If you’ve used an online solar rebate calculator to estimate your STC discount, there’s a reasonable chance it quietly overstated your rebate by around 17%. Not through dishonesty — through a data error that’s more common than it should be. Here’s what’s going on, and how to check any quote you’ve been given.
The STC rebate, in brief
Australia’s federal solar rebate works through Small-scale Technology Certificates (STCs). Your installer calculates how many STCs your system earns using a simple formula:
System size (kW) × Zone rating × Deeming years = STCs
Those STCs are then valued at the current market price (capped at $40 by regulation) and applied as an upfront discount on your system, handled by your installer — you never see or trade the certificates yourself.
The formula looks simple. The problem is in the middle term: the zone rating.
The mistake: using the wrong zone for Melbourne
The Clean Energy Regulator divides Australia into four solar zones, each with a different rating that reflects how much sun that area gets:
| Zone | Rating |
|---|---|
| Zone 1 | 1.622 |
| Zone 2 | 1.536 |
| Zone 3 | 1.382 |
| Zone 4 | 1.185 |
Melbourne’s CBD, and large parts of the metro area, sit in Zone 4, rated 1.185 — including our own postcode here in Mount Waverley. But checking the Clean Energy Regulator’s official postcode table shows Victoria isn’t a clean split between “Melbourne = Zone 4” and “regional = Zone 3.” It’s more of a checkerboard: some outer and western Melbourne postcodes sit in Zone 3, while much of the eastern and northern metro area is Zone 4. Ballarat, for example, is Zone 3.
That patchwork is exactly why postcode-specific accuracy matters. In checking a number of publicly available solar rebate calculators, we found at least one defaulting to the Zone 3 rating (1.382) for general Australian calculations rather than asking for a postcode at all. If a calculator shows you a number without asking where you live, there’s a real chance it’s not using your actual zone.
What that error actually costs you, in the numbers
Take a common system size — 6.6kW — installed in 2026, with a 5-year deeming period:
Using the correct Zone 4 rating (1.185):
6.6 × 1.185 × 5 = 39.1 → rounds down to 39 STCs
Using the incorrect Zone 3 rating (1.382):
6.6 × 1.382 × 5 = 45.6 → rounds down to 45 STCs
That’s 6 extra STCs shown on a calculator that doesn’t actually exist — at a market price of around $38–40 each, that’s roughly $228–240 of rebate that was never really there. If a quote is built around that inflated number, the gap shows up later, usually as a “the rebate came in lower than expected” conversation after the paperwork is done.
To be clear: this isn’t unique to any one calculator, and it’s usually a data-entry oversight rather than anything deliberate. But it’s exactly the kind of error that erodes trust in an industry that already has a reputation problem with inflated numbers.
How to check a quote yourself
You don’t need to take anyone’s word for it — including ours. Here’s the check:
- Find your postcode’s zone on the Clean Energy Regulator’s official postcode zone table (search “CER postcode zone ratings solar”).
- Confirm your system size in kW and the year of installation (this sets the deeming years — 5 for 2026, dropping to 4 from 1 January 2027).
- Run the formula yourself: system size × zone rating × deeming years, rounded down.
- Compare that STC figure to what’s on your quote. A small variance is normal (STC market price moves daily), but the STC count itself should match exactly.
If a quote’s STC count doesn’t match what you calculate, it’s worth asking the installer directly which zone and deeming period they used.
Where we land on this
We built our own quoting process around getting this right by postcode, specifically because we’d rather show you an accurate number upfront than a bigger one that changes later. If you want a second opinion on a rebate figure you’ve already been quoted elsewhere, we’re happy to run the numbers with you — no obligation, no pressure to switch.