Solar vs. Staying on the Grid: The Honest Maths

The honest answer: for most NZ homeowners with a sunny-ish roof, a 6-7 kW solar system breaks even somewhere between year 7 and year 11, and from that point onward it generates roughly $1,500-$2,500 of net household value per year compared with staying on the grid. Staying on the grid isn't "free", it's a 25-year subscription to whatever the wholesale market and your lines company decide to charge. Solar isn't a magic escape from power bills either; you still need a grid connection, you still pay daily fixed charges, and you still buy power at night. What solar actually does is convert an open-ended monthly bill into a mostly-fixed upfront cost, which is a very different financial shape. Below we'll lay both options side by side using realistic NZ numbers, so you can see exactly where the break-even sits for a household like yours.

This article is for the homeowner who's done a couple of late-night Google sessions and now wants the maths laid out honestly, no upsell. We're going to compare two scenarios for the same household: Plan A is doing nothing and staying on the grid for the next 25 years. Plan B is installing a typical residential solar system today and living with it for those same 25 years. Same house, same power use, two very different financial paths.

What "Solar vs. the Grid" Actually Means for NZ Homeowners

First, let's clear up a myth that costs Kiwis money every year: going solar does not mean leaving the grid. Around 99% of residential solar installs in New Zealand are grid-tied, meaning your house stays connected to the network and you simply generate some of your own power during the day.

When the sun's up and your panels are producing more than you're using, the surplus exports to the grid and your retailer pays you a buy-back rate for it. When it's cloudy, dark, or you're running the heat pump at 7pm, you pull power from the grid just like always. The grid acts as your giant, free-of-charge "battery", which is genuinely one of the better deals in modern infrastructure.

So the real comparison is not "grid vs. no grid". It's:

• Plan A (Status Quo): Buy 100% of your power from a retailer at retail rates, forever.
• Plan B (Solar): Buy a chunk of your annual power upfront as hardware on your roof, top up from the grid for the rest, and sell surplus back.

Both plans involve the grid. Only one involves a roof investment.

The Honest Numbers: A Side-by-Side for a Typical NZ Home

Let's use a realistic example household. We'll call them the Patels: a family of four in suburban Hamilton, with WEL Networks as their lines company. They use about 8,000 kWh per year, which is bang-on average for a NZ household according to EECA data. Their roof faces roughly north, no major shading.

Plan A: Stay on the Grid (25 Years)

The Patels currently pay around 30c per kWh for variable charges and a daily fixed charge of about $1.80. Their annual power bill works out to roughly:

• Variable: 8,000 kWh × $0.30 = $2,400
• Fixed: 365 × $1.80 = $657
• Annual total: ~$3,057

That's today. The honest bit: power prices don't stay still. Looking at MBIE's historical residential electricity price data, retail prices have risen roughly 2-4% per year on average over the past two decades, sometimes much more in single years.

If we assume a fairly conservative 3% annual increase, the Patels' total power spend over 25 years on Plan A is approximately $111,000. If we assume 4%, it's closer to $127,000. Either way, the "do nothing" option is not free; it's a six-figure commitment paid in monthly instalments to whichever retailer they're on.

Plan B: Install a 6.6 kW Solar System

The Patels get quotes (using something like our 3 free quotes service) and settle on a 6.6 kW system with a quality string inverter, no battery to start with. Realistic installed cost in 2024-2025 NZ pricing is around $13,500-$16,000 for a good-quality install. We'll use $14,500 as our working number. For where the per-watt market sits right now, see our breakdown of the current cost per watt for NZ solar installations.

In Hamilton, a 6.6 kW system typically produces about 8,800 kWh per year, slightly more than the Patels actually use. But because they're at work and school during peak production hours, they'll self-consume roughly 35-45% of that (call it 40%) and export the rest.

Year 1 maths looks like this:

• Self-consumed: 3,520 kWh × $0.30 saved = $1,056 saved
• Exported: 5,280 kWh × ~$0.12 buy-back = $634 earned
• Remaining grid purchases: ~4,480 kWh × $0.30 = $1,344 still spent
• Fixed daily charges: $657 (unavoidable, you're still connected)

So the Patels' year-one net power cost on Plan B is roughly $1,344 + $657 - $634 = $1,367, compared with $3,057 on Plan A. That's a $1,690 net benefit in year one, before counting the upfront $14,500.

