Solar Finance Calculator
Compare the true cost of ‘doing nothing’ – using and paying the same amount in electricity – versus the cost of financing a solar installation and see how long until a financed solar system pays for itself from your power savings alone.
The ‘Do Nothing’ calculator option
The ‘do nothing’ scenario is the status quo option where you continue to pay your power bills, much as you are today. It feels like the “free” option because you’re not investing in anything extra – but power bills will keep arriving, and they tend to rise over time. This calculator shows how much you will spend in total on electricity over the years ahead if you ‘do nothing’.
The ‘Go Solar’ finance calculator
The ‘Go Solar’ finance calculator helps you to understand the financing period required to maintain a cost-neutral cashflow position – it’s the moment in time where a financed solar system has fully paid for itself using only the power savings it generates. Each month, your solar savings are used as the loan repayment, so your household cashflow is unchanged while the loan is repaid.
Example: a $12,000 solar system that saves $1,800 per year in power bills, financed on Westpac’s 0% Green Loan offer, reaches cost-neutrality in about six years. During those years the solar savings can cover your loan repayments entirely, so that you pay nothing extra out of pocket. After that period, the system continues to generate you additional savings until the end of its operational life.
Assumptions
- CPI-based inflation: 2.9% per year, the 10-year average of NZ Consumers Price Index inflation, 2016–2025 (Stats NZ).
- NZ electricity price inflation: 5.0% per year. This figure is assumed as an average of previous years' electricity price inflation (~3% per year, MBIE 2006–2022) and the more recent ~12% per year electricity price inflation observed over the previous two years (Stats NZ, year to March 2026).
- Do Nothing Option: your monthly bill is assumed to increase once per year at the selected inflation rate. The total is the sum of all bills over the period you remain in your home.
- Cost-neutral logic: each month, the loan repayment is set exactly equal to that month's solar savings (Annual Solar Return ÷ 12). The financing period shown is the time until the loan balance reaches zero.
- Solar savings growth: the Annual Solar Return is inflated each year at the NZ electricity price inflation rate above (5.0% p.a.), because every unit of grid power you avoid buying gets more valuable as power prices rise.
- ANZ Good Energy / BNZ Green / ASB Better Homes: 1% p.a. fixed for the first 3 years, on up to $80,000. Any balance remaining after the promotional period accrues interest at your mortgage rate.
- Westpac Greater Choices: 0% p.a. for the first 5 years, on up to $50,000. Any balance remaining after the promotional period accrues interest at your mortgage rate.
- Kiwibank Sustainable Energy Loan: variable rate assumed equal to your mortgage rate for the whole term. Kiwibank contributes $2,000 cashback over 4 years ($800 at the end of year 1, then $400 at the end of each of years 2–4) for loans over $5,000; this is credited against the loan balance.
- 0% Interest Q-Card: 0% for 48 months. A $55 establishment fee and a $50 annual account fee are added to the balance. Any balance remaining after the interest-free period accrues interest at the Q-Card standard rate of 27.99% p.a.
- Loan caps: where the System Cost exceeds a bank's cap, the excess is assumed to be financed at your mortgage rate, and repayments are applied to the higher-rate portion first.
- User Defined: the entire loan accrues interest at the mortgage rate you enter, for the whole term.
- Interest is calculated monthly on the outstanding balance. Bank offer terms were verified from public sources in July 2026 and may change — always confirm current rates and eligibility (e.g. 20% home equity for mortgage top-ups) directly with the lender.
This calculator is a guide only and does not constitute financial advice. Figures are estimates based on the assumptions above. Please seek advice from your bank or a financial adviser before making financing decisions. © ZEN Energy Systems New Zealand Limited.