What size solar panel and battery do you need? Add up the watt-hours your gear uses each day, divide by the peak sun hours in your worst month, and allow about 25% for losses. This solar panel calculator for NZ does that maths with monthly sun data for nine centres, for baches, sheds, vans, cameras and a Starlink Mini.
The catch is winter. The sun sits low and the days are short, so a system that is ample in January can run flat in June; size it for the months you actually need it. Further down we cover grid-tied home solar and payback, and if you are unsure how much power your house uses, start with our home energy monitoring guide.
Key takeaways
- Panel watts = daily Wh ÷ (winter peak sun hours × 0.75). A 360 Wh a day load in Auckland needs about 140 W of north-facing panel for year-round use.
- In June, a tilted north-facing panel gets roughly 2.4–3.5 peak sun hours in most centres (about 1.5 in mountain-ringed Queenstown), but a flat panel gets only 1.1–2.0.
- Battery Wh = daily Wh × days of backup ÷ usable share (about 0.8 for lithium, 0.5 for lead-acid). Divide by the system voltage for amp-hours.
- Keep DIY systems at extra-low voltage: WorkSafe says anything above 120 V DC or 50 V AC, and any 230 V wiring, needs a licensed electrician.
- For home solar, EECA puts a 5 kW system at about NZ$11,500 installed, paying back in 7–10 years if you use most of what it makes.
NZ solar panel and battery calculator
Size the panel, battery and charge controller for a bach, shed, van, camera or Starlink Mini, using monthly sun data for nine NZ centres.
1. What will it power? Enter watts and hours a day for each load; set unused rows to 0.
| Load | Watts | Hours a day | Wh a day |
|---|---|---|---|
| Total per day | – | ||
2. Where and how?
Sun data: EU Joint Research Centre PVGIS (PVGIS-ERA5, 2005–2023), average daily irradiation on the panel for each month, in kWh per m² (peak sun hours). Panel = daily Wh ÷ (sun hours × 0.75), allowing 25% for heat, dirt, wiring, controller and battery losses. Battery = daily Wh × days ÷ usable share. Controller = panel W ÷ battery V × 1.25, rounded up to a common size. Shade, snow and long cloudy spells change everything, so treat the results as a starting point and size up if in doubt.
In this guide
How to size a solar panel and battery
Every off-grid calculation follows the same five steps. Here they are with a small example: a Starlink Mini, some LED lights and device charging at an Auckland-area bach used all year.
- Add up your daily watt-hours. Multiply each load’s watts by its hours of use. Starlink rates the Mini’s average draw at 25–40 W, so 30 W for 6 hours is 180 Wh; add 20 W of LED lights for 4 hours (80 Wh) and 50 W of charging for 2 hours (100 Wh) for 360 Wh a day.
- Find the peak sun hours for your worst month. For a north-facing panel near Auckland that is June, at 3.45 hours (kWh per m² per day).
- Size the panel. 360 ÷ (3.45 × 0.75) = 139 W, so fit about 140 W, for example one 150 W panel.
- Size the battery. Two days of backup on lithium: 360 × 2 ÷ 0.8 = 900 Wh, or 75 Ah at 12 V. A 100 Ah, 12 V lithium (LiFePO4) battery covers it with room to spare.
- Size the charge controller. 140 W ÷ 12 V × 1.25 = 14.6 A, so a 15 A MPPT controller, or 20 A if you might add a panel later.
The 0.75 factor covers the real-world losses between sunlight and stored energy: hot panels, dust and bird droppings, cable losses, the charge controller and the battery’s own charging losses.
Battery Ah to Wh, and what you can actually use
Watt-hours are amp-hours multiplied by voltage. What you can use depends on the chemistry: lithium iron phosphate (LiFePO4) tolerates deep discharge, while lead-acid lasts far longer if you only use about half.
| Battery | Nominal voltage | Stored energy | Planning figure for usable energy |
|---|---|---|---|
| 100 Ah LiFePO4 | 12.8 V | 1,280 Wh | about 1,020 Wh (80%) |
| 100 Ah AGM lead-acid | 12 V | 1,200 Wh | about 600 Wh (50%) |
| 200 Ah LiFePO4 | 12.8 V | 2,560 Wh | about 2,050 Wh (80%) |
| 100 Ah LiFePO4 | 25.6 V | 2,560 Wh | about 2,050 Wh (80%) |
Peak sun hours in NZ, month by month
Peak sun hours are the daily solar energy landing on a panel, expressed as hours of full 1,000 W per m² sunshine. The figures in this guide and the calculator come from the European Commission’s Joint Research Centre PVGIS tool (PVGIS-ERA5 data, 2005–2023), which models sunshine for any point on Earth, including terrain shading from surrounding hills.
