Solar output in summer vs winter in Perth: what to expect each month

Perth has some of the most consistent solar radiation in Australia — but the seasonal variation still matters for understanding your energy bills. Here's a month-by-month guide to expected solar output in Perth, and why the summer advantage is smaller than most people assume.

Why Perth's seasonal variation is smaller than other cities

Perth's Mediterranean climate means:

• Summer (Dec–Feb): Long days, high sun angle, intense radiation — but also high panel temperatures (reducing efficiency) and frequent summer afternoon cloud/haze
• Winter (Jun–Aug): Shorter days, lower sun angle — but cooler panel temperatures (improving efficiency) and typically clearer morning/afternoon skies

The efficiency gain from cooler winter panels and Perth's clear winter skies partially offsets the shorter days and lower sun angle. The result: Perth's winter generation is less dire compared to, say, Melbourne's winter.

Month-by-month generation guide for Perth

For a 6.6kW north-facing solar system at Perth latitude (~32°S):

| Month | Avg daily generation (kWh) | Notes | |---|---|---| | January | 30–36 | Peak sun, but high temperatures reduce panel efficiency | | February | 28–34 | Still strong, slight cloud increase late Feb | | March | 24–29 | Excellent — cooler temp, still long days, clear skies | | April | 19–24 | Days shortening; often excellent generation | | May | 14–18 | Days shorter, sun angle lower | | June | 11–15 | Lowest month — short days, low sun angle | | July | 12–16 | Similar to June, often clearer skies | | August | 14–19 | Spring approaching, days lengthening | | September | 18–23 | Strong spring generation, clearer days | | October | 23–29 | Excellent month — long days, moderate temps | | November | 27–33 | Near-peak output, pre-summer heat | | December | 30–37 | Peak potential — longest days |

Annual total (approximate): 9,000–11,500kWh/year for 6.6kW north-facing in Perth metro.

These ranges reflect typical clear-day generation with normal soiling and performance losses (PR 0.78–0.85). Individual system results vary based on orientation, shading, panel temperature, and local microclimate.

What drives the month-to-month variation

1. Day length: From 9.8 hours of daylight in June to 14.5 hours in December. More daylight hours = more potential generation. This is the dominant driver.

2. Sun angle: In December, the sun reaches ~80° elevation at solar noon in Perth. In June, the maximum elevation is ~37°. Lower sun angle means panels collect solar radiation at a steeper incidence angle — less efficient per unit area.

3. Panel temperature: Panels lose approximately 0.4–0.45% output per degree Celsius above 25°C (for PERC panels). In January, panel temperature may reach 65–70°C at midday — a 17–20% efficiency penalty. In July, panel temperature may peak at 40–50°C — a 6–11% penalty. This temperature correction partially offsets winter's lower sun angle and shorter days.

4. Cloud cover and haze: Perth is famous for its clear skies, but February and March often see afternoon sea breezes with cloud build-up. December afternoons can have Perth's characteristic "fremantle doctor" bringing cloud and humidity. June–August typically have Perth's clearest mornings. The pattern isn't always "summer = more sun hours and clearer sky."

What seasonal variation means for your electricity bill

Bill pattern for a solar household (A1 tariff, no battery):

• Summer bills: potentially very low (or zero) if solar surplus is large — but household consumption is also high (AC)
• Winter bills: higher import charges as solar generation drops by 50–65% from summer peak
• Shoulder months (March–May, September–November): often best net bill outcome — good solar, moderate consumption

Key insight: Your winter electricity bill will always be higher than summer, but less dramatically so than in Melbourne or Sydney. The difference between Perth's best (January) and worst (June) solar month is approximately 3× — compared to 5× or more in Melbourne.

How to use the seasonal pattern

Budget planning: Expect summer electricity bills to be near-zero or small (especially with a battery), and winter bills to be 2–4× higher. Annual average is a more useful planning metric than monthly.

Checking your monitoring: If your June generation is below 11kWh/day average on a clear-day basis (for a 6.6kW system), investigate. If December is below 25kWh/day average, investigate. Comparing each month to the expected range identifies whether shortfalls are weather-related (normal) or system-related (investigate).

Midday Saver strategy in winter: On Midday Saver, the 8.85c/kWh super off-peak window (9am–3pm) still applies in winter. Your solar may not cover as much of the house load in winter, but the tariff structure still rewards shifting appliances to midday.

North vs west vs east facing in different seasons

North-facing: Best annual total. Season variation follows the pattern above — strongest summer, 60–65% of summer generation in midwinter.

West-facing: Better afternoon generation (2pm–6pm), which aligns with Perth's afternoon heating. Slightly lower annual total than north (-17–23%), but the seasonal pattern is similar.

East-facing: Better morning generation (7am–12pm). Aligned with morning use peaks. Annual total -17–23% vs north. Winter mornings in Perth can be foggy or hazy, slightly reducing east-facing winter advantage.

East-west split: Flatter daily generation curve — generates from 7am through to 6pm but at lower peak wattage. Self-consumption ratio often improves with east-west splits because generation is spread throughout the day rather than concentrated at noon.

Your monitoring app tracks actual generation by month. Compare your system's monthly totals to the ranges above to verify you're in the expected zone. If you're consistently below the bottom of the range on clear months, it's worth investigating.