Blackcurrent · Pre-sales sizing
Monthly average daily generation for a north-facing fixed array anywhere in New Zealand. Pick a region, a system size and a roof pitch — the table and chart update straight away.
DC nameplate capacity of the array.
All system losses combined — inverter, temperature, soiling, cabling, mismatch. 0.80 is a fair default for a well-installed NZ system.
| Month | Peak sun hours on the array (h/day) |
Daily average generation (kWh/day) |
Month total (kWh) |
Share of the year |
|---|---|---|---|---|
| Year | — | — | — | 100% |
Every region in the estimator is a real NIWA climate station with three decades of measured radiation behind it. Nothing here is interpolated, and nothing is modelled from sunshine hours.
NIWA publishes mean daily global radiation for its climate station network as 1991–2020 normals — the monthly average of daily total solar energy landing on a horizontal surface, in MJ/m²/day. The estimator uses all 28 New Zealand stations in that dataset, converted to kWh/m²/day.
These are pyranometer measurements, not satellite estimates or model output. That matters: it is why the numbers below occasionally contradict the reputation of a place.
| Station | Annual (kWh/m²/yr) |
January (kWh/m²/day) |
June (kWh/m²/day) |
Seasonal spread |
|
|---|---|---|---|---|---|
Over a full year the two are close — Wellington comes in about 5% under Auckland, which is less than most people expect. But the annual figure hides the whole story. In December they are identical. In June, Wellington produces a quarter less.
The cause is latitude, not cloud. Wellington sits 4.4° further south, so its midwinter sun peaks at 25.3° above the horizon against Auckland's 29.7°, over a shorter day. Wellington actually records more annual sunshine hours than Auckland — sunshine hours and solar radiation are not the same measurement, and only one of them generates electricity.
What it means for sizing. A Wellington site sized on annual average will feel materially shorter of generation through winter than an Auckland site with the same array. If the load is winter-weighted — heating, dairy, glasshouses — size against the June row in the table, not the annual total.
Nelson's reputation rests on sunshine hours — the count of hours the sun is not obscured. On radiation, which is what a panel converts, Nelson records 1462 kWh/m²/yr and sits below Auckland. Kaitaia, Lake Tekapo and Blenheim all beat it.
A place can log many hours of weak, low-angle winter sun and still collect less energy than somewhere with fewer but stronger hours. Quote radiation in a sales conversation, not sunshine hours.
Four steps run each time you change an input. Everything is a monthly-average method — one representative day per month, following Duffie & Beckman.
H, in kWh/m²/day.Kt = H / H0, where H0 is the radiation that would arrive with no atmosphere at all.Rb, the geometric ratio of beam falling on the tilted surface against the horizontal. Diffuse is added isotropically, plus 20% ground reflectance. This is where a winter gain appears: tilting north lifts low midwinter sun much more than it lifts high summer sun.Every loss between the panel face and the meter, as one number: inverter efficiency, cell temperature, soiling, cabling, module mismatch, and availability. 0.80 is a fair default for a well-installed New Zealand system. Below 0.75 suggests a real problem; above 0.85 is optimistic outside a cool, clean, well-ventilated site.
Use it for sizing conversations, indicative payback maths, and sense-checking a third-party yield figure that looks wrong. Not a substitute for a site-specific design.
At 30° north-facing with a 0.80 performance ratio the model returns 1293 kWh/kWp/yr for Auckland, 1397 for Blenheim and 1110 for Invercargill — all inside the 1100–1400 band commonly quoted for New Zealand. Optimal pitch comes out between 27° in the Far North and 37° in Central Otago, matching standard guidance.
blackcurrent-energy/solar-yield-estimator.