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AS 4509.2-2010
Why Year-Round Performance Requires Seasonal Analysis
A system sized to the annual average solar resource will produce surplus in summer and deficit in winter. For a grid-connected hybrid system, winter deficit is covered by the grid. For a true off-grid system, the winter deficit must be covered by the battery (days of autonomy) and the generator. AS 4509.2 specifies using the worst-month data as the design baseline.
Monthly Energy Balance Table (Example: Sydney, 4 kW Array, 10 kWh Battery)
Month | PSH (tilted 30°N) | Array Production (kWh) | Daily Load (kWh) | Daily Surplus/Deficit (kWh) |
|---|---|---|---|---|
January | 6.2 | 22.3 | 12.0 | +10.3 |
April | 4.8 | 17.3 | 10.0 | +7.3 |
June (worst) | 3.0 | 10.8 | 11.0 | -0.2 |
September | 4.5 | 16.2 | 10.0 | +6.2 |
December | 6.8 | 24.5 | 12.0 | +12.5 |
In June, this system nearly breaks even — the generator would run approximately 1–2 times per week to maintain battery SoC. In summer, there is large surplus that can run additional loads or export if grid-tied.
Adding Design Margins
Pure mathematical energy balance often gives a system that works "on paper" but fails in practice because of:
Cloudy day sequences (3–5 consecutive days with 30% of average PSH)
Load growth over time (appliances added, occupancy changes)
System inefficiency losses not captured in the basic calculation
AS 4509.2 recommends applying an overall system efficiency factor of 0.75–0.80 to the calculated solar harvest to account for battery round-trip efficiency, controller losses, cable losses, and temperature derating. Always design for 15–20% headroom above the minimum break-even calculation.
Engr. Jason Morales — Founder, SolarEnergyPH
