How to Size a Solar System: 3kW, 5kW, or 10kW?

The short answer: most standalone NZ homes land in the 5 kW to 6.6 kW sweet spot. A small flat or retired couple with low daytime use can often do well on 3 kW. A family with an EV, a heat pump or two, and a hot water cylinder running on solar should look at 8 kW to 10 kW. The right size isn't the biggest system you can fit; it's the one that matches your daytime consumption, your roof's usable area, and your future plans (EV, battery, heat pump hot water). Get those three things right and your payback looks great. Get them wrong and you're exporting half your power for a buy-back rate that won't excite anyone.

This guide is for Kiwi homeowners trying to make sense of installer quotes that range from a tidy 3 kW system to a chunky 10 kW one, often for the same house. We'll walk through how to read your power bill properly, the rules of thumb that actually hold up in NZ conditions, and what changes when you add an EV or battery into the mix.

No jargon dumps. Just the maths you need, and a few honest warnings about where installers tend to oversell.

What "Sizing" Actually Means for NZ Homeowners

When an installer quotes you a "5 kW system," they're talking about the combined DC capacity of the solar panels on your roof. A 5 kW array, on a sunny Auckland or Canterbury day around midday, will produce roughly 4 to 4.5 kW of AC power after inverter losses. Over a full year, that same system typically generates somewhere between 6,500 and 7,500 kWh, depending on roof orientation, tilt, and shading.

For context, the average NZ household uses about 7,000 kWh per year according to EECA and MBIE energy data, though this varies wildly. A small flat in Wellington might use 4,000 kWh. A four-bedroom Hamilton house with two EVs, a spa pool, and electric hot water can easily push past 15,000 kWh.

Here's the catch most installers gloss over: annual generation doesn't equal annual savings. What matters is how much of your solar you use yourself, versus how much you export to the grid for a lower buy-back rate. We'll come back to this.

The three numbers that actually determine your system size

• Daytime consumption (kWh): how much you use between roughly 9am and 4pm, when the sun is doing its work.
• Usable roof area: how many panels you can physically fit on north, east, or west facing roof without shading.
• Future load: are you planning an EV, a hot water heat pump, or a battery in the next 5 years?

Step One: Read Your Power Bill Like a Pro

Before you talk to any installer, dig out 12 months of power bills (or download them from your retailer's portal). Most NZ retailers, including Mercury, Genesis, Contact, and Meridian, give you a year's history in CSV or PDF form.

You're looking for two numbers:

• Total annual kWh consumption (sum of all monthly use)
• Daily average kWh across summer and winter separately

A house using 20 kWh per day on average has very different solar needs from one using 50 kWh per day. The first is comfortably served by a 5 kW system; the second needs 8 to 10 kW and probably a battery to soak up midday surplus.

The daytime-use question

If you're on a standard tariff, your bill probably won't break out daytime versus night-time use. You can estimate it: most households consume 30% to 45% of their power between 9am and 4pm. That climbs to 55%+ if someone's home during the day, runs a heat pump for daytime heating, or has hot water cylinder timers shifted to midday.

If you're on a time-of-use plan with Octopus, Electric Kiwi, or Contact's Good Nights, your retailer app will show this split clearly. That's gold for sizing.

The Three Sizing Tiers: 3 kW, 5 kW, and 10 kW

Let's walk through who each size actually suits, with realistic NZ numbers.

3 kW Systems: The "Just Right" Small Home

A 3 kW array is roughly 6 to 8 panels and produces around 4,000 to 4,500 kWh per year in most of the North Island, slightly less in Otago and Southland.

Best fit for:

• Retired couples or single-occupant homes using under 4,500 kWh annually
• Townhouses or smaller flats with limited north-facing roof
• Households with low daytime use (everyone out at work and school)
• Budget-conscious buyers wanting a starter system they can expand later

Where it falls short: if you add an EV, heat pump hot water, or work-from-home patterns, a 3 kW system runs out of headroom fast. You can technically add panels later, but adding 2 or 3 panels to an existing string inverter is fiddly and sometimes not possible without changing the inverter.

5 kW to 6.6 kW Systems: The Standard NZ Family Home

This is where the bulk of residential installs in Auckland, Tauranga, Hamilton, Wellington, and Christchurch land. Around 10 to 14 panels, generating 6,500 to 9,000 kWh per year depending on roof orientation and region.

The "6.6 kW" figure you see quoted a lot reflects the practice of slightly over-panelling a 5 kW inverter (a 5 kW inverter paired with 6.6 kW of panels). This is standard, fully legal under NZ regulations, and squeezes more generation out of cloudy mornings and late afternoons. It's a smart default, not a dodgy trick.

