreement with your electricity retailer to purchase surplus electricity from you).

Configuration

A photovoltaic array is made up of solar PV panels that contain solar cells. The cells consist of layers of semi-conductor material (typically silicon), generally sandwiched between glass and another robust material and are sealed against moisture.

Solar radiation striking the cells cause electrons to move between the semi-conductor layers, creating an electric current. Cells are connected to produce a voltage output from the panel.

Capacity

The electricity generation capacity of photovoltaic panels is measured in Watts peak (Wp), which is the panel’s power output rating under standard test conditions.

Panels come in output capacity sizes up to 350 Wp and can be configured in any array size. An array of panels with a 2,000 Wp rating may produce between 4 kWh and 10 kWh per day on sunny days with good solar gain (New Zealand households use an average of 22 kWh of electricity per day). Residential installations range from 500 W to 10 kWp with most between 1–5 kWp of output.

PV systems should ideally be considered for use in conjunction with other options, such as solid fuel heaters for space heating.

Maximising sunlight absorption

The capacity of any given photovoltaic system is directly proportional to the amount of sunlight absorbed, which depends on these factors:

  • Solar irradiance – This is generally higher at more northern latitudes, in summer, in clearer air and when there is less shading. Avoid shading – shade on even a single cell can disproportionately affect the power output of a panel. Photovoltaic cells can still generate electricity in cloudy conditions, though at a lower output.
  • Solar panel area – Approximately 1 kWp requires 5–17 m2 of solar panel, depending on type.
  • Solar panel orientation – In New Zealand, the sun follows an arc to the North. Solar panels should, in general, be oriented to the North. It may also be necessary to change the orientation because of shading, aesthetic reasons, lack of available space or poor building orientation. Facing the panels away from true North will result in a drop-off in performance. To assess the significance of this reduction, check here.
  • Solar panel tilt angle – The tilt angle is the angle of the solar panels to the ground. For a grid-connected system that aims to generate the maximum amount of energy on an annual basis, the tilt angle should be at the local latitude minus 10º. Off-grid systems are usually designed to maximise output in winter when power need is greatest, so tilt angle should be local latitude plus 10º. Some systems allow the tilt angle to be adjusted maximise efficiency throughout the year. Where the PV array is oriented away from North, a lower tilt angle may be more effective and this will extend the time the panels receive sunlight.

The NIWA webtool solarview provides a convenient way to examine the available solar resource. This webtool can produce a skymap for locations around New Zealand, providing estimates of the solar energy available for different times of the year as well as direction and tilt.

 

Tilt angle 
Tilt angle

The tilt angle is the angle of the photovoltaic panels to the ground. The ideal angle will be different in summer and winter, and depends particularly on how the system is used.

 

Types of Solar cell

There are two common types of solar cell panel:

  • Crystalline silicon solar cells have a solid silicon wafer as the semi-conductor. The cells are sandwiched between tempered glass and a backing of tough ethylene vinyl acetate (EVA). These cells are protected from moisture. They need to remain cool as their output efficiency can drop by about 0.5% for every degree Celsius above a standard test temperature of 25ºC. They typically incorporate a gap of approximately 100 mm behind the panels to allow for cooling.
  • Amorphous silicon thin film solar cells have silicon in a thin film as the semi-conductor. The silicon thin film is deposited on a low-cost substrate such as glass or a thin metal foil. The coating on top may be a flexible material (as opposed to glass), and they may use a flexible mounting system. This type of cell is generally cheaper. They are being developed for integration with materials so they can be part of the building fabric.

Array frames

Array frames allow the solar panels to be tilted to the optimum angle for receiving solar energy. They can be: 

  • fixed (permanently oriented in one direction, frequently at the roof angle)
  • adjustable (so the orientation can be changed to suit the time of year)
  • tracking (which move to follow the sun).

Tracking array frames are normally controlled by an electric motor or a refrigerant gas. They are designed to provide more electrical power output throughout every day of the year (there may be some power used to provide the tracking, but this will normally be less than the additional power output obtained). However, they work better in dry desert climates than in New Zealand’s wetter and cloudier climates. They are more expensive than the alternatives, require more maintenance and may be less reliable. Therefore, it is almost always more economical to use more array frames to increase power output than to employ tracking frames.

The array frame must be installed to ensure it: 

  • meets wind and seismic loading requirements
  • is isolated to prevent electrochemical corrosion with different metals in the solar panels or the building fabric – New Zealand metal roof manufacturers specify a 100 mm gap so that panel installations allow for roof washing and do not void roof warranties
  • allows adequate airflow behind the panels to provide cooling – approximately a 100 mm gap for crystalline silicon panels.

While PV panels in array frames are still the most popular option in New Zealand, there is now another choice. With building-integrated photovoltaics (BIPV), the roof cladding, wall cladding etc. have solar cells built into them. Fully integrated systems act as solar panels and also provide the required building element functions of strength, weathertightness and durability. The first building elements with this feature used in New Zealand homes have been roof tiles.

Electricial connections

The output of a PV panel is DC electricity.  DC electricity needs to be converted to AC electricity before it can be used within the house or sent back into the electricity grid. DC electricity is converted into AC electricity by a device known as an inverter. An inverter used in PV systems also include additional control functions as well.

The wiring between the PV array and the inverter needs to be isolated from other household wiring and needs to be in its own trunking.

Consents and permits

Consents for installations may be required if the roof is penetrated or if an installation may have an impact on a neighbour’s property. This is at the discretion of the local council. The property owner will need to apply to the local lines company to allow the solar PV system to be connected to the grid.

In the case of grid-connected systems, in order to receive money for the surplus generation, an agreement needs to be reached with the electricity retailer. Systems must meet the requirements of AS/NZS 4777.2:2015 Grid connection of energy systems via inverters – Part 2: Inverter requirements and AS/NZS 5033:2012 Installation and safety requirements of Photovoltaic (PV) arrays.

Stand-alone power systems must meet the requirements of AS/NZS 4509 and battery installations must meet AS 4086 as well as NZS 4219.

The design and installation of the system should be carried out by skilled tradespeople to ensure safety and energy-efficient outcomes, and work must meet the requirements of AS/NZS 3000:2018 Wiring rules.

Insurance

Insurance coverage is as for all other electrical equipment, although the building owner may want to check the limits of coverage with their insurer.

Costs

The costs of PV systems have dropped enormously in the last decade, but they still need to be located, designed and installed properly to show an acceptable financial return. More information about financial returns of PV systems can be found in the BRANZ Factsheet Sustainable Construction #2 Photovoltaic (PV) design.

The University of Canterbury’s EPECentre has developed an online calculator that tells consumers whether a PV system will be cost-effective for them. It considers geographic region, roof slope, current electricity use, the cost of a PV system and other factors. It then gives an estimate of the years it would take for the cost of a PV system to be repaid, and the earnings or losses that would be incurred. It can be found on the Energywise website.

In remote locations, the cost of connection to the electricity distribution grid (which can be between $20,000 and $25,000 per kilometre) makes photovoltaic systems more immediately economic.

Monitoring

Many PV systems allow remote monitoring to provide feedback on performance and to provide alerts if something goes wrong. Monitoring can be integrated with various smart devices such as smartphones, tablet computers and PCs. Usually, the systems rely on wifi and broadband.

 

Updated: 07 June 2018