Solar panel overshadowing calculator

1. Gather the essential information

• 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.

Note:

• 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.

To work out what your roof angle is, there are apps available online that can calculate this via a smart phone or computer, for example, https://pitchgauge.com or https://www.ursupplier.com/tools/angle_measurement/

Alternatively, your installer may have noted this on your original quote. If not, you could also take a photo of the cross section of the roof and use a protractor to calculate the roof pitch.

3. Fill out the information in the ‘Yellow’ coloured boxes of the calculator

• 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.

5. Number of panel rows – either 1 (single row) or 2 (two rows)

6. Orientation – what compass direction do your panels face?

To check, look at your property on a map, refer to a mapping website such as Google Maps, or use a compass.

7. Solar panel technology type:  Select what matches your system

• 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.

If you don’t know, and you cannot ask your supplier, select standard solar rooftop system.

8. Set back angle

Estimate the distance from the bottom of the roof to the bottom edge of the solar panels, as shown on the diagram below. If the panels extend to the bottom of the roof, please enter zero. You may be able to measure this or estimate it by looking at a map or using measuring tools in aerial photography such as Google Maps or Google Earth.

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]

This is the distance from the bottom edge of the solar panels to the overshadowing building. You may be able to measure this or estimate it by looking at a map or using measuring tools in aerial photography such as Google Maps or Google Earth.

Note that this distance may not correspond directly to a building 'setback' due to the solar panel position on the roof.

11. Left distance from system edge and right distance from system edge

Estimate how far the overshadowing building extends beyond the left and right edges of the solar system. If the solar panels continue beyond the edge of the overshadowing building, this distance should be recorded as negative (see example below, where the solar panels extend 2 metres beyond the edge of the overshadowing building).

Note that the calculator assumes that the overshadowing building is 'box-shaped'.

12. Usage on your bill

This number can be found on your electricity bill and is often referred to as the average daily usage (Use this figure).  

If the bill just shows a total amount of energy rather than a daily average, you need to divide this by the number of days in the billing period to get the daily average. In the example bill below we divide 893.52 kWh by 92 days = 9.7 kWh per day.  This input affects the assumption on how much solar generation is used onsite versus exported to the grid.

13. Import tariff

This is the rate that you pay for electrical energy from the grid, charged in cents per kWh. The calculator only works with a single number for this input, so if you have been charged different rates for different amounts of energy (e.g. on a time-of-use tariff), please enter a rough average tariff representing what you paid overall.

14. Supply charge

This is an amount you are charged every day, regardless of how much energy you import from the grid.  

In the example below the amount is 116.95 c/day, however this retailer granted a special discount of 8.71 c/day, so the effective supply charge is 108.24 c/day. Applying GST (multiply by 1.1) and the 7% pay-on-time discount (multiply by 0.93)1 as above we get 110.7 c/day.

15. Feed-in tariff

This is a rate at which your retailer pays you for energy you export (feed in) to the grid, defined in cents per kWh. The example below uses 12 c/kWh. GST is not payable on this rate.

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.

Planning system standards covering overshadowing of rooftop solar systems are outlined under Clauses 54.03-5 and 55.03-5.

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,