NZ Solar Yield Estimator

Region

System size (kWp)

DC nameplate capacity of the array.

Array angle (° from horizontal)

Performance ratio

0.80

All system losses combined — inverter, temperature, soiling, cabling, mismatch. 0.80 is a fair default for a well-installed NZ system.

Annual generation

—kWh

Average day

—kWh

Across the whole year

Best month

—kWh/day

Leanest month

—kWh/day

Average daily generation by month


Month	Peak sun hours on the array (h/day)	Daily average generation (kWh/day)	Month total (kWh)	Share of the year
Year	—	—	—	100%

EstimateFigures are a long-run monthly average, not a forecast. Any single month will land above or below — a dull winter can run 30% under the average.
Irradiance — NIWA's measured mean daily global radiation, 1991–2020 normals, for 28 climate stations. Each region in the list is a real station, not an interpolation: annual totals run from 1163 kWh/m²/yr at Masterton to 1547 at Kaitaia.
Tilt — beam radiation is transposed onto the tilted plane with the standard monthly-average R_b ratio; diffuse is split out using the Erbs correlation and added isotropically, plus 20% ground reflectance.
Conversion — plane-of-array irradiation × system size × performance ratio.
AssumptionThe array faces true north at a single fixed pitch, with an unshaded horizon. A north-east or north-west roof loses roughly 3–8%; east or west, 15–20%. Shading, snow and inverter clipping are not modelled.
Sanity check — the model returns 1293 kWh/kWp/yr for Auckland at 30°, 1397 for Blenheim and 1110 for Invercargill, consistent with published NZ specific yields.
Station sitingFigures describe the station, not the whole region. Masterton and Timaru read low, Lake Tekapo and Queenstown high — those are real measurements at those sites. Pick the nearest station by climate, not just by distance.