| Centre | Best fixed tilt | June, tilted | June, flat | January, tilted | Yearly output per kW of panels |
|---|---|---|---|---|---|
| Auckland | 35° | 3.45 | 1.96 | 6.12 | 1,440 kWh |
| Hamilton | 36° | 3.19 | 1.81 | 5.84 | 1,340 kWh |
| Napier | 37° | 3.25 | 1.74 | 6.21 | 1,450 kWh |
| Wellington | 39° | 3.01 | 1.55 | 6.62 | 1,550 kWh |
| Nelson | 39° | 3.22 | 1.61 | 6.36 | 1,480 kWh |
| Christchurch | 41° | 3.01 | 1.40 | 6.41 | 1,490 kWh |
| Queenstown | 35° | 1.54 | 1.06 | 6.01 | 1,230 kWh |
| Dunedin | 42° | 2.54 | 1.17 | 5.82 | 1,360 kWh |
| Invercargill | 42° | 2.43 | 1.09 | 5.54 | 1,270 kWh |
Sun hours are kWh per m² per day on a north-facing panel. Yearly output assumes a grid-tied system with PVGIS’s standard 14% losses. PVGIS-ERA5 is modelled on a coarse grid, so local fog, cloud and hills can shift real results; NIWA’s free SolarView tool gives an address-level estimate that includes your local horizon.
Which way should solar panels face in NZ?
Face them true north, not magnetic north: a compass points roughly 20° or more east of true north in NZ, so use a phone map or a compass app set to true north. PVGIS puts the best year-round tilt at 35–42° across these centres. For a system that must get through winter, err steeper, because a steep panel catches more of the low June sun and sheds rain and dust. Flat panels on vans and boats are convenient, but in June they collect roughly half of what a tilted panel does in most centres.
Worked examples for a bach, a camera and a campervan
| Example | Daily use | Design sun hours | Panel | Battery (12 V lithium) | MPPT controller |
|---|---|---|---|---|---|
| Solar camera, Christchurch, all year | 120 Wh | 3.01 (June, tilted) | 60 W | 38 Ah for 3 days (fit 50 Ah) | 10 A |
| Starlink Mini at a Queenstown crib, all year | 360 Wh | 1.54 (June, tilted) | 320 W | 75 Ah for 2 days (fit 100 Ah) | 40 A (or 20 A at 24 V) |
| Campervan, Nelson, summer only | 640 Wh | 4.58 (March, flat) | 190 W | 134 Ah for 2 days (fit 150 Ah or more) | 20 A |
A solar-powered camera
A camera and 4G router drawing an average of 5 W around the clock use 120 Wh a day. In Christchurch that needs a 60 W panel for June, and three days of backup rides out a southerly. Our solar security camera guide covers the all-in-one cameras that come with their own small panel, which suit lighter loads.
Starlink Mini at the crib
Running a Starlink Mini for 12 hours a day at 30 W takes 360 Wh. Around Queenstown, where the surrounding mountains cut winter sun to about 1.5 peak sun hours, that means roughly 320 W of tilted panels, or 460 W laid flat. The Mini accepts 12–48 V DC, so it can run straight from a 12 V battery through a suitable DC cable, avoiding inverter losses. See our Starlink Mini guide for the kit itself, and our Starlink NZ overview for how the service works in rural areas.
A campervan in summer
A 12 V compressor fridge averaging 20 W, plus lights and charging, might total 640 Wh a day. With flat roof panels in Nelson from November to March, the design month is March (4.58 hours), giving about 190 W of panels and a 150 Ah or larger lithium battery.
Off-grid solar in NZ: practical tips
- Frost and lithium. LiFePO4 batteries must not be charged below freezing. In Central Otago, Canterbury and high-country huts, choose a battery with low-temperature charge protection or a built-in heater, or keep it inside the insulated part of the building.
- Shade matters more than you think. A shadow across part of a panel can cut its output sharply. Keep panels clear of trees and ridgelines, especially for the low winter sun, and clean off lichen and salt spray near the coast.
- Go 24 V as systems grow. Above about 400 W of panels, a 24 V battery halves the current, which means thinner cables and a smaller controller.
- Fuse everything. Put a fuse or DC breaker close to the battery on every positive cable; a lithium battery can deliver enormous current into a short circuit.
- Pick low-power electronics. An ESP32 that deep-sleeps between readings runs for weeks on a small battery, while a Raspberry Pi draws power constantly; our Arduino vs ESP32 vs Raspberry Pi comparison covers the trade-off, and a Pi 5 can use its real-time clock wake alarm to sleep between jobs.
