How the calculator works
Most solar calculators multiply a yearly output figure by your power price. That overstates the saving, because only the solar you use in the house saves the full retail rate. Power you export earns the buy-back rate, which is usually a third of the price you pay. So this calculator works out, hour by hour, when your panels produce and when your household uses power.
- Monthly output comes from PVGIS 5.3, the European Commission’s solar model, run for seven Canterbury towns, with 14% system losses and the same settings as our Christchurch and Canterbury solar output data. North-east, north-west and split east-west roofs use a sun-position model checked against the PVGIS east, west, flat and steep-roof runs.
- Each month’s output is spread across the day using the sun’s path over Christchurch, including daylight saving, then checked so the month still adds up to the PVGIS total. Sunny, average and dull days are modelled separately, because averaging them hides the surplus on sunny days.
- Your household use follows the daily pattern you choose, with more heating in winter mornings and evenings if you heat with electricity. A hot water diverter, a daytime EV charger and a battery each take surplus solar before it is exported.
- The saving is the power you no longer buy at your import price, plus exports at your buy-back rate. Payback includes your chosen power price rise and 0.5% a year of panel ageing.
Default figures and where they come from
| Setting | Default | Source or reason |
|---|---|---|
| Solar output | PVGIS monthly kWh by town | European Commission JRC PVGIS 5.3, ERA5 2005–2023 |
| Solar system price | About $1,480 plus $1,978 per kW | A line fitted to median 2026 quoted prices from 279 NZ quotes, published by My Solar Quotes ($2.04 to $2.54 a watt depending on size) |
| Battery price | $1,300 per kWh | Implied by the same 2026 quotes: solar-plus-battery systems cost about $12,000 more for 9 kWh |
| Power price | 32c a kWh incl. GST | Close to published Christchurch plan prices; change it to the rate on your bill |
| Buy-back rate | 12c a kWh | Mid-range flat rate in 2026; compare plans with our buy-back calculator |
| Battery efficiency | 90% round trip, 5 kW | Typical lithium home battery |
| Hot water | 6 kWh a day, more in winter, heated whenever it needs to | A typical family electric cylinder without a timer; choose overnight or solar heating if that is how yours works |
| Power price rise | 3.5% a year | MBIE: the average residential unit price roughly doubled from 18.9c to 38.3c a kWh over 20 years, about 3.6% a year |
| EV use | 0.17 kWh per km | Typical small to mid-size EV including charging losses |
Reading your results
Payback under eight years is a strong result in New Zealand. EECA’s 2025 study of half-hourly data from 21,038 Christchurch homes found simple paybacks of roughly 7 to 10 years. Installer quotes tend to look better: My Solar Quotes reports a median quoted payback of 6.9 years for solar alone and 9 years with a battery across 2026 quotes. The “Why an installer’s quote may show more” panel in the results explains the usual reasons for the gap. If your figure is much longer, check the roof direction, the system size and your daytime use.
The size chart matters more than the headline. Payback is usually shortest at the size that covers your daytime use. Beyond it, extra panels mostly export at the buy-back rate. That can still be worth it over 25 years, but it should be a choice, not the installer’s default. Our guide to sizing a solar system explains what a well-sized quote shows.
Winter is the weak point. A Canterbury home can export most of its December output and still import most of its July power. If cutting winter bills is your goal, read how far a battery goes on a Canterbury winter night before paying for a large battery.
A battery should earn its place. Compare the payback with and without one. If backup during outages is the reason, price it as insurance, and read battery or no battery.
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Solar calculator questions
How accurate is this solar calculator?
Monthly output comes from PVGIS, the European Commission’s solar model, run for Canterbury locations with 14% system losses. Annual output for a real roof is usually within about 10% of that once shade is accounted for. The savings depend most on how much solar you use at home, which the calculator estimates from the daily pattern you choose. Your retailer’s half-hour data gives a better answer, and a good installer should model it.
How many solar panels do I need in New Zealand?
Divide the system size by the panel wattage. A 6 kW system is about 14 panels of 440 W. The size chart shows which size pays back fastest for your household, because beyond a point extra panels mostly add exports at a low buy-back rate.
Why does the calculator show lower savings than some installer quotes?
Many quotes value every kWh of solar at the full retail price, or at an average price that includes the daily charge, and some assume steep yearly price rises. Exported power earns only the buy-back rate, often 8 to 16 cents a kWh, so a calculator that splits self-use from exports gives a lower saving. We checked ours against EECA's 2025 study of 21,038 Christchurch homes, and on EECA's assumptions it is not more pessimistic than EECA's results. Our quote checker tests a quote for this.
Does solar work in a Canterbury winter?
Yes, but output falls sharply. PVGIS puts a 5 kW north-facing Christchurch system at 316 kWh in June against 782 kWh in January. Winter is when household use peaks, so most homes still buy most of their winter power from the grid.
Is a battery worth it?
Turn the battery option on and compare the payback. A battery raises self-use, but at current battery prices it usually lengthens payback unless you export a lot of power, are on a plan with a high evening export rate, or value backup power.
Do I need to enter my address or email?
No. The calculator runs in your browser and does not send your inputs anywhere. You only share details if you choose to request quotes.
Free, no-obligation quotes
Turn this estimate into real quotes.
We pass your usage, roof and preferred system size to installers who work in your part of Canterbury, so you can compare their designs and prices side by side.
Get my free quotesSources used
- European Commission Joint Research Centre, PVGIS 5.3 (PVGIS-ERA5, 2005–2023), via our Canterbury solar output dataset
- My Solar Quotes: What is the payback on solar for NZ homeowners (2026 data)
- EECA: Understanding the value of residential solar PV and storage in New Zealand (June 2025)
- My Solar Quotes: Using less power, paying more for it (MBIE sales-based electricity costs)
- EECA: Planning the right home solar system for you