South-facing solar panels Perth: when does it make sense?

Perth solar installers recommend north-facing panels as the default because north faces the sun throughout the day. But many Perth homes have the best available roof area facing south — either by orientation or because the north roof is shaded, has a chimney, or is already occupied. Here's what south-facing actually means for Perth solar economics.

The output penalty

In Perth (latitude ~32°S), a north-facing roof at ~20–25° tilt captures the most solar energy year-round. A south-facing roof at the same tilt captures substantially less because it faces away from the sun for most of the day, receiving only morning and afternoon scatter plus some diffuse irradiance.

Approximate annual output comparison (6.6kW system, Perth):

| Orientation | Annual estimated output | vs north-facing | |---|---|---| | North-facing, 20–25° tilt | ~9,500–10,800 kWh/yr | Baseline | | East-facing, 20–25° tilt | ~7,600–8,600 kWh/yr | ~80% of north | | West-facing, 20–25° tilt | ~7,400–8,400 kWh/yr | ~78% of north | | South-facing, 20–25° tilt | ~5,500–6,800 kWh/yr | ~58–63% of north |

A south-facing system typically generates 35–42% less than the same north-facing system in Perth. This is a larger penalty than east or west-facing.

When south-facing is still worth installing

Despite the output penalty, south-facing solar can make economic sense in several Perth scenarios.

You have no usable north roof

If the north-facing section of your roof is:

• Shaded by a tree, neighbouring structure, or dormer
• Too small to accommodate a meaningful array (< 3kW)
• Occupied by a skylight, chimney, or solar hot water collector
• Not structurally suitable

…then south-facing may be the only viable option. Generating 5,500–6,500 kWh/yr from south is substantially better than generating zero from north.

As a second array to supplement north

If you already have a full north-facing array (say 6.6kW) and want to expand capacity, a south-facing addition can make sense:

• South generates in early morning and late afternoon when north output is lower
• Spreads generation more evenly across the day
• May allow you to time an EV charge or pool pump to a broader solar window
• Combined north + south gives a flatter generation curve than north alone

Perth homes with large south-facing roof sections and high energy demand (EV, pool, ducted AC) can benefit from this approach.

Flat or low-pitch roofs

Perth has many low-pitch roofs (5–10°). On a nearly-flat roof, the orientation penalty is substantially reduced — a south-facing, 5° pitch roof loses much less than a south-facing, 20° pitch roof. On a flat roof with adjustable racking, panels can sometimes be oriented north regardless of the roof structure.

When south-facing is not recommended

As the primary array with good north options

If you have a clear, unshaded north-facing roof and the choice is simply about cost or convenience, always choose north. The 35–42% output gap is too large to ignore on purely economic grounds.

In WA Battery Scheme rebate calculations

The WA Battery Scheme rebate ($130/kWh, max $1,300) doesn't care about panel orientation — it's based on battery capacity. But battery payback calculations depend on how much solar generation is available to charge the battery. A south-facing system generating 40% less solar has a materially different battery charging profile than a north-facing system. If your installer quotes battery savings based on north-facing assumptions for a south-facing system, the numbers will be wrong — ask them to re-run with the south-facing output estimate.

South-facing output by season in Perth

South-facing panels follow the opposite seasonal pattern to north-facing:

| Season | North-facing output | South-facing output | |---|---|---| | Summer (Dec–Feb) | Very high | Lowest relative penalty — sun is higher in sky | | Autumn / spring | High | Moderate penalty | | Winter (Jun–Aug) | Moderate | Highest relative penalty — sun is lowest, most of day behind the roof |

The winter penalty for south-facing is severe because Perth's winter sun tracks low across the northern sky. A south-facing roof in Perth winter may produce very little — just early morning and late afternoon scatter.

Design considerations for south-facing arrays

Tilt angle: For south-facing panels in Perth, a lower tilt angle (10–15°) is generally better than the standard north-facing recommendation of 20–25°. Lower tilt reduces the direct-east and direct-west exposure loss.

String configuration: South-facing panels should be on their own MPPT input, not strung together with north-facing panels. Mixing orientations on a single MPPT causes severe output reduction because the lower-performing south panels drag down the north ones. Any reputable Perth installer will ensure separate MPPTs or separate strings for mixed-orientation systems.

Monitoring expectations: Set monitoring system baselines based on south-facing estimated output. Comparing a south-facing system to a default north-facing benchmark will make the system appear under-performing when it is operating correctly.

Shading: South-facing roofs in Perth are less affected by morning and afternoon neighbour shading (which typically comes from the north side of neighbouring structures). This can occasionally be an advantage — a south-facing roof in a dense suburb may be cleaner of shading than the north roof.

Sample Perth economics (south-facing)

Assume a 6.6kW system on a south-facing, 20° pitch roof:

| Factor | North-facing | South-facing | |---|---|---| | Annual output | ~10,000 kWh | ~6,200 kWh | | Synergy A1 rate (33.26c/kWh, self-consumption saving) | ~$3,326/yr max savings | ~$2,061/yr max savings | | Payback (assuming 70% self-consumption) | ~8–11 years | ~12–16 years |

The payback is longer but not necessarily prohibitive, particularly if the south-facing system is displacing high-value daytime loads or there is no viable north alternative.

South-facing solar in Perth carries a real output penalty — typically 35–42% less than north-facing. It remains worthwhile when north roof is unavailable or inadequate, as a second array to broaden generation hours, or on low-pitch roofs where the orientation penalty is reduced. The economics work on longer payback timelines. The key rule: if you have a clear north roof, use it first. If you don't, south-facing is still viable — but ask your installer to quote it based on south-facing modelling, not the default north-facing assumption.