This calculator provides a ‘rule of thumb’ estimate of whether roof top solar panels will be significantly affected by shade from a neighbouring building.
The calculator estimates any changes in solar panel energy generation and the impact on electricity bills that might occur due to overshadowing.
This calculator can be used in a variety of situations, for example when:
- considering a new solar panel installation that might be shaded by an existing building
- considering a new solar panel installation and looking ahead to potential overshadowing from a likely future neighbouring development
- assessing if an existing solar installation will be shaded by a proposed neighbouring development.
Instructions on how to use the calculator are below.
Select and download the calculator below that best matches your rooftop angle
Using the calculator and interpreting the results
Follow these steps for guidance on how to use the calculator.
- Details of your solar panel system, such as number of panels, number of kilowatts
- Your energy bill
- Details of the overshadowing building
- if assessing the impact of a proposed development, check your council website for the detailed plans and permit application or contact the council for more information.
- if assessing the impact from an existing development, find an aerial image of your house and the neighbouring property. This can be sourced from Google Maps, Google Earth or other aerial image sources.
- the calculator assumes both properties are on flat ground and the overshadowing building is box shaped
- where the overshadowing property is at a higher elevation (slope) the overshadowing will be greater than the tool estimates.
2. Select the calculator that most closely matches the slope of your roof.
Select from the calculators on the main page above.
- Type over the pre-populated figures. These are just typical numbers included so the tool works in case you forget to add a figure.
- For help, hover your mouse above the cells showing a red mark to get more information.
- A grey triangle will appear when you click on some yellow cells. Click on this triangle and a list of options will be revealed. You’ll need to select the value that matches your situation most closely (these are pre-set options – the calculator won’t work if you try to input something else).
4. Size or capacity of your solar panel system in kilowatts (kW)
For example, a 4 kW system is equivalent to about 16 modern panels, or about 22 older pre 2015 panels.
- Standard rooftop solar system - the panels are connected to a wall-mounted appliance called an inverter.
- Micro inverter rooftop solar system – microinverters are located underneath each panel on the roof.
9. Height above panels (a)
This is the height from the bottom edge of the solar panels to the top of the overshadowing building.
The height of a building storey is generally around 3.3 metres, but this varies from just under this figure to well over it.
For situations where the overshadowing house is on much higher ground, select the next highest 'height above panels' option.
10. Horizontal distance from panels (standoff) [see (b) in diagram below]
Existing building setbacks in the planning scheme generally ensure about 70% of the yearly energy generation of solar panel systems is maintained when a neighbouring building is built. If this calculator estimates greater shading impacts on energy generation, a more detailed assessment may be needed by a building designer or solar energy system supplier.
Further information on the results of your calculation and what they mean are provided below.
The calculator results show the estimated annual solar energy generation and month by month figures for an unshaded and a shaded solar system, as well as the reduction in solar energy generation due to shading.
The results are shown as a table and a graph. Example:
Shading can be reduced by increasing the setback of the adjacent building, locating solar panels higher on the roofline (closer to the roof peak) or locating panels on a roof face that is less subject to shade.
You can test the results of any changes by re-running the calculator. The aim is to achieve design changes that result in bringing the red line as close as possible to the blue line, especially over summer when most sunlight is available.
Current planning provisions generally maintain 70% of annual solar system generation (compared to no overshadowing) due to setback requirements. If the calculator estimates a lower figure, the need for a more detailed assessment is indicated and a discussion with the developer, council or building surveyor might be appropriate.
This section shows the estimated impact that occurs due to shading on the day of the spring equinox, 22nd September. The equinox is the date when the day and night are both 12 hours long.
For most purposes the annual impacts of overshadowing provide the best guide as to whether this shading is something you should worry about. However, planning authorities sometimes refer to overshadowing impacts on the spring equinox day, so this information may be useful.
Estimates are provided showing the total solar generation in kilowatt-hours for both an unshaded and a shaded solar system. The percentage reduction in electricity generated is calculated too.
Planning schemes define setbacks between buildings.
Standard side setbacks are equivalent to an angle of 58.8 degrees created by line drawn from the bottom of the roof (with solar panels) and the closest part of the overshadowing adjacent building.
Where there is a north facing window, the setbacks applying to the new building are greater than standard setbacks and extend for a distance of 3 metres from the edge of each side of the window. The equivalent setback angle is 47.5 degrees
For development proposals that require a planning permit this is assessed on a case by case basis by the relevant planning authority, with reference to planning scheme standard A7 and B10 under Clauses 54.03-5 and 55.03-5 Energy efficiency protection objectives.
Where the development only needs a building permit, comparable setbacks apply.
If the Calculator estimates a set back angle greater than 58.8 degrees (or over 47.5 degrees where there is a north facing window – and the solar panels are located above the window) then more detailed assessments by the council planner or building surveyor may be necessary to check that the required setbacks are being achieved. The tool provides an estimate only as it is limited by the fixed number of standoff distances that can be selected.
Standard A7 and B10 specify (in part) that new buildings should be:
'Sited and designed to ensure that the performance of existing rooftop solar energy facilities on dwellings on adjoining lots in a General Residential Zone, Neighbourhood Residential Zone or Township Zone are not unreasonably reduced. The existing rooftop solar energy facility must exist at the date the application is lodged'
This also applies to overshadowing from buildings at zone boundaries (e.g. in a commercial zone) where the shadows are cast on existing solar systems in these residential zones.
Generally, a development should be designed to ensure at least 70% of annual solar energy generation of an existing system is maintained. After considering the other factors listed in this clause and the planning scheme, design measures should be applied to achieve better outcomes and reduced energy generation losses due to overshadowing,
Page last updated: 22/11/19