How much solar do you actually need?
Bigger is not automatically better. Here is how system size is really decided, and why the cheapest quote for the biggest system is often the worst value.
Ask three installers to quote your house and you will likely get three different system sizes. That is not because two of them are wrong. It is because sizing is a judgement about how you use power, and most quotes never look at that properly.
Generation is the easy part
In Sydney, a well-oriented system produces roughly 1,400 kWh per kW per year. A 6.6 kW system therefore makes around 9,200 kWh annually. That number is fairly predictable and every installer will quote something similar.
What varies enormously is how much of that you actually use rather than export.
Self-consumption decides the answer
Power you use yourself is worth what you would have paid for it — around 35 cents. Power you export is worth whatever your feed-in tariff is, which is usually a small fraction of that. The gap between those two numbers is the entire argument for sizing carefully.
A household that is empty until 6pm might use only 20% of its generation directly. A business running refrigeration through the day might use 80%. The same system produces radically different returns in those two cases.
So why do quotes push bigger systems?
Because a larger system is a larger sale, and the extra kilowatts look impressive on the generation figure. That is fine if you will use the output, or if you plan to add a battery or an electric vehicle. It is poor value if the extra generation just gets exported at a low rate.
The honest questions are: what does your daily load profile look like, is that likely to change, and what will you do with the surplus?
Roof space sets a ceiling
Allow roughly 5 m² of usable roof per kW installed once you account for spacing and access. Orientation matters more than most people expect: north is best for total output, while west-facing panels generate later in the day, which suits households that consume power in the afternoon and evening.
A reasonable starting point
Work out your daily consumption, decide honestly how much of it happens in daylight, and size so that most of your generation is consumed on site. Then consider whether a battery changes the picture — because it usually does.
Our solar calculator does this arithmetic with published assumptions, and the battery calculator handles the second half of the question.
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