What Size Solar System Do I Need?

Some will tell you to size a solar system that is ‘just right’ to cover your daytime usage.

A system that’s not too big, and not too small.

On this page we will side with other commentators such as Finn Peacock, (Electrical Engineer and founder of solarquotes.com.au) to say that the correct solar size is as many panels as will fit, and you can afford.

We’re not saying this to increase our profits here at Solar Pro Bendigo. We’re customer outcome focused first. Always have been. Always will be. Our director is putting 20kW + on his new build at the time of writing.

Let us explain why:

1.    Daytime usage is hard to define

The problem with sizing purely for ‘daytime usage’ is that it’s difficult to define.

• Day time usage varies from weekdays to weekends
• It also varies with the seasons
• Solar production also varies day on day and through the seasons

Further, sizing for ‘day time usage’ even if calculated correctly won’t serve you well long term.

2.    Small systems can’t grow with you

If you size your system purely for your daytime usage it:

• Won’t produce enough to cover future increases in daytime usage (look at your bill and look at your usage vs this time last year. I bet it has increased)
• Won’t produce enough to charge a battery in the future
• Won’t produce enough to put ‘free fuel’ in your electric vehicle (yes, your electric vehicle)

Remember these solar panels are warranted for 25 years, so you need to have a long-term outlook on your situation

3.    Small systems are less cost-effective

Pricing is changing all the time, but at the time of writing a 6.6kW System after the Solar Victoria Rebate (not including the loan) is installed for $6,100 (with a high quality Fronius Inverter + Consumption Meter)

That works out to be $924 per kilowatt, fully installed.

A 10kW system, also with a Fronius inverter, inclusive of Solar Victoria Rebate is installed for $9,250

That works out to be $893 per kilowatt, fully installed.

For an extra $3000, you’ve almost doubled your system size.

How is this possible?

This is because of the economy of scale when installing solar.

The switchboard work, and running cables through your roof cavity is almost exactly the same regardless of system size. The only difference is the number of panels, and slightly more labour for a bigger system.

On top of this, the more panels you install – the more STCs (federal solar rebate) you receive, which means these extra panels are basically discounted by the government.

Expanding the system later – not so fast

The last thing to note is around the idea of expanding a system later.

Whilst in theory you could install a 10kW inverter with 5kW of panels with the plan of adding more panels later, in reality Australian Standards for solar installations are changing every 1-2 years.

Each change means installers are not allowed to mix installations.

E.g we can’t install the second set of panels to a 2025 standard when the first set of panels and inverter were installed to a 2021 standard.

The only way to expand systems is to install your second set of panels on a new, second inverter. 5kW Fronius inverters cost about $2000, whereas their 8kW inverter is approx. $2750

So you save over $1000 by getting the inverter right the first time. Not to mention doubling up on labour costs, travel etc.