Best fit for:

• 3-4 bedroom homes using 7,000 to 11,000 kWh per year
• Families with kids home in school holidays and weekends
• Households running a heat pump and gas-free hot water
• People planning one EV in the next few years (with care around charging windows)

If you're not sure where you fit, this is the safe default. It's also the size most likely to qualify for green loans from Westpac, ANZ, BNZ, and Kiwibank without overstretching.

8 kW to 10 kW Systems: The Power User

Around 18 to 24 panels, producing 11,000 to 14,500 kWh per year. You'll need significant roof area, ideally north-facing, and you'll be on a single-phase or three-phase connection depending on your lines company's rules.

Best fit for:

• Larger homes (4-5 bedrooms) using 12,000+ kWh annually
• Households with one or two EVs and a willingness to charge during the day
• Lifestyle blocks with workshops, water pumps, or outbuildings
• Homes that have, or are planning, a battery to soak up midday surplus

The honest warning: without a battery or an EV that charges during the day, a 10 kW system on an average family home exports a huge amount of generation. You'll get the buy-back rate (which you can compare via our Dynamic Tariff & Buy-Back Engine) rather than offsetting your retail rate. The financial logic only really works if you're consuming what you generate.

The EV Factor: What Changes When You Plug In a Car

An EV is the single biggest variable in solar sizing for modern NZ households. A typical EV driven 15,000 km per year adds roughly 2,500 to 3,500 kWh of annual electricity use, depending on the vehicle.

That's not small. It's the equivalent of adding another half-house of demand. But here's the good news: if you can charge your EV during the day (working from home, retired, weekend-heavy driving), that demand soaks up solar that would otherwise export for a low buy-back rate.

Rules of thumb for EV owners:

• One EV, daytime charging possible: step up from 5 kW to 6.6 kW or 8 kW
• One EV, night-only charging: a 5 kW solar system plus a smart time-of-use plan (Octopus Intelligent or similar) often beats a bigger array
• Two EVs: seriously consider 8 kW to 10 kW, ideally with a smart EV charger that can follow solar output

If you want to dig into smart EV charging and the way dynamic tariffs stack with solar, that's covered in detail across our Hardware & Tech silo.

Roof Reality: What You Can Actually Fit

You can't size purely on consumption; the roof has the final say. A few NZ-specific things to factor in:

Panel size and roof area

Modern N-type TOPCon panels (the current default in NZ for quality installs) typically run 440-460 watts and measure around 1.9m x 1.1m. That means:

• A 5 kW system: roughly 22-24 square metres of roof
• A 10 kW system: roughly 44-48 square metres of roof

If you're curious why N-type panels have taken over from older P-type, we've broken that down in our N-Type vs P-Type comparison.

Orientation and tilt

In NZ, north-facing is best. East and west are still very productive (around 85-90% of north-facing yield) and often give you a better daily generation curve. South-facing isn't usually worth it unless the tilt is very shallow.

Most NZ roofs are pitched at 20-30 degrees, which is close to ideal for our latitude. NIWA publishes solar irradiance data by region if you want to dig into local sun-hour averages.

Shading: the silent system killer

A single tree shading one panel for two hours a day on a basic string inverter can drag down the whole string. If your roof has partial shading from neighbouring two-storey homes (very common in older Auckland and Wellington suburbs), ask your installer about panel-level optimisation: micro-inverters (Enphase) or DC optimisers (SolarEdge, Tigo). It costs more, but it can rescue an otherwise compromised site.

What This Means for You: Three Persona Snapshots

The ROI Pragmatist

Don't oversize. The maths punishes you. Every extra kW you install adds cost but, if it's just exporting at buy-back rates, the return on that marginal kW is poor. Aim for a system where 50-70% of your generation is self-consumed. Run your specific numbers through our Solar System Cost & ROI Calculator before signing anything.

The Tech-Savvy Optimiser

Size for your future load, not today's. If you're planning an EV in 12 months and a hot water heat pump in 24, a 5 kW system will leave you wishing you'd gone bigger. Pair the system with a hybrid inverter (Sungrow, Goodwe, Fronius GEN24) so you can add a battery later without replacing the inverter.

The Eco-Conscious Family

Size for total household decarbonisation. That usually means 6.6 kW to 8 kW, sized to cover hot water, heating, and at least one future EV. Choosing Tier-1 panels matters here for long-term reliability; we explain what that label actually means in our Tier-1 Solar Panels guide.