What you can wire yourself
WorkSafe’s guidance on DIY solar is that systems above 50 V AC or 120 V ripple-free DC, and anything involving 230 V terminations, must be installed by a licensed electrician, and the homeowner wiring code (ECP 51) does not cover PV systems. Small 12 V and 24 V systems like the examples above stay under those limits. Stand-alone systems should follow AS/NZS 4509, and WorkSafe recommends a registered electrician for any PV installation.
Home solar in NZ: panel cost and payback
Grid-tied rooftop solar is a different calculation: the goal is to cut your power bill, and the grid is your battery. EECA’s figures, updated December 2025, are the best NZ benchmark:
| System size | Panels | Typical installed price (incl. GST) |
|---|---|---|
| 3 kW | about 7 | NZ$8,500 |
| 5 kW | about 12 | NZ$11,500 |
| 10 kW | about 24 | NZ$20,000 |
| Battery add-on | – | NZ$5,000–15,000 extra |
EECA estimates a 5 kW system saves a home with electric heating and hot water about NZ$1,047 a year in Southland and up to NZ$1,467 in Marlborough. If your household uses most of what the panels generate, it expects 7–10 years to pay back the upfront cost. The key is self-consumption: retailers pay far less for exported power than they charge you for imported power, so run the hot water cylinder, dishwasher, washing machine and EV charger during the day, using timers or smart plugs where it helps. From the PVGIS figures above, each kilowatt of well-placed panels produces roughly 1,230–1,550 kWh a year, so a 5 kW system makes around 6,100–7,700 kWh.
Grid-connected systems must be designed and installed by licensed electricians to AS/NZS 5033 and AS 4777.1, and your installer applies to the local lines company for approval to connect. Measuring your daytime use first, with smart-meter data or a CT clamp monitor, is the best way to pick the right size.
Common mistakes when sizing solar panels and batteries
- Sizing for summer. A system sized on January sun will fall short from May to August.
- Laying fixed panels flat. Fine on a van, but a fixed install loses up to half its winter output.
- Forgetting inverter standby draw. An inverter uses power even with nothing plugged in, so switch it off, or run 12 V appliances directly where you can.
- Treating lead-acid amp-hours as usable. A 100 Ah AGM battery gives about 50 Ah before it starts wearing out quickly.
- Using a PWM controller with large panels. Most household-size panels run at 30–40 V; an MPPT controller converts that extra voltage into charging current, where a cheap PWM controller wastes it.
- Skipping days of backup. A run of grey days is normal in an NZ winter; one day of storage is rarely enough for gear you depend on.
Our take
- Size off-grid systems for June unless you only use them in summer, and tilt fixed panels north.
- For small setups, 12 V lithium plus an MPPT controller is the simplest reliable combination; move to 24 V as the panel array passes about 400 W.
- Allow two to three days of backup for anything you rely on, such as a camera or Starlink at a remote property.
- For your house, get quotes against EECA’s benchmarks and size the system to your daytime use, not your total bill.
Sun data is modelled and your site may differ; prices were checked in September 2026 and change often. This is general guidance, not an electrical design. Nicegear is independent, sells nothing and has no paid placements.
Sources and further reading
FAQs
What size solar panel do I need to charge a 100 Ah battery?
It depends on how much you use each day. Replacing about 1,000 Wh a day, most of a 12 V 100 Ah lithium battery, needs roughly 390 W of north-facing panel in Auckland in June but only about 220 W in January.
How many solar panels do I need for a house in NZ?
EECA’s benchmarks range from about 7 panels (3 kW) to 24 panels (10 kW), with 12 panels (5 kW) a common middle size. Size it to your daytime electricity use, because exported power earns much less than imported power costs.
How long does solar take to pay back in NZ?
EECA estimates 7–10 years if your household uses most of the electricity the panels generate. Payback is slower if much of the output is exported at low buy-back rates.
Can I install solar panels myself in NZ?
You can build small extra-low-voltage systems, such as 12 V or 24 V off-grid setups under 120 V DC. WorkSafe requires a licensed electrician for systems above 120 V DC or 50 V AC, anything involving 230 V terminations and all grid-connected solar.
How do I convert battery Ah to Wh?
Multiply amp-hours by the battery’s voltage: a 100 Ah battery at 12.8 V stores 1,280 Wh. Then allow for usable depth of discharge, about 80% for lithium and 50% for lead-acid.
Can solar power a Starlink Mini?
Yes. Starlink rates the Mini at 25–40 W on average with a 12–48 V DC input, so 8 hours a day at 30 W is 240 Wh, which needs about 100 W of tilted panel in Auckland in June plus a battery for night-time use and cloudy days.
How many peak sun hours does NZ get?
According to PVGIS, a north-facing tilted panel gets about 5.5–6.6 peak sun hours a day in January and 2.4–3.5 in June in most centres. Flat panels get more in midsummer but only 1.1–2.0 in June.
