What size solar system do I need?

The calculator takes your units for a year and divides them by what 1 kW of solar panels makes in a year in your region, on a roof with the direction and pitch you pick. It then rounds up to whole panels. The regional figures are long-run averages for one place in each region, from the Global Solar Atlas (World Bank Group, solar data by Solargis), with about a fifth lost in the panels, wiring and inverter. They are not a measurement of your roof. Many New Zealand roofs make less, and a roof that makes less needs a bigger system than the calculator shows.

The wattage of one panel, from its datasheet. The calculator starts with a 475 watt two-sided (bifacial) panel, counted on its front side only: fixed flat to a pitched roof, very little light reaches the back. Change it to match the panels you are looking at.

Each month is the system's size times the atlas's long-run average for that month, for your region, roof direction and roof pitch. Summer is December to February, autumn March to May, winter June to August and spring September to November, and each season's total is its three months added up. Figures are rounded to 10 kWh so the months add up to the seasons and the year. Facing north, a steeper roof makes more in winter. A real year can be sunnier or cloudier.

Not hour by hour. It is sized to make about as much electricity over a year as you use. Solar makes nothing at night and less in winter, so a home with a system this size still draws from the grid at times.

It is a guide only, and it can come out on the small side. Many things decide what a solar system actually makes, including your roof, shading and region. Ask us for a design sized to your home.