Size your solar system

Find out what size rooftop solar system you can get to suit your electricity use and budget and that makes best use of your roof.

The right size rooftop solar system for your home or business depends on the:

  • available sunny roof area on your property
  • amount of electricity you use and when you use it
  • price you pay for electricity and the feed-in tariff
  • the amount of sunshine in your climate region
  • your budget.

Working out the best system size can be complicated. Solar retailers and installers usually use specialist solar quoting software to determine a suitable system size and design, and estimate the projected savings. Talk to your solar retailer or installer about the accuracy of these estimates and ask them what assumptions their calculations are based on.

SunSPOT

SunSPOT solar and battery calculator

Get an estimate of a suitable rooftop solar system size for your home or business needs.

SunSPOT is a not-for-profit solar calculator built specifically to help householders and small businesses with reliable, free estimates.

SunSPOT will provide a guide to the installed system cost, projected annual bill savings, emissions avoided and the payback period. You can add a battery and find out how it will impact the savings and the cost.

Get your solar estimate. Have an electricity bill handy to improve the accuracy of results.

System size refers to the total capacity of the panels

The size of a rooftop solar system refers to the total power-generating capacity of all the solar panels, measured in kilowatts (kW).

The system size depends on the number of solar panels and the rated capacity of the panels.

System size is measured in kilowatts (kW). One kilowatt (1 kW) = 1000 Watts.

For example, a typical home solar system might include 19 x 350 Watt panels, so the system size would be 6,650 Watts or 6.65 kW.

Inverter sizing

In many systems, the inverter is sized to be smaller than the panel output. For example, a 6.6 kW solar system is often paired with a 5 kW inverter. 

Because the panels are only rarely generating at their full rated capacity, this can be a good way to get the best value from the inverter and often makes good economic sense.

Talk to your solar retailer or installer about the inverter specifications for inverter to panel size requirements. If the system size (total rated solar panel output) is more than the inverter manufacturer’s specifications, you will not be able to access the Australian Government’s Small-scale Renewable Energy Scheme rebate.

The available sunny roof area

The amount of available sunny roof area can often be a limiting factor when deciding what system size to install, particularly for household solar systems in urban areas.

One residential solar panel is often around 1.7 m2 in area. A common 6.6 kW system might take up 29 – 32 m2 of roof space, depending upon the rated capacity of the panels. Panels can be installed in portrait or landscape orientation to make the best use of the available roof space.

Learn more about how your roof affects the design of your solar system.

Tip: You can find out how much sunny roof area you have available on your home or business, and the number of panels that can fit within it, with SunSPOT.

Your electricity usage

The more electricity you use, the bigger the solar system you need. The financial benefits of solar also depend on when you use electricity.

On your electricity bill, look for your 'average daily use' in kilowatt-hours (kWh). This is the total amount of electricity used divided by the number of days in the billing period (which is often 90 days).

On average, Australian homes use 11–23 kWh per day.

The average daily usage for your home or business is probably different in summer and winter. You can check your average daily usage for each season by looking at your quarterly or monthly electricity bills.

Tip: Your solar system will generate less electricity in winter than in summer, so it is important to consider how much electricity you use at different times of year.

Your electricity bill will also show how much electricity you use during peak and off-peak times, if you have a time of use pricing plan.

Use smart meter data for the most accurate estimate of your electricity usage

A smart meter records electricity usage in your home or business every 15 or 30 minutes. This gives an accurate record of how much electricity you’re using and when you’re using electricity.

If you have a smart or interval meter, ask your distribution network service provider for 12 months of interval data. Learn how to ask your distribution network service provider for your interval data.

You’ll find the name of your distribution network service provider under ‘Faults and Emergencies’ on your electricity bill. You can also find the distributors in each state and territory listed on the Australian Energy Regulator’s website.

Tip: Add your bill information or interval meter data to the SunSPOT solar and battery calculator to get an estimate of a suitable system size.

Consider your future electricity needs

A rooftop solar system will last 20 years or more, so you should consider your future electricity needs when buying or upgrading your system.

