How to Invoice Solar Installations with STC Rebates: Correct Format and GST Treatment

The STC (Small-scale Technology Certificate) rebate is the most commonly mishandled element of solar installation invoicing in Australia. Getting it wrong affects GST calculations, causes issues with solar finance applications, and creates confusion for clients who expect to see the rebate clearly on the invoice.

This guide explains the correct way to show STC rebates on solar installation invoices, with the right GST treatment.

What is the STC rebate?

The STC scheme lets householders receive a discount on their solar system by assigning their STCs to the installer. The installer trades those certificates for cash (or a rebate from the retailer), and passes the discount to the customer at point of sale.

From an invoicing perspective, the STC rebate appears as a negative line item on the invoice — reducing the customer's payable amount.

The correct STC rebate line item format

Equipment and labour subtotal: $8,800.00

Small-scale Technology Certificate (STC) rebate — 32 certificates × $38 = −$1,216.00

Post-rebate subtotal: $7,584.00

GST (10%): $758.40

Total payable: $8,342.40

The key points:

• GST is calculated on the post-rebate subtotal, not the pre-rebate total
• The STC rebate must be shown as a negative line item
• The number of STCs and the assignment price should be stated explicitly

Common STC rebate invoicing mistakes

Mistake 1: GST calculated on the full pre-rebate amount If your total is $8,800 and you calculate GST on that before subtracting the STC rebate, you are overcharging the customer and the invoice is technically incorrect.

Mistake 2: Absorbing the STC rebate into the system price Some installers quote a system price that already has the STC rebate factored in, without showing it separately. This creates problems for solar finance (lenders want to see the gross price and the rebate separately), and does not give the client the transparency they are entitled to.

Mistake 3: Using an incorrect STC price STC prices fluctuate. Use the actual price you agreed to assign the certificates at — not an estimate or last year's price.

How many STCs does a system generate?

STCs = System size (kW) × Zone factor × Deeming years

For example, a 6.6kW system in Zone 3 (most of eastern Australia) with 8 deeming years remaining:

6.6 × 1.185 × 8 = approximately 62.5 STCs (rounded to 62)

You can verify using the Clean Energy Regulator's STC calculator. SMASH lets you add the STC count directly in your voice description.

Solar finance invoice requirements

When the client is using solar finance (Plenti, Brighte, etc.), the lender typically requires:

• Full itemised invoice with equipment and labour listed separately
• STC rebate shown as a negative line item
• GST calculated on the post-rebate amount
• Installer ABN and CEC accreditation number
• Invoice addressed to the client (not the finance company)

SMASH for solar installers generates this format automatically from a voice description. The invoice is compliant with most solar finance lender requirements.

Using voice invoicing for solar installations

"6.6kW system at the Dee Why property. 18 panels LG 370W, SolarEdge 5kW inverter with optimisers, standard residential install. Two crew, one day. Deduct 32 STC rebate."

SMASH builds the invoice with equipment lines, labour, the STC rebate as a negative line item, GST on the post-rebate total, and a payment link — all in under 60 seconds.

Frequently asked questions:

Is GST calculated before or after the STC rebate? After. GST is calculated on the post-rebate subtotal.

Do I need to show the number of STCs on the invoice? Yes. State the number of certificates and the assignment price — both for transparency and for solar finance lenders.

Can the STC price vary between jobs? Yes. The spot price for STCs fluctuates. Use the actual price you assigned the certificates at on that particular install.

What if the client does not want to assign their STCs? If the client retains their own STCs, remove the rebate line item. The invoice shows the full gross cost and the client arranges their own STC assignment.