If you've started collecting solar quotes, you've probably noticed that every company wants to talk about system size — 5kW, 6.6kW, 10kW, 13.2kW. But what do those numbers actually mean for your household, and how big should you really go?
Why System Size Matters
System size (measured in kilowatts, kW) describes the maximum output of your solar panels. Bigger systems generate more electricity — but the right size for you depends on how much power your household uses, when you use it, and how much suitable roof space you have. An oversized system exports its surplus cheaply; an undersized one leaves savings on the roof.
Typical Residential Sizes in New Zealand
- 5–6.6kW: The sweet spot for most Kiwi families — enough generation to cover daytime use with surplus to sell back or store.
- 8kW–10kW: Larger homes, or households with an EV, spa or multiple heat pumps.
- 13.2kW+: High-usage homes running an EV, pool and battery, where maximum self-sufficiency is the goal.
How Much Power Will a System Generate?
As a rule of thumb, a well-sited system in New Zealand generates around 3.9 kWh per day for every kilowatt installed, averaged across the year — noticeably more in summer, less in winter. So a 6.6kW system produces roughly 26 kWh per day, a 10kW system around 39 kWh, and a 13.2kW system just over 50 kWh. Location, roof orientation, tilt and shading all move the number, which is why a proper site assessment beats any online calculator.
Five Things That Change the Answer
- When you use power: Solar is most valuable when you consume it directly. Households home during the day get more from every panel.
- Roof orientation and shading: North-facing roofs perform best in New Zealand; east and west still work well with the right design.
- Battery plans: Adding storage lets a larger system power your evenings, not just your afternoons.
- Future loads: An EV, spa pool or workshop can double your usage — size for the household you're becoming, not just the one you are.
- Buy-back rates: Retailer buy-back plans differ widely. A good rate improves the economics of going larger.
Bigger Is Often Better Value
The cost per kilowatt falls as systems get larger, because fixed costs — scaffolding, the inverter, installation and certification — are spread across more panels. If your roof and budget allow, sizing up modestly now is usually cheaper than adding panels later.
Get a Design, Not a Guess
The honest answer to "what size do I need?" is: it depends on your roof and your power bill. EFS Solar designs every system from an on-site assessment, installs with licensed electrical workers, and backs the hardware with warranties of up to 30 years — with $0 upfront options available. Get a free quote and we'll size it properly.