Common Pitfalls (What Installers Won't Always Tell You)

A few traps we see often when reviewing quotes for readers:

• "Bigger is always better" pitch. A 10 kW system on a household using 6,500 kWh per year exports masses of power for low buy-back rates. The payback on the last 5 kW of that install is often double the payback on the first 5 kW.
• Ignoring inverter sizing. If you ever want a battery later, you need a hybrid inverter from day one (or be prepared to add an AC-coupled battery, which is more expensive). Many entry-level string inverters are not battery-ready.
• Hand-waving the export limit. Some lines companies (Vector in Auckland, Orion in Christchurch, Wellington Electricity, Powerco, Aurora) have export caps, typically 5 kW or 10 kW on single-phase connections. A bigger array with an export limit is fine if you self-consume, but ask the installer to confirm the cap in writing.
• Quoting annual generation, not annual savings. These are very different numbers. A good quote shows estimated self-consumption percentage and a year-one savings estimate, not just kWh produced.
• Skipping the conversation about future EVs. If you're even thinking about an EV in the next 5 years, mention it. It changes the sizing recommendation materially.

Frequently Asked Questions

What size solar system do I need for a 4-bedroom NZ house?

Most 4-bedroom homes land in the 5 kW to 6.6 kW range, using around 8,000 to 11,000 kWh per year. If you've got an EV or are planning one, step up to 8 kW. If the house is gas-free with heat pump hot water, 6.6 kW to 8 kW is a safer target.

Is a 10 kW system overkill for a residential home?

For most homes, yes. A 10 kW system makes financial sense if you use 13,000+ kWh per year, run two EVs, have a pool or spa, or plan to add a battery. Without those, you're likely exporting half your generation for a low buy-back rate.

Can I start small and add panels later?

Yes, but with caveats. If your inverter has spare capacity and your roof has room, adding a few panels later is feasible but rarely cost-effective (installers charge for the truck-roll, scaffolding, and reconfiguration). It's usually smarter to size correctly upfront, especially for the second string of panels.

How many panels make 5 kW?

With modern N-type panels at around 440-460 watts each, a 5 kW array is 11-12 panels. The popular 6.6 kW configuration is typically 15 panels.

Does my roof orientation matter that much?

Yes, but not as much as people think. North-facing is best in NZ, but east and west deliver around 85-90% of north-facing yield and often give you a more usable generation curve (morning and afternoon peaks aligning with household demand). South-facing on a steep pitch is the only orientation that's generally not worth pursuing.

Do I need a battery to make solar worthwhile?

No. Most NZ homes get good payback from solar alone, especially if someone's home during the day or you can shift loads (dishwasher, washing machine, hot water cylinder) to midday. A battery is an optimisation, not a requirement. We cover this in more detail across our Hardware & Tech silo.

What's the difference between a 5 kW and a 6.6 kW system?

Usually the inverter is still 5 kW; the "6.6 kW" refers to the panel array being slightly over-sized to the inverter. This is a standard, legal NZ practice (the inverter caps output) that captures more generation in cloudy or shoulder conditions. It typically adds only a small amount to the install cost for meaningful extra annual generation.

How does an EV change my sizing?

An EV adds roughly 2,500 to 3,500 kWh of annual demand. If you can charge during the day, this is solar gold (you're self-consuming what would otherwise export). If you can only charge at night, a smart time-of-use plan often beats simply going bigger on solar.

Will my lines company let me install a 10 kW system?

Generally yes, but with export limits on single-phase connections. Vector, Orion, Wellington Electricity, Powerco, Aurora, and Unison all have published rules; your installer should handle the application. Three-phase connections allow higher export limits if your home has one.

How do I know if an installer is sizing my system honestly?

Ask them to show you the self-consumption estimate and year-one savings figure, not just the annual kWh generation. A trustworthy installer will ask you about your power bill, daytime occupancy, EV plans, and future hot water plans before they recommend a size. If the first thing they pitch is "the biggest we can fit," push back.

Where to Go From Here

Sizing is the single biggest decision in any solar quote, and it's where most homeowners either save thousands or quietly overspend. Once you've got a rough size in mind, the next steps are working out your realistic budget and payback (our Solar ROI Calculator is built for exactly this), then comparing what live buy-back rates are doing in your region.

From there, it's worth understanding the hardware that goes into a properly sized system: panel choices like DAS Solar and Tongwei N-type panels, panel tier ratings, and inverter choices that leave the door open for a battery later. The full picture sits in our Hardware & Tech pillar.

And when you're ready to put it to a real quote? Don't rely on a single installer's number. Three quotes from vetted Kiwi installers will tell you very quickly whether your sizing instinct is right and where the local sweet spot lives.