A solar quote can look like a simple price on a page. But behind a good one is a detailed calculation. Behind a bad one is a guess. Knowing the difference could save you thousands.
Step 1: They Look at Your Electricity Bill
The first thing any competent installer should ask for is your electricity bill — ideally a full year's worth or at least the last 3 months. They're looking for:
- Your daily average consumption in kWh
- Your usage pattern (flat rate or time-of-use?)
- Whether you already have solar
- Your current rate per kWh and any controlled load tariffs
If an installer gives you a quote without seeing your bill, that's a red flag. They're sizing the system off a national average, not your home.
Step 2: Roof Assessment
Orientation and tilt matter. North-facing panels at a 15–25° pitch (roughly matching most roof pitches) are optimal in Australia. East and west panels produce 15–20% less overall, but generate useful power outside the midday window.
Shading is critical. A tree, chimney, or neighbouring building casting shadow on even a portion of your roof can significantly reduce output — and the impact depends on what inverter technology is used. A good installer will use satellite imagery or an on-site assessment to map potential shading.
Step 3: Solar Irradiance Data
Australia's Bureau of Meteorology publishes detailed solar irradiance data by location. Professional quoting software — tools like PVSell, Solargraf, or the PVGIS database — uses this data to estimate exactly how many kilowatt-hours your proposed system will generate per year, adjusted for your specific roof orientation and pitch.
This is the number that drives everything else. If the installer's software says your 6.6kW system will generate 9,500 kWh/year, and you currently buy 8,000 kWh/year, then you'll be generating more than you use — and you'll need to decide how valuable that excess export is.
Step 4: Self-Consumption Modelling
Not all solar you generate gets used directly. Some is consumed in real time, some is exported. Your self-consumption rate depends on when you use electricity. If you're home during the day, self-consumption is higher. If you work 9–5 and nobody's home, you might only self-consume 25–30% of what your panels generate — the rest gets exported at low feed-in rates.
Better installers model your self-consumption ratio based on your usage profile. This is the key to an accurate savings estimate.
Step 5: Applying the Rebate
The federal government's STC scheme reduces the upfront cost of your system. Most installers quote post-rebate prices (i.e., the rebate has already been taken off). Always clarify whether you're looking at a pre-rebate or post-rebate figure.
The STC value varies by system size, zone (solar irradiance region), and the number of years remaining in the scheme (it ends in 2031). For a 6.6kW system in 2026, the STC rebate is roughly $1,500–$2,500 depending on your location.
The Shortcuts Bad Quotes Take
Watch out for these:
- No bill review — sizing based on "average" usage without seeing your actual consumption
- No shade assessment — assuming your roof is clear without checking satellite imagery
- Overstated self-consumption — quoting 50% self-consumption when most working households are closer to 25–35%
- Inflated feed-in tariff assumptions — using 10–15c/kWh when your retailer pays 4–6c
- Excluding GST or approvals costs — check the fine print for what's included
How to Pressure-Test a Quote
Ask for the annual generation estimate in kWh and compare it across quotes. Ask what self-consumption percentage they've assumed. Ask what feed-in tariff rate they've used. These three numbers will tell you immediately whether you're looking at a realistic estimate or a sales fantasy.
Or start with your bill. GridBeater analyses your actual electricity consumption, state, and tariff structure to give you an independent estimate — before you've even spoken to an installer.
Get an independent solar savings estimate from your bill → Free at GridBeater