Comparing ‘Interest‑Free’ Solar Deals With Using Your Home Loan

Most ‘interest‑free’ solar deals in Australia aren’t really free. The cost is usually baked into higher system prices, account fees and short terms. To compare them fairly with using your home loan, you need to strip every option back to the same basics: cash price, fees, term, interest and total dollars you’ll repay.

In this guide, we’ll walk through a simple, decision‑grade method you can use this week to compare a shiny “0% interest” offer against topping up, splitting or refinancing your home loan.

Start every ‘interest‑free’ comparison with a clean cash quote and the full fee list.

1. The quick answer: how to compare ‘0%’ solar vs mortgage

To compare an ‘interest‑free’ solar promotion with using your home loan:

1. Get a clean cash quote for the same system, from the same installer if possible.
2. Compare that to the “interest‑free” financed price, including establishment, monthly and account‑keeping fees.
3. Work out the effective extra cost of the finance (financed price + all fees – cash price).
4. Divide that extra cost by the loan amount and term to get a rough effective interest rate.
5. Compare that rate (and the total repaid) with a separate 5–10 year home‑loan split at your current mortgage rate plus 3%.

In many cases, the ‘0%’ deal works out like paying a 10–20%+ p.a. rate on a normal loan, especially once you include higher system prices and fees. A short 3–5 year term can still be fine if your cashflow handles the payments and you want the debt gone quickly. But if the repayments feel tight, a dedicated 5–10 year principal‑and‑interest home‑loan split can be safer, as long as you don’t stretch the term out to 25–30 years.

2. What “interest‑free” solar really means in practice

2.1 Where the cost usually hides

Most 0% solar offers are built on:

• Higher system prices than a genuine cash quote.
• Account and payment processing fees (sometimes $5–$10 per month).
• Upfront establishment or platform fees.
• Merchant fees charged to the installer, which they pass through in the price.

So instead of charging you interest openly, the provider recovers its margin via the price and fees.

This is why we always say in our red‑flags guide [/insights/red-flags-solar-finance-contracts]: never compare finance options without a clean cash price for the same system.

2.2 Why term length matters more than the sticker rate

With solar, the term can matter more than the rate:

• Solar systems typically have a 15–25 year useful life.
• Most ‘interest‑free’ deals run 3–7 years.
• A standard mortgage runs 25–30 years.

If you stretch a $12,000 system over 30 years at a low rate, you can easily pay more total interest than a 5–7 year high‑rate loan – but the monthly repayments are much lower.

That’s why our broader solar borrowing guidance is to:

• Keep solar/battery borrowing on a separate 5–10 year home‑loan split, not blended into the main 25–30 year term.¹
• Model all your home and investment debt at current rates + 3%, and keep repayments under roughly 30–35% of after‑tax income.²

3. Step‑by‑step: turning a solar promotion into a real interest rate

3.1 Start with a worked example

Imagine you’re quoted:

• Cash price: $12,000 for a 10 kW system
• “Interest‑free” promotion:
  • Advertised as 0% over 5 years
  • Financed price: $13,800 (same system)
  • Establishment fee: $299
  • Monthly account fee: $8
  • Term: 60 months

Step 1: Calculate total cost of the “interest‑free” deal

• Extra price over cash: $13,800 – $12,000 = $1,800
• Monthly fees: $8 × 60 = $480
• Establishment fee: $299

Total extra cost of finance = $1,800 + $480 + $299 = $2,579

Total paid under ‘0%’ deal = $13,800 + $480 + $299 = $14,579

Step 2: Approximate the effective interest cost

Treat the cash price ($12,000) as the amount borrowed.

• Extra cost of finance: $2,579
• Over 5 years, that’s about $2,579 ÷ 5 = $516 per year in finance costs on an average balance of roughly $6,000.

A rough effective rate ≈ $516 ÷ $6,000 ≈ 8.6% p.a.

A proper calculator would land a bit higher or lower depending on the repayment profile, but it’s clearly not 0%.

3.2 Compare with a home‑loan split

Assume:

• Current mortgage rate: 6.50% p.a. variable (illustrative only)
• You set up a separate 7‑year P&I split for solar (to align with useful life and our guidance on shorter splits).

Use a basic repayment formula (or a calculator) for $12,000 over 7 years at 6.50%:

• Monthly repayment ≈ $179
• Total repaid ≈ $179 × 84 = $15,036
• Total interest ≈ $3,036

On raw dollars, the 0% deal over 5 years ($14,579) looks slightly cheaper than the 7‑year mortgage split ($15,036). But the home‑loan split has:

• A longer term, so lower monthly strain.
• A rate that can move up or down.
• Security over your home – raising the stakes if you hit trouble.

Now stress‑test that mortgage rate at 9.5% (6.5% + 3% APRA‑style buffer). The same $12,000 over 7 years at 9.5% is about:

• Monthly repayment ≈ $196
• Total repaid ≈ $16,464
• Total interest ≈ $4,464

Once you stress‑test, the ‘0%’ deal is clearly cheaper in total dollars – but the monthly repayment will be higher because the term is only 5 years.

This is why you must compare both:

• Total dollars repaid; and
• Monthly cashflow impact under realistic stress‑tested scenarios.