Why Dayboro's Valley Microclimate Means Your Solar Panels Produce Less Than Brisbane Averages

If you have solar panels in Dayboro, King Scrub, Mount Mee, or anywhere in the D'Aguilar Range hinterland, you have probably noticed something: your system produces less than the installer promised. Not by a massive amount — but consistently, stubbornly less. And the reason is not your panels. It is where you live.

We have been measuring solar radiation and actual panel output from a local 11.5 kW system since December 2025. After 59 days of data, the numbers tell a clear story: Dayboro's valley location produces roughly 10–15% less solar energy than what generic calculators based on Brisbane weather data predict. On foggy winter mornings, the gap can be 40–60% for the first few hours of the day.

This is not a defect. It is physics. And if you understand it, you can make better decisions about battery sizing, electricity usage timing, and whether that quote from the solar installer is realistic for your actual roof.

The Valley Fog Problem

Dayboro sits in a valley at approximately 130 metres elevation, surrounded by the hills and ridgelines of the D'Aguilar Range. This geography creates a phenomenon called cold air drainage — on clear nights, cooled air flows downhill and pools in the valley floor. When that cold air hits the warmer, moister atmosphere at dawn, you get fog. Sometimes thick enough that you cannot see the end of the driveway.

Every solar calculator you find online — SolarQuotes, Solar Choice, solarcalculator.com.au, even the government-backed SunSPOT tool — pulls its solar radiation data from the Bureau of Meteorology. The nearest BoM stations with solar instruments are at Brisbane Airport (5 metres elevation, on a coastal plain) and Archerfield (15 metres, flat suburban). Neither station experiences valley fog. Neither station is surrounded by hills that shade the horizon.

When those calculators tell you a 6.6 kW system in postcode 4521 will produce 25 kWh per day, they are using solar radiation data measured in a completely different microclimate. The number is correct for Brisbane Airport. It is not correct for Dayboro.

What the Data Actually Shows

We run a numerical weather prediction model — the Dayboro Model — specifically calibrated for this valley. It produces hourly forecasts of solar radiation, temperature, cloud cover, and humidity. We validate those forecasts against actual power output from a local Sigenergy system with panels on two roof orientations.

Sources: PVGIS ERA5 (1994–2023) — APVI Brisbane Solar Potential 2018 — SolarQuotes orientation guide — Regen Power orientation data — BoM IDCJAC0016 — ScienceDirect: PV-battery self-sufficiency Australia (2025)

Two numbers jump out. First, the average Dayboro yield of 2.97 kWh/kW is well below the Brisbane optimal of 4.69 — a gap of 37%. That is the valley microclimate in a single number. Second, the SW-facing panels produce just 1.44 kWh/kW/day against a Brisbane generic of ~3.8. That is not just orientation — it is orientation plus afternoon ridge shadowing plus older panel technology combining to gut the output. More on that below.

The summer average of 2.97 kWh/kW/day will also drop in winter with shorter days, lower sun angles, and more persistent valley fog. We expect the annual figure to land closer to 2.5 kWh/kW/day — we will know for certain once we have twelve months of data.

The Physics Behind the Forecast

Converting a weather forecast into a solar panel output prediction is not guesswork. It is applied physics with well-established models. Here is how it works.

Step 1: Decomposing Sunlight

The weather model produces Global Horizontal Irradiance (GHI) — the total solar energy hitting a flat surface on the ground, in watts per square metre. But your panels are tilted on a roof. To calculate what actually hits the panel surface, you first split GHI into:

• Direct Normal Irradiance (DNI) — the beam from the sun's disc. Casts sharp shadows. Its contribution to your panel depends on the angle between the sun and the panel surface.
• Diffuse Horizontal Irradiance (DHI) — scattered light from the entire sky dome. On a cloudy day, almost all production comes from DHI. On a clear day, only 15–20%.

We use the Erbs et al. (1982) correlation to split GHI into DHI and DNI, using the clearness index as input — the ratio of measured GHI to what would arrive at the top of the atmosphere if there were no atmosphere at all.

Step 2: Tilted Surface Irradiance

With DHI and DNI separated, we calculate Plane of Array (POA) irradiance — how much solar energy actually hits your panel face. The isotropic sky model (Liu & Jordan, 1963) adds three contributions: direct beam (DNI × cosine of incidence angle), diffuse sky (DHI × view factor depending on tilt), and ground reflection (GHI × albedo 0.2 for grass/soil).

Step 3: Panel Age and Technology

Panels degrade over time. The calculator applies published degradation rates per technology type — 0.5%/year for monocrystalline, 1.5%/year for polycrystalline, 0.6%/year for thin film — with a floor of 30% to handle very old panels. Enter your installation year and the calculator works out the production penalty automatically.

Panel Orientation in the Dayboro Valley

Most Dayboro homes have rooflines running roughly northeast–southwest, following the valley's natural orientation. This means panels typically face either NE or SW. Neither is the optimal north-facing direction, but the difference between them is stark.

The NE-facing panels produce 3.11 kWh per kilowatt per day. The SW-facing panels manage 1.44 kWh/kW — less than half. The generic Brisbane estimate for NE facing is around 4.4–4.6 kWh/kW, and for SW around 3.8 kWh/kW. The Dayboro measured values are substantially below both benchmarks, particularly the SW result, which we attribute to a combination of westward ridge shadowing in the late afternoon and the older panel technology.

Interestingly, on days with morning cloud clearing to afternoon sunshine — a common pattern in the valley — west and north-west orientations can actually outperform true north. The afternoon sun hits west-facing panels when the sky is clearest, while north-facing panels had their best hours during the cloudy morning. The hourly forecast shows this day-by-day, which is where it gets genuinely useful.

What This Means for Battery Sizing

A battery sizing calculator that uses Brisbane solar averages will overestimate your daily surplus by 10–15%. That might lead you to buy a smaller battery than you need, or to overestimate your payback period.

From our measured data, the 11.5 kW system achieves 77% self-sufficiency with an 11.5 kWh battery. Peer-reviewed Australian research (ScienceDirect, 2025) puts the typical ceiling for PV+battery households at around 76% — so the Dayboro result is actually tracking right at that ceiling, which is encouraging. But a Brisbane-based generic estimate using optimal orientation data might project 80–85% for the same system size. The gap translates to real dollars on the quarterly bill.

Ongoing Measurement and Self-Learning

The prediction system is not static. Every day it compares its predictions against actual measured output from the local reference system and adjusts its calibration. This self-learning approach means the forecasts improve over time as the model accumulates more data across different weather patterns, seasons, and solar angles.

After the first months of validation, the model achieves daily accuracy within 5–10% on clear, stable days and 20–30% on variable weather days. We expect this to tighten as we build a full year of seasonal comparison data through winter 2026.

All of this feeds into the Dayboro Weather Station's broader purpose: giving local residents genuinely useful, hyperlocal information based on measured data from where they actually live — not averaged estimates from a coastal plain 35 kilometres away.