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

For most New Zealand homes, the right answer is a 5kW to 6.6kW solar system, and here's why: a 6.6kW array generates roughly 8,500 to 9,500 kWh a year across the upper North Island, and around 7,500 to 8,500 kWh in the lower South, based on NIWA solar radiation data and EECA's generation estimates. That comfortably covers the daytime load of an average household, which Stats NZ and the Electricity Authority put at around 7,000 to 8,000 kWh of total annual use. A 3kW system suits a small, low-use home or a flat. A 10kW system only makes sense if you're charging an EV, running heat pumps hard, or have genuinely high daytime consumption. Size it to what you actually use during the day, not to the size of your roof.

Why getting the size right matters more than anything else

Sizing is the single decision that makes or breaks your solar economics. Go too small and you're still buying a chunk of expensive grid power at the worst times. Go too big and you're exporting power to your retailer for a fraction of what you paid to generate it.

The reason is buy-back rates. When your panels produce more than your home is using, the surplus flows to the grid and your retailer pays you for it. Trouble is, that buy-back rate is usually far lower than the retail rate you pay. As a rough guide, NZ buy-back rates sit somewhere between 7 and 17 cents per kWh depending on the retailer and plan, while you're paying 28 to 38 cents per kWh to buy power back. We break down how to read those rates properly over here: the cost and ROI calculator.

So every kWh you use yourself is worth two to four times more than every kWh you export. That single fact drives the entire sizing question. The goal isn't maximum generation. It's maximum self-consumption.

Step one: work out what you actually use, and when

Forget the brochure. The first thing to do is dig out your power bills and find your annual kWh usage. Most retailers (Genesis, Mercury, Contact, Meridian) show this on the bill or in your online account. Add up twelve months and you've got your total.

The average NZ household lands around 7,000 kWh a year, per Electricity Authority figures, but the spread is huge. A retired couple in a well-insulated Tauranga townhouse might use 4,500 kWh. A family of five in a draughty 1960s weatherboard place in Mount Eden with two heat pumps running all winter could be over 12,000 kWh.

But the total only tells you half the story. What matters for solar is when you use it.

The daytime question almost nobody asks first

Here's the bit installers often skip past: solar only offsets the power you use while the sun is up. If your house is empty from 8am to 5pm and everyone showers, cooks and watches telly after dark, a big array will spend its day exporting at the low buy-back rate.

Be honest with yourself about your daytime load. Ask:

• Is someone home during the day? Shift workers, retirees, people working from home, and stay-at-home parents shift the maths heavily in solar's favour.
• What runs in daylight hours? Fridge and freezer (always on), a heat pump on a cold but sunny winter day, the hot water cylinder if you put it on a timer, pool pump, dehumidifier.
• Can you shift load to the sun? Dishwasher, washing machine and dryer on delay timers; charging an EV during the day; heating hot water at midday instead of overnight.

A household that's home all day and shifts its big loads to daylight might self-consume 50 to 70 percent of what a 6.6kW system makes. A household out all day with no load-shifting might self-consume only 25 to 35 percent. Same panels, wildly different payback. That gap is the self-consumption trap, and it's the reason a system that pencils out beautifully for one house is a dud for the identical house next door.

What each size actually does

Generation depends heavily on where you are. NIWA data shows the upper North Island gets meaningfully more annual sun than Southland, but clear-sky regions like Central Otago do better in winter than you'd expect. As a working set of numbers for a well-oriented, unshaded north-facing roof:

3kW system (roughly 7 to 8 panels)

• Generation: around 3,800 to 4,500 kWh a year nationally.
• Suits: small homes, flats, one or two occupants, low daytime use, or a tight roof.
• Indicative installed cost: roughly $7,000 to $9,000 in 2025, per current installer pricing.

A 3kW system is a sensible entry point for a modest household that still wants to take the edge off daytime usage. It's rarely the best value per dollar, though, because the fixed costs of an install (scaffolding, inverter, labour, paperwork with your lines company) are spread across fewer panels.

5kW to 6.6kW system (roughly 12 to 16 panels)

• Generation: around 6,500 to 9,500 kWh a year depending on region and orientation.
• Suits: the typical NZ family home. This is the sweet spot for most.
• Indicative installed cost: roughly $9,000 to $13,000 in 2025.

