The Short Answer
For most Australian homes a 6.6kW solar system will be enough to cover their daily electricity use of about 20kWh. This is also the most common size that solar companies suggest.
Even though a 6.6kW system will probably be enough for your needs, if you have enough roof space, it might be a good idea to get as many panels as you can fit. Adding panels later can be harder than it seems and it can be relatively cheap to buy a bigger system since you are already spending the upfront amount for a 6.6kW
A More Detailed Answer
If the above generalisation is not enough for you, I don’t blame you. We are the Solar Nerds after all. If so, there is only one way to exactly answer the question, what size solar system do I need?
You’ll need to match the energy you use (consumption) with the amount of electricity you generate (generation). When doing this calculation, you can use averages for a simple answer, or you can get exact data for a more precise answer. We have explained both ways below
If you want to get into the weeds like this, we strongly recommend using the SunSPOT tool. SunSPOT was made by solar experts from UNSW and is a not-for-profit solar research tool.
Simple Step-by-Step Guide
The simple method leverages your electricity bills to calculate average energy consumption and uses average peak sunlight hours to estimate solar production. This approach is ideal for homeowners seeking a straightforward way to size their solar system without delving into detailed data analysis.
1. Gather Your Electricity Bills
a. Collect 12 Months of Bills:
- Obtain your electricity bills for the past year to account for seasonal variations in energy usage.
b. Calculate Average Consumption:
- Total Annual Consumption: Sum up your monthly electricity usage to get your total annual consumption in kilowatt-hours (kWh).
- Average Daily Consumption: Divide the annual consumption by 365 to find your average daily energy usage.
2. Determine Average Peak Sunlight Hours
a. Understand Peak Sunlight Hours (PSH):
- Peak Sunlight Hours represent the number of hours per day when solar irradiance averages 1,000 watts per square meter. In Australia, this typically ranges from 4 to 6 hours depending on your location.
b. Find Local PSH Data:
- Refer to resources like the Australian Bureau of Meteorology or regional solar maps to identify the average PSH for your area.
3. Calculate the Required Solar System Size
NOTE: This doesn’t feel simple. We know. SunSPOT is a free tool built by researches at UNSW. It can do this calculation for you.
a. Use the Solar Sizing Formula:
System Size (kW)=Average Daily Energy Consumption (kWh)PSH×System Efficiency\text{System Size (kW)} = \frac{\text{Average Daily Energy Consumption (kWh)}}{\text{PSH} \times \text{System Efficiency}}
System Size (kW)=PSH×System EfficiencyAverage Daily Energy Consumption (kWh)
- System Efficiency: Account for typical system losses (approximately 15-20%).
b. Example Calculation:
- Average Daily Consumption: 30 kWh
- PSH: 5 hours
- System Efficiency: 80% (accounting for 20% losses)
System Size=305×0.8=7.5 kW\text{System Size} = \frac{30}{5 \times 0.8} = 7.5 \text{ kW}
System Size=5×0.830=7.5 kW
Complex Step-by-Step Guide
For homeowners who prefer a more detailed and accurate assessment, the complex method involves analyzing granular energy consumption data and overlapping it with solar production data. This approach provides a precise sizing tailored to your specific usage patterns and solar potential.
If you are using this method, the SunSPOT tool will handle all of this for you, but we go into detail so you understand how it is doing what it is doing.
1. Obtain Detailed Energy Consumption Data
We’ve dedicated a whole article to explaining how to access your meter data, however in short you’ll need to log In to your electricity retailers online portal and download your meter data. The export of your energy consumption data will be in CSV format, typically available in half-hour intervals.
2. Gather Solar Production Data
Utilize sources like Australian Solar Resource Assessment, PV Watts, BOM to obtain detailed solar irradiance data specific to your location. You can also generate or access solar production profiles based on your geographical location, roof orientation, and tilt angle.
NOTE: If you do this through SunSPOT, it will automatically calculate the solar production data for you.
3. Overlay Consumption with Solar Production
a. Synchronize Data Sets:
- Time Alignment: Ensure that your half-hourly consumption data aligns with the corresponding solar production data.
b. Perform Overlapping Analysis:
- Match Production to Consumption: Identify how much of your energy consumption can be directly offset by solar production during each interval.
- Excess Production: Determine periods when solar production exceeds consumption, which can be fed back into the grid under Australia’s net metering policies.
c. Change system size to find the perfect fit
- Once you have this analysis, if you change the size of the solar array so it will cover more or less of your consumption. Depending on your goals the ideal scenario will be different.
- If you want to pick the best size system from a purely financial perspective, you’ll need to calculate the return on investment of each of these variations (you’ll need prices from the solar company to do this) and choose the one with the quickest return on investment.
NOTE: If you do this through SunSPOT, it will automatically overlay the consumption data with the solar production data for you.
Economies of Scale vs. Standard Recommendations
Maybe a simple 6.6kW system will be enough, or you’ve gone through the above and sized your system perfectly. Even still there are benefits to getting a larger system:
- Cost Efficiency: Bigger systems often have a lower cost per watt. Once you’ve invested in a 6.6kW system, it might be worth spending a bit more for a larger one.
- Future-Proofing: If you expect higher energy use in the future, a bigger system can save money in the long run.
- Energy Independence: Larger systems can work better with battery storage.
If you have enough roof space, consider these options:
| System Size (kW) | Number of Panels (400W each) | Approx. Roof Area Required (m²) |
| 3kW | 8 | 13.6 |
| 5kW | 12 | 20.4 |
| 6.6kW | 17 | 28.2 |
| 8kW | 20 | 33.6 |
| 10kW | 25 | 42.5 |
Battery Storage: Adding to Your Solar System
Adding a solar battery can store extra energy, but it is expensive and might not save as much money as you hope. Still, it has some benefits:
- Use More Solar Energy: Use more of the energy you produce and rely less on the grid.
- Energy Security: Batteries provide backup power during outages or at night.
- System Size Impact: A larger solar array is needed to store enough energy for evening use.
Common Misconceptions About Solar System Size
- “Bigger is Always Better”: While larger systems have benefits, they may not be needed for every home and can have higher upfront costs.
- Feed-in Tariffs Make Oversizing Worth It: With feed-in tariff rates going down, oversizing just to export extra energy often doesn’t make financial sense.
- Batteries Solve Sizing Issues: Batteries improve system performance but need careful planning. They are still expensive and might not be the best option if you only want to save money.
Conclusion
Figuring out what size solar system do you need means looking at your energy use, roof space, sunlight, and future needs. For most Australian homes, a 6.6kW solar system is the best mix of cost and efficiency. For a more precise answer you’ll need to match the energy you use (consumption) with the amount of electricity you generate (generation).
