Kilowatt Peak

What kWp measures

Kilowatt peak is the maximum output a solar panel or system can produce under precisely defined test conditions: 1,000 watts of solar irradiance per square metre (roughly equivalent to full midday sun on a clear day), cell temperature at 25°C, and air mass of 1.5 (which accounts for the angle the light travels through the atmosphere).

These conditions are called Standard Test Conditions (STC), and they exist as a reference point so you can compare panels from different manufacturers on equal terms. In practice, your panels will only briefly hit STC conditions - if ever - but the kWp rating is still the most useful shorthand for system size.

The gap between rated and actual output

A 6.6 kWp system doesn’t consistently produce 6.6 kW. Output varies with:

Irradiance. Early morning, late afternoon, and overcast days all produce less power per square metre than the STC benchmark.

Temperature. Solar cells are rated at 25°C but in Australian summer conditions - panels sitting on a roof in 35°C ambient air - cell temperature can reach 55–65°C. Most silicon panels lose roughly 0.3–0.4% of their rated output per degree above 25°C, so a 65°C panel is producing about 12–16% less than its kWp rating even in full sun.

System losses. Inverter efficiency, wiring resistance, soiling, and panel mismatch all reduce the output from cells to the grid connection. A well-designed system with modern equipment runs at about 80% overall efficiency - meaning a 6.6 kWp system will produce roughly 5.3 kW at the meter in ideal conditions.

kWp vs kW (installed vs operating)

You’ll see both terms in solar quotes and system monitoring. kWp refers to the panel capacity (nameplate rating). kW refers to the actual power being produced at a given moment. Your monitoring system will show kW output in real time - and on a clear winter afternoon with panels at 30°C, a 6.6 kWp system might be producing its closest-to-rated output of the year.

Common system sizes in Australia

Australian residential solar installations typically range from 6.6 kWp to 13.3 kWp, with most new installs trending toward the larger end:

• 6.6 kWp: 16 × 415W panels. Common entry point for an average household
• 10 kWp: ~24 panels. Suitable for homes with EVs or pool pumps
• 13.3 kWp: ~32 panels. Maximises a typical north-facing roof section

The inverter size is often quoted alongside - a 5 kW inverter with 6.6 kWp of panels is a common configuration in Australia, where network rules can limit export to 5 kW on single-phase connections.