How to Calculate How Many Solar Panels You Need
The number of solar panels required for a given system size is straightforward to calculate. You divide the total system capacity in watts by the wattage of each individual panel, then round up to the nearest whole panel:
Number of panels = System size (W) / Panel wattage (W)
Because panels are sold in whole units, you always round up. This means the actual installed system will be slightly larger than the target. For example, a 6 kW target using 400 W panels requires exactly 15 panels (6,000 / 400 = 15.0), giving an actual system size of 6.0 kW. A 5 kW system with 350 W panels requires 15 panels (5,000 / 350 = 14.28, rounded up to 15), giving an actual system of 5.25 kW.
Estimating Energy Output
Once you know the system size, you can estimate annual energy production using peak sun hours. Peak sun hours represent the number of hours per day during which solar irradiance averages 1,000 watts per square metre. The formula is:
Daily output (kWh) = System size (kW) x Peak sun hours x Efficiency factor
The efficiency factor (typically 0.75 to 0.85) accounts for real-world losses including panel heat, inverter conversion losses, wiring resistance, dust, and slight shading. A factor of 0.80 is a reasonable default for a well-installed residential system.
Annual output (kWh) = Daily output x 365
How Much Roof Area Do Solar Panels Need?
Modern residential solar panels are typically around 1.70 m tall and 1.0 m wide, giving a panel area of approximately 1.70 m2. The total roof area for panels alone is:
Roof area (m2) = Number of panels x Panel area (m2)
In practice, installers allow additional clearance between panels (typically 50 to 100 mm) for airflow and mounting hardware. Add around 10 to 15 percent to the panel area to estimate the total roof space required. The roof should ideally face north (in the Southern Hemisphere) and have a pitch of 15 to 35 degrees for optimal output in New Zealand.
Worked Example
Default inputs: 6 kW system, 400 W panels, Auckland (4.2 peak sun hours), 0.80 efficiency factor, panel area 1.70 m2.
| Step | Calculation | Result |
|---|
| Panels needed | 6,000 W / 400 W | 15 panels (exact) |
| Actual system size | 15 x 400 W | 6.00 kW |
| Total roof area | 15 x 1.70 m2 | 25.5 m2 |
| Daily gross output | 6.00 kW x 4.2 hrs | 25.20 kWh |
| Daily net output | 25.20 kWh x 0.80 | 20.16 kWh |
| Annual output | 20.16 kWh x 365 | 7,358 kWh |
This matches the calculator's default output when all fields are left at their defaults.
Peak Sun Hours Across New Zealand
| Region | Avg Peak Sun Hours (hrs/day) | Notes |
|---|
| Southland | 3.5 | Lower solar resource, longer winters |
| Dunedin | 3.8 | Coast provides some benefit |
| Wellington | 4.0 | Wind can reduce dust but shade can be variable |
| Auckland | 4.2 | Good year-round solar, mild winters |
| Christchurch | 4.5 | High sunshine hours, cold clear winters |
| Nelson / Marlborough | 4.8 | Highest solar resource in NZ |
These figures are annual daily averages. Summer months will produce significantly more; winter considerably less. Your installer will use month-by-month irradiance data for a more precise yield estimate.
Choosing the Right System Size
A common starting point for New Zealand households is a 6 kW to 10 kW system. A 6 kW system in Auckland produces around 7,300 kWh per year, which covers most of the annual electricity consumption of a typical family home (average household consumption is around 8,000 to 9,000 kWh per year, but a well-insulated home may use 6,000 to 7,000 kWh). Adding a home battery allows you to store excess daytime generation for evening use.
Related Calculators
Sources and method: Panel count formula: panels = system kW x 1,000 / panel wattage (rounded up). Energy output formula: daily kWh = system kW x peak sun hours x efficiency factor; annual kWh = daily kWh x 365. Peak sun hours derived from NIWA solar radiation atlas data for NZ regions (niwa.co.nz/climate/information-and-resources/sunlight-hours). Efficiency factor range (0.75 to 0.85) consistent with IEA PVPS methodology.
This calculator provides indicative estimates only. Actual energy output depends on panel orientation, roof pitch, shading, local microclimate, inverter type, and installation quality. Consult a registered solar installer for a site-specific assessment before purchasing a system.