Why does the industry quote 6.6kW so often? It's a quirk of inverter sizing. Most homes get a 5kW inverter, and the rules allow you to oversize the panel array up to about 133 percent of the inverter rating. Putting 6.6kW of panels on a 5kW inverter squeezes more generation out of the most affordable component (the inverter) and produces more usable power in the shoulders of the day, morning and late afternoon, when you're more likely to be home. It's genuinely good value engineering, not a sales gimmick.

10kW system (roughly 22 to 26 panels)

• Generation: around 13,000 to 17,000 kWh a year.
• Suits: large homes with high daytime use, EV charging, a pool, or a battery; lifestyle blocks; anyone genuinely running big daytime loads.
• Indicative installed cost: roughly $15,000 to $22,000 in 2025.

A 10kW system can be a cracking decision or a money pit, and the deciding factor is almost always whether you can absorb that much power during the day. Worth knowing too: some lines companies cap what you can export without extra approval. Vector in Auckland, Orion in Canterbury and others may require additional sign-off for larger systems or limit export to the grid, so a 10kW array doesn't automatically mean you can sell 10kW back. Your installer should confirm your network's rules before you commit.

The EV changes everything

If you own or are about to own an electric vehicle, throw the standard sizing rules out. An EV is the single best thing that can happen to your solar payback, because it's a large, flexible daytime load you control completely.

A typical EV in NZ travels around 11,000 to 14,000 km a year (Ministry of Transport figures), using roughly 2,000 to 2,800 kWh of electricity. If you can charge it during the day off your own panels, you're effectively running your car on power worth a fraction of petrol, and self-consuming surplus that would otherwise export at the low buy-back rate.

This is the worked example that makes it click. Take a Hamilton family (WEL Networks) with a 6.6kW system, home in the evenings only. Without an EV they self-consume maybe 35 percent and export the rest at the low buy-back rate. Add an EV charged on a timer between 10am and 3pm, and suddenly that daytime export becomes self-consumed power offsetting petrol. Self-consumption jumps toward 60 to 65 percent, and the whole system pays for itself years sooner.

For an EV household, 8kW to 10kW is often the right call rather than 6.6kW, specifically because you've got somewhere genuinely valuable to put the extra generation. The key is a charger that can be scheduled, or better, one that ramps charging to match what your panels are producing in real time.

Should you size for a battery now or later?

A battery lets you store daytime surplus and use it at night, which lifts self-consumption dramatically, often to 70 to 90 percent. But batteries are still the most expensive part of any solar setup in NZ, and whether they pay for themselves depends heavily on your buy-back rate and usage pattern.

If a battery is on your radar, size the array a little larger than your daytime load needs, because you'll want surplus to charge it. A 6.6kW to 8kW array pairs well with a typical home battery. We go properly deep on whether a battery actually pencils out, and how to size one, as part of the wider hardware picture: our main guide to solar hardware and tech.

A quick rule-of-thumb table

Rough starting points, to be refined against your actual bills and your installer's design:

• 1 to 2 people, out all day, low use: 3kW to 5kW.
• Average family, some daytime presence: 5kW to 6.6kW.
• Larger family, heat pumps, home during the day: 6.6kW to 8kW.
• EV in the driveway: 8kW to 10kW.
• EV plus battery plus high use plus a pool: 10kW and up.

The traps that catch people out

Sizing to the roof, not the load

Plenty of quotes max out the available roof space because more panels means a bigger sale. A 10kW system on a house that uses 6,000 kWh a year and is empty all day will export most of its generation at the buy-back rate. You'll have spent thousands more for power you're nearly giving away. Always tie the size back to your daytime usage.

Ignoring orientation and shading

A north-facing roof is ideal in NZ, but east-west splits can actually suit some households better because they spread generation across morning and afternoon, when people are home. A single chimney, a neighbour's poplar, or a two-storey house next door can knock a surprising chunk off output. This is also where panel choice starts to matter: some panels handle shade and low light better than others. If your roof has any shading, it's worth understanding the difference between cell types before you buy, which we cover in our look at N-type versus P-type cells in the NZ climate.

Counting winter generation like summer

Solar output in NZ swings hard with the seasons. A system might make four times as much in December as in June. NIWA radiation data shows the difference is steepest in the south and the cloudier west. Solar will take a solid bite out of your winter bill, but it will not zero it. Anyone promising otherwise is selling you something. Be wary of any quote that uses annual average generation to imply your winter bills disappear.

