The six factors that decide how much power your solar panels actually make
Two identical 6.6kW systems on two Melbourne streets can produce wildly different amounts of power. The difference is rarely the panels. It is these six factors, roughly in order of importance.
1. Shading
The most brutal factor. Even partial shade on one panel can drag down a whole string, because panels wired in series behave like a hose with a kink in it. A chimney shadow at 3pm, a neighbour's gum tree in winter, a TV antenna: all matter. Good installers do a shade analysis before quoting; microinverters or optimisers can rescue partially shaded roofs at extra cost. If a salesperson never asks about shade, that is a red flag.
2. Orientation
North-facing roofs collect the most energy across the year in Melbourne. But east and west are far from useless: they produce around 10 to 15% less in total, and west-facing panels earn their keep by generating into the late afternoon peak when you are actually home using power. A split east-west array often beats pure north for self-consumption. South-facing panels are the only genuinely poor choice, losing 25% or more.
3. Tilt
Melbourne's latitude means the theoretical ideal tilt is around 30 to 38 degrees. In practice, panels laid flat on a typical 20 to 25 degree roof pitch lose only a few percent. It is rarely worth paying for tilt frames on a pitched roof; on a flat roof they matter more, both for output and for letting rain wash the panels.
4. Temperature
Counterintuitive but true: panels hate heat. Output drops roughly 0.3% for every degree the cell temperature rises above 25 degrees, and a dark roof in January can push cells past 60 degrees. This is why Melbourne's mild climate partly compensates for its cloud, and why a crisp sunny autumn day often beats a scorching February one. Panel spec sheets list this as the temperature coefficient; lower is better.
5. Inverter sizing and quality
The inverter caps your output like a speed limiter. Australian rules allow panel capacity up to 133% of inverter capacity (6.6kW of panels on a 5kW inverter is the classic combo), and that oversizing is usually smart, because panels rarely hit their lab rating in the real world. A failing or undersized inverter, though, silently throttles everything. This is where quality genuinely pays.
6. Dirt and degradation
Melbourne rain handles most cleaning on pitched roofs. Dust, possum traffic and bird droppings cost a few percent; flat panels collect more grime. Panels also age: good ones lose about 0.4% of output per year, cheap ones more. That is the number hiding in the performance warranty.
What this means when you buy
You control shading response (optimisers), orientation choices, and hardware quality. You cannot control the weather, so ignore any quote promising exact dollar savings. Ask every installer how they modelled YOUR roof: if the answer involves a satellite shade analysis and a conservative estimate, you are talking to a professional.
Compare Melbourne installers by rating and reviews on our home page, and see our guides for what solar actually costs.