Buy-back rates vary widely by retailer and tariff structure, so treat the 12c figure as illustrative; for live, retailer-by-retailer rates, our Dynamic Tariff & Buy-Back Engine is the source of truth. Some retailers like Octopus and Ecotricity offer dynamic structures that materially shift this maths.

Where Break-Even Lands

Stacking up Plan B's annual savings (which grow as retail rates climb) against the $14,500 upfront, simple payback for the Patels lands around year 8 or 9. From year 9 onward, every kWh the panels generate is essentially printing money compared with the Plan A baseline.

Over the full 25 years, Plan B's net spend (upfront + ongoing reduced bills - export income) comes in around $45,000-$55,000, depending on inflation assumptions. That's a $55,000-$75,000 lifetime advantage over Plan A.

For your specific roof, postcode, and power use, plug your numbers into our full cost and ROI breakdown, which walks through this maths with adjustable inputs.

What This Means for You (By Persona)

If You're the ROI Pragmatist

The headline number you care about is payback period, and the honest one is 7-11 years for most NZ homes, longer if you over-size or pay a premium for boutique kit, shorter if you self-consume more (e.g. work from home, EV, electric hot water).

The hidden lever is finance. If you draw from a low-interest green loan through Westpac, ANZ, BNZ, or Kiwibank, the monthly repayment can be close to (or less than) your current bill, meaning you're cash-flow positive from day one even before break-even. Check eligibility through our Green Finance Qualifier Tool.

If You're the Tech-Savvy Optimiser

The Patel example is conservative because it assumes static tariffs. If you move to a dynamic time-of-use plan with a retailer like Octopus Energy NZ, you can shift loads (dishwasher, EV charging, hot water) into solar production windows or low-cost overnight wholesale windows. Self-consumption rates can move from 40% up to 60-70%.

That single optimisation can shave 1-2 years off payback. Add a battery later (when you're ready, not because someone pressured you), and the maths shifts again.

If You're the Eco-Conscious Family

The ROI maths matters, but you're also thinking about the 25-year stability. Plan A means your kids' household running costs are at the mercy of wholesale electricity prices, drought-year hydro shortages, and whatever Transpower's grid upgrade costs land on retail bills.

Plan B locks in a big chunk of your future power costs at today's prices. That's not just an environmental decision; it's a hedge against an uncertain future. Roughly 8,000 kWh per year of fossil-displacing generation is also a measurable contribution, given the marginal generation on the NZ grid is still often gas-fired during dry years.

Common Pitfalls in the "Solar vs. Grid" Comparison

Both pro-solar and anti-solar arguments are full of dodgy maths. Here are the traps to watch:

• The ultra-short payback claim: Promises of breaking even in just a few years are almost always built on impossible assumptions (e.g. 80%+ self-consumption, no daily fixed charges, ignoring degradation). If a salesperson promises this, walk away.
• The "solar is a rip-off because exports pay peanuts" claim: Buy-back rates have improved significantly in the last three years, and self-consumption (not export) is where the real value lives. Designing your system around self-consumption changes the conversation entirely.
• Ignoring inflation in Plan A: Comparing today's bill to a solar system "without escalation" makes solar look worse than it is. Retail power prices have risen consistently for two decades per MBIE data.
• Over-sizing to "future-proof": A 13 kW system on a house with 8,000 kWh annual use just exports massive volumes at low rates. Size for self-consumption, not for ego.
• Assuming a battery is mandatory: Batteries currently add $8,000-$15,000 and stretch payback. They make sense for some households, but plenty of Kiwis get strong economics from a PV-only install and add storage later.
• Forgetting you still pay daily fixed charges: Even with a beautiful solar setup, you'll still owe your lines company the daily standing charge for being connected. Quote-givers who hide this are not your friends.

What About the SolarZero / Subscription Model?

You might be thinking, "Can't I get solar with no upfront cost and just pay a monthly fee?" Historically, that was SolarZero's pitch. The reality has been complicated for many households. If you're weighing a subscription-style offer against ownership, read our honