About this tool

I built this calculator while investigating solar panels for my own home. The existing tools available didn't quite answer the questions I wanted answered — namely:

• What size system can I fit on the north-facing section of my roof?
• Is this the right size given my usage patterns?

Hopefully it's useful to others facing the same questions.

Methodology

Solar generation is calculated using historical flat-plane solar irradiance data for the selected region, adjusted for roof tilt, orientation, and system size. The model estimates the electricity a solar system would generate across a typical year.

Savings are estimated by valuing electricity consumed directly from the solar system at the retail electricity price, and surplus generation exported to the grid at the assumed buy-back rate. Self-consumption is the primary driver of solar financial performance in New Zealand, and the calculator is designed to make that relationship visible.

Historically, right-sizing a system to match household usage was important — the low value of grid exports meant oversizing reduced returns. Installation costs have fallen significantly in recent years, and in many cases today the optimal system is simply the largest that fits within budget, roof space, and network export limits. Even so, understanding how generation aligns with usage remains worthwhile.

Note on batteries

At the time of writing, residential battery systems are often marginal on financial returns alone. However, batteries can provide energy resilience and support the electricity network by reducing a customer's contribution to network peaks. As electricity pricing and flexibility markets evolve, the financial case for batteries is likely to strengthen.

Disclaimer

This tool is provided for informational purposes only and should not be considered financial, engineering, or installation advice. Results are estimates based on typical regional solar radiation data and a number of simplifying assumptions — they do not account for site-specific shading, financing costs, maintenance, inverter replacement, or future tariff changes.

While reasonable care has been taken in building the calculator, no guarantee is made regarding the accuracy of results or projected savings. Site-specific advice from a qualified solar installer or energy professional should be obtained before making any investment decision.