What size solar power system do I need?

by Mar 11, 2022Local Solar Installers, Solar, Toowoomba

What size solar system do I need? Solar system sizing

In a nutshell: To work out what size solar PV system you require, you need to analyse your household’s daily electricity consumption. Your monthly or quarterly electricity bill measures your household’s electricity consumption in kilowatt-hours (kWh). Using your current average daily electricity consumption, as well as your expected FUTURE consumption, we size and design a system to suit you and your needs. Sizing a solar power system should not be a ‘One Size Fits All’ process. Our team of accredited solar designers can help work out what size system will suit you best.

This article covers:

How is the size of a system calculated?

What your system size depends on

Recommended solar system size

Solar system size limits

What else do you need to consider?

First things first – How is the size of a solar system calculated?

Solar systems are usually described by the total capacity of panels installed (not the size of the inverter).  See the examples below:

  • A solar system with 15 x 400W solar panels is referred to as a 6kW (or 6000W) system. (15 x 400W) ÷ 1000 = 6kW
  • To work out the size of a solar system with 12 x 380W solar panels, 12 x 380 = 4,560W →  4,560W ÷ 1000 = 4.56kW. Therefore, this system size is referred to as 4.56kW.

Note: 1000W = 1kW

The size of your solar PV system will depend on:


The physical unshaded area available for the installation of the solar panels.


Another important factor to think about is money. How much you are prepared to spend?


What portion of your electricity can/do you use through the day time.

Total usage per day (assuming there is both off-peak and primary tariffs)

Power bill per quarter

Recommended solar system size

10-20 kWh


3.5kW - 4.5kW

20-30 kWh


4.5kW - 5.5kW

30-40 kWh


6.6kW (max size for 5kW inverter)

40-60+ kWh


8.0kW - 9.0kW+

NOTE: According to Energy Australia, a standard 4-person Australian family (without a swimming pool) uses an average of 21kWh a day.

Future Usage

The above figures are our recommended solar system size likely to deliver the most cost effective solution based on your current usage. However, if you plan on increasing your energy usage in the future, this will impact what size system you will need. Increasing your energy usage can look like:

  • Adding a pool or spa
  • Starting working from home
  • Getting an Electric Vehicle (EV)
  • Starting a family
  • Installing or using the air-con more
Know your limits

Solar size limits for Ergon/Energex

Single-phase power 

Easy limit

5kW inverter with 6.67kW of panels

Maximum limit

10kW inverter with 13.33kW of panels

What is meant by the ‘easy limit’ in single phase homes?  If you have a total inverter capacity greater than the easy limit, Ergon will make you limit the export to a maximum of 5kW, or in some extreme cases, they could just not allow the system to be installed at all.

The size limit rules for three-phase power aren’t as straightforward… 

  1. The easy limit for three-phase premises is 15kW (inverter capacity). 
  2. The next easy(ish) limit is 30kW (inverter capacity).
  3. Systems between 30kW and 100kW typically require network protection equipment to be installed and REPQ engineering sign-off. These requirements add considerably to the system cost.
  4. Systems over 100kW are a completely different ball game…
Other things to consider

Are you looking at installing just panels, or a battery as well?

If you are planning on adding batteries in the near future, it is super important that you mention this to your installer/designer! Solar batteries store unused energy produced by your solar panels, to be used at night, in times of inclement weather or during a black-out or power outage situation. With batteries, you need a solar system large enough to create enough surplus energy to charge your batteries. 

The cost of a 3kW system is not half of a 6kW system…

Many customers feel that while they are going to the effort of getting solar, they may as well spend the bit extra and get a bigger system to future proof themselves. There is absolutely nothing wrong with this! In fact, there are quite a few scenarios where we would recommend this:

  • Your budget allows it and you have enough physical space to fit a larger system
  • You would like to use your air con more, install a pool/spa or start a family etc
  • You are interested in or considering adding solar batteries
  • You are considering getting an Electric Vehicle (EV)

In fact, over the past 14 years of installing solar power systems, we’ve never had a customer tell us they wish they got a smaller system. We have however, done many solar system upgrades over the years, for people who “should have just gone with the bigger system in the first place”…

Nigel Phillips
Nigel Phillips

Director/Technical Manager at Electrical Sensations

Nigel has decades of experience in domestic, commercial and industrial electrical work, as well as 14 years of experience in the Solar Industry. As a Clean Energy Council (CEC) accredited solar designer and installer for both grid and off-grid solar power and battery systems, Nigel has the knowledge and expertise to create solutions to suit every client. Nigel is also one of the 23 solar experts in CEC’s Installer Reference Group, which assist the CEC in improving and shaping the small-scale PV and battery industry. Troubleshooting, fault finding and thinking outside the box are some of Nigel’s most valuable strengths.


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