Assuming all the kit is equal

Two 6.6kW systems can perform very differently over 20 years depending on the panels and inverter. The "Tier 1" label gets thrown around as a quality badge, but it's actually a financial bankability ranking, not a guarantee about your warranty. It pays to know what it really means before you let it sway your decision: here's what Tier 1 actually tells you. And if you want a real-world example of how modern N-type panels stack up, we put some through their paces in our review of DAS Solar and Tongwei N-type panels.

How to lock in the right size

Here's the practical sequence:

• Pull twelve months of bills and find your annual kWh.
• Estimate your daytime share. Be ruthlessly honest about when the house is occupied and what runs during daylight.
• Factor in change. Buying an EV soon? Adding a heat pump? Retiring and about to be home all day? Size for where you're heading, not just where you are.
• Check your network's export rules. Ask your installer to confirm what Vector, Orion, Wellington Electricity, Powerco, Aurora or your local lines company allows.
• Ask the installer to model self-consumption, not just generation. A good quote shows estimated self-consumption percentage and the value of exports separately. If a quote only shows total generation, push back.
• Get more than one design. Sizing is where quotes diverge most. Comparing a few will tell you fast whether someone's overselling.

When you're ready to see real designs for your specific roof, we'll line you up with installers we've checked out ourselves: get three free quotes here.

Frequently Asked Questions

What size solar system does the average NZ home need?

For most households, a 5kW to 6.6kW system is the sweet spot. It covers the daytime load of an average home using around 7,000 kWh a year (per Electricity Authority figures) without producing so much surplus that you're exporting heavily at low buy-back rates.

Is a bigger solar system always better?

No. A larger array only helps if you can use the extra power during the day. If your home is empty from 9 to 5 and you don't shift loads into daylight, a 10kW system will export most of its generation at the buy-back rate, which is far less than retail. Size to your daytime usage, not your roof area.

How much power does a 6.6kW system make in New Zealand?

Roughly 8,500 to 9,500 kWh a year in the upper North Island and around 7,500 to 8,500 kWh in the lower South Island for a well-oriented, unshaded north-facing roof, based on NIWA solar radiation data and EECA generation estimates. Shading, orientation and tilt all change the figure.

Will solar get rid of my power bill?

Not for a grid-connected home, and especially not in winter. Solar can take a substantial bite out of your bill, but you'll still draw grid power at night and on dark winter days. NZ output can be around four times higher in summer than winter. Anyone claiming solar eliminates your bill is overselling.

Do I need a bigger system if I have an EV?

Usually yes. An EV is a large daytime load you control, which makes it one of the best things for solar economics. For EV households, 8kW to 10kW often makes sense rather than 6.6kW, especially if you can schedule charging for the middle of the day to soak up generation that would otherwise export at a low rate.

Should I add a battery to my solar system?

It depends on your buy-back rate and usage. A battery lifts self-consumption to around 70 to 90 percent by storing daytime surplus for the evening, but it's the most expensive part of a setup. If a battery is likely, size your array a touch larger so there's surplus to charge it, ideally 6.6kW to 8kW or more.

Can I add more panels later?

Sometimes, but it's rarely simple or low-cost. Your inverter has a fixed capacity, your roof has finite suitable space, and adding panels later means a second round of scaffolding and labour. If you're likely to get an EV or expand, it's usually better value to size up at the start than to retrofit.

Does my lines company limit how big my system can be?

It can limit how much you export. Networks like Vector (Auckland) and Orion (Canterbury) may require extra approval or cap export for larger systems, so a big array doesn't always mean you can sell all that surplus back. Your installer should confirm the rules for your specific network before you sign anything.

The Bottom Line

The right size isn't the biggest one that fits on your roof, and it isn't a one-size-fits-all number. It's the system that matches the power you actually use while the sun is up. For most Kiwi homes that's 5kW to 6.6kW. Add an EV and you're looking at 8kW to 10kW. The deciding factor every time is self-consumption: the more of your own generation you use, the faster the whole thing pays off.

Once you've got a feel for size, the next questions are usually cost and payback, and the quality of the gear itself. Run your own numbers with the cost and ROI calculator, and when you want to understand what separates good kit from budget kit, start with our guide to solar hardware and tech. Get those three things right, sizing, payback and gear, and you'll buy well.