Solar inverter sizing for Perth: the 133% rule, DC:AC ratio, and clipping

A "6.6kW" Perth solar system usually means 6.6kW of panels wired to a 5kW inverter. That is not a typo or a cut corner. It is standard design. Panels almost never hit their rated output in real Perth conditions, so pairing a bigger array with a smaller inverter captures more energy across the day for very little loss.

The number to look at on a quote is the DC:AC ratio: panel capacity divided by inverter capacity. For most Perth north-facing homes, a ratio of 1.3 to 1.4 is normal and correct. The Clean Energy Council caps the array at 133% of the inverter for a no-battery system, which is where the "6.6kW on a 5kW inverter" pairing comes from. Below we cover what the ratio means, why oversizing works here, and the handful of cases where a high ratio actually costs you.

What is the DC:AC ratio?

The DC:AC ratio is the panel capacity (measured in DC, at standard test conditions) divided by the inverter's AC output rating.

• 6.6kW of panels on a 5kW inverter is a ratio of about 1.32
• 10kW of panels on a 6.6kW inverter is a ratio of about 1.52
• 10kW of panels on a 10kW inverter is a ratio of 1.0

A higher ratio means more panels feeding a given inverter. A ratio of 1.0 means the array and inverter are matched one to one, which is unusual for a modern residential system.

What is the 133% rule, and is it a hard limit?

For a system without a battery, the array can be sized up to 133% of the inverter's capacity and still qualify for the upfront certificate discount. Put the other way, the inverter has to be at least 75% of the array size. Those are the same rule: 1 divided by 0.75 is 1.333.

Worked through:

• A 5kW inverter takes up to 6.65kW of panels. That is where "6.6kW" comes from, a common size that sits just under the cap.
• A 10kW inverter takes up to 13.3kW of panels, which is where the "13.3kW" system size comes from.

For a no-battery system, going over 133% can get your certificate claim knocked back, so accredited installers stay under it. With a battery on the system, the cap can be exceeded, because the extra panels have somewhere to send the surplus. If a quote pushes past 1.33 on a battery-less system, ask the installer to confirm the certificate eligibility before you sign.

Why do Perth systems fit more panels than inverter?

Two reasons: panels rarely produce their rated output, and the energy you gain in the shoulders of the day outweighs the little you lose at the peak.

Panels run below their nameplate rating. A panel's wattage is measured under standard test conditions: 1,000 W/m² of light, a 25°C cell temperature, and a set air mass. Perth rooftops are hotter and dustier than a lab. On a hot summer afternoon a panel sits well above 25°C, and hotter cells produce less power. Dust and grime shave off a bit more. Add it up and a real Perth system delivers meaningfully less than its nameplate, roughly around 1,500 kWh per year for each kW installed. So a "6.6kW" array almost never puts 6.6kW into the inverter at once.

The shoulders of the day matter more than the peak. Oversizing lifts generation in the morning and afternoon, when the sun is lower and the array is nowhere near its ceiling. Only during a narrow window around midday, on a clear summer day, would a big array try to push past what the inverter can handle. When that happens the inverter caps its output at its maximum. That capping is called clipping.

How much does clipping actually cost?

On a typical Perth system with a ratio around 1.32, clipping loss over a year is small. The inverter only runs flat out for a handful of hours across the whole year, and the extra generation you get every morning and afternoon far outweighs those few clipped peaks. For most north-facing homes the trade is firmly in your favour: a little clipped at noon, a lot more banked in the shoulders.

If you want the deeper version of this, solar inverter clipping in Perth: what it is and when it matters walks through the day-by-day picture and the dollar value of a clipped kilowatt.

When does a high DC:AC ratio actually cost you?

Clipping only becomes a real loss in a few situations:

1. The ratio is very high. Push well past 1.5 without a battery or shade to soak up the surplus, and the clipped hours start to add up.
2. The whole array faces north at optimal tilt. North-at-tilt gives the sharpest midday peak, so a north-only array hits the inverter ceiling more often than a split-orientation one.
3. There is no battery to absorb the surplus. With a battery, energy that would have been clipped gets stored instead of thrown away.

The flip side is useful to know: spreading panels across east and west, or off true north, flattens the midday peak. That lower peak clips less often, even at the same ratio, because generation is smeared across more of the day instead of piling up at noon. So a split-orientation roof can carry a higher ratio comfortably.

For most Perth north-facing homes, a ratio of 1.3 to 1.4 is well inside the sweet spot. The economics of a few more panels beat the small clipping loss.

How do multi-MPPT inverters change the sizing?

Many modern inverters have two or more independent MPPT (maximum power point tracker) inputs. Each MPPT manages its own string of panels with its own input limit.

Take a 10kW inverter with two 5kW MPPTs. You can load up to about 6.65kW of panels on each input, for up to 13.3kW of panels total, while each string still respects the 133% cap on its own.

Multiple MPPTs earn their keep when your roof is not one simple plane:

• Two roof faces, for example north and west panels on separate strings
• Shade hitting one part of the array but not another
• Different numbers of panels in different orientations

If any of that describes your roof, microinverters vs string inverters vs power optimisers for Perth homes covers which setup handles mixed orientations and shade best.

How do I check the ratio on my own quote?

Four numbers, in order:

1. Panel capacity (kW). Add up the panels. For example, 27 panels rated 415W is about 11.2kW.
2. Inverter capacity (kW). The inverter's rated AC output.
3. DC:AC ratio. Panel capacity divided by inverter capacity.
4. Sanity-check the ratio. For a north-facing Perth home, 1.3 to 1.4 is normal and correct. Above 1.5 is worth a question, especially with no battery. Below 1.0 is unusual: the inverter is oversized for the array, which can point to a planned panel addition or a conservative design, so ask why.

A ratio of 1.3 to 1.4 on a north-facing Perth install is standard and correct. If you are adding panels to a system you already own, adding more solar panels to an existing system in Perth covers how the ratio and your inverter headroom decide what you can bolt on.

My monitoring app shows a flat peak. Is something wrong?

No. A flat plateau in your monitoring app, say a level 5,000W line from late morning to early afternoon instead of a smooth curve, is clipping. The inverter is simply holding at its maximum output. It is expected behaviour, not a fault.

You can put a rough number on it by comparing what the system should have generated in those hours against what it actually did. If clipping looks like it is costing you a real slice of your annual generation, the ratio may be too high for your particular roof, so raise it with your installer. On a standard Perth system, the clipped amount is small.

If you want to go deeper on any of the specs behind these numbers, how to read solar inverter specifications: a Perth buyer's guide explains what each rating on the datasheet actually means.

The short version

A DC:AC ratio around 1.32, the "133% rule", is standard for Perth solar. Slightly more panels than the inverter can pass at its peak, with a small annual clipping loss that is outweighed by stronger morning and afternoon generation. When you compare quotes, check the ratio: 1.3 to 1.4 is normal, above 1.5 needs a reason, and below 1.0 is worth querying.

Related guides

• Solar inverter clipping in Perth: what it is and when it matters: the day-by-day view of what clipping costs.
• Microinverters vs string inverters vs power optimisers for Perth homes: the right setup for mixed orientations and shade.
• Adding more solar panels to an existing system in Perth: how inverter headroom decides what you can add.
• How to read solar inverter specifications: a Perth buyer's guide: what every rating on the datasheet means.
• Perth solar panel installation: what size system do you need?: sizing the array before you worry about the inverter.