Your electricity usage may increase if you:

  • buy more electric appliances
  • switch from gas to electric hot water, cooking and heating
  • install a pool or spa
  • increase your air-conditioning use
  • build an extension on your home or business
  • buy an electric vehicle.

These will all have a significant impact on your electricity usage. When getting quotes, let the solar retailer or installer know about plans like these.

Electricity pricing

The more you pay for electricity, the more you can save with solar. The type of electricity pricing plan also has an impact.

For example, a bigger solar system may be cost effective if it:

  • reduces how much electricity you need to buy from your electricity retailer and that electricity is expensive
  • increases how much you export to the grid and your electricity retailer pays you a high feed-in tariff.

Read more about how solar pays for itself.

Learn more about electricity pricing plans.

SunSPOT

You can test the impact of different electricity pricing plans on your solar savings with the SunSPOT solar and battery calculator.

Have one or more recent electricity bills handy when you get your SunSPOT estimate. You can also use SunSPOT to explore what would happen to your savings if electricity prices go up or down.

The regional climate and annual sunshine hours

The climatic conditions in your region affect how much electricity your rooftop solar system will generate.

A 6.6 kW system in Sydney might generate, on average, about 26 kWh of solar electricity on a sunny day. In Brisbane it could be 28 kWh. In Hobart where there is less annual sunshine, it’s likely to be closer to 23 kWh.

If you are in an area with less hours of sunshine, you may need a bigger rooftop solar system to meet your needs.

Very hot temperatures can also lower the generation of solar systems marginally, but the impact is less important than the amount of sunshine falling on the panels.

Network limitations

In most areas there are limits on the size of the rooftop solar system inverter that can be connected to the grid and/or the amount of electricity that can be exported to the grid from rooftop solar. These limits are set by the local distribution network service provider, the company that owns and operates the network infrastructure, poles and wires.

The limits often depend on the phase of your electricity connection. A single-phase supply provides 240V electricity to your property through 3 wires. A three-phase supply uses 5 wires and provides more electricity to run more or larger appliances. Read more about phases.

A connection limit restricts the size of the inverter that can be connected to the grid. If the connection limit is, for example, 10 kW per phase, you could connect a 10 kW inverter if your grid connection is single-phase. If you have a three-phase connection you could install a three-phase inverter up to 30 kW.

An export limit restricts how much electricity you can send to the grid. This may be applied to each phase of your grid connection and may be lower than the connection limit. If your inverter is bigger than the export limit, the installer will need to set up export limiting which may add to the system cost.

For example, the distribution network service provider may allow a 10 kW single-phase inverter to be connected, but with the export limited to 5 kW. Any excess generation will be curtailed (wasted).

Some distribution network service providers have dynamic export limits, which only restrict solar exports at times when the grid is under pressure from high solar exports. This reduces how much generation is curtailed.

To learn more about the network limits in your area you can:

Get an independent system size estimate with SunSPOT

The not-for-profit SunSPOT solar and battery calculator can provide you with a free rooftop solar system size estimate.

SunSPOT was developed by photovoltaic (solar) engineers from the:

The Australian Government is a key partner in the SunSPOT project.

Unlike quotes from solar sales companies, a SunSPOT estimate does not make recommendations about brands or models of solar panels, inverters or batteries. Nor will it share your details with third parties.

You can get an instant estimate by answering a few simple questions. You can add more details from your bill or upload interval meter data for an even more accurate estimate.

Sunspot will take into account:

  • your available roof area, and any shading impacts
  • how much electricity you use, and when
  • the price you pay for electricity and your feed-in tariff
  • your region’s climate
  • your budget.

SunSPOT has been purpose-built by solar engineers at the University of NSW to help householders and small business owners get independent solar and battery estimates and advice.  

A SunSPOT estimate will reveal:

  • a suggested system size
  • a guide to the installed system cost (including Australian Government rebates)
  • how many panels can fit on your roof
  • shading impacts of trees and buildings
  • estimated annual bill savings
  • likely payback period
  • carbon emissions avoided each year
  • the cost and savings when adding a battery to the system.

Get your SunSPOT estimate.