Solar Irradiance

What irradiance measures

Solar irradiance is the instantaneous power of sunlight hitting a surface - measured in watts per square metre (W/m²). On a clear summer day at solar noon in most of Australia, irradiance at a north-facing surface reaches 900–1,100 W/m². Standard Test Conditions (STC), used to rate solar panels, define 1,000 W/m² as the reference point.

Irradiance is the snapshot. Solar radiation (or solar energy) is the accumulation over time - measured in kWh/m² per day or per year.

Australia’s solar resource

Australia has one of the best solar resources in the world. The annual average horizontal irradiance across the continent runs from around 4 kWh/m²/day in Tasmania to over 7 kWh/m²/day across the interior and northwest. For comparison, the UK averages around 2.7 kWh/m²/day, and Germany around 3.1 kWh/m²/day - which is why Germany’s solar industry was a policy push while Australia’s is simply economically obvious.

Approximate annual average horizontal irradiance for major cities:

• Darwin: 6.4 kWh/m²/day
• Perth: 5.5 kWh/m²/day
• Brisbane: 5.2 kWh/m²/day
• Adelaide: 5.0 kWh/m²/day
• Sydney: 4.6 kWh/m²/day
• Melbourne: 4.4 kWh/m²/day
• Canberra: 4.5 kWh/m²/day
• Hobart: 4.0 kWh/m²/day

Irradiance vs temperature: the trade-off

High irradiance is good. High temperature is not. Solar panels lose output efficiency as temperature increases - roughly 0.3–0.4% per degree above 25°C for silicon panels. Darwin and central Australia have very high irradiance but also very high ambient and cell temperatures that partially offset the irradiance advantage.

This is why Perth and Adelaide often outperform Darwin on a per-kWp basis in annual yield: they get high irradiance without extreme temperatures that degrade panel efficiency during peak generation hours.

Diffuse vs direct irradiance

Total irradiance has two components. Direct normal irradiance (DNI) is the beam of sunlight coming directly from the sun’s disc - this is what concentrating solar technologies need and what PV panels capture most efficiently. Diffuse irradiance is sunlight scattered through the atmosphere and clouds. On overcast days, panels still generate because diffuse irradiance continues to reach the surface - just at lower intensity.

Standard flat-panel solar PV captures both direct and diffuse irradiance, which is why output on a cloudy day is 10–30% of clear-day output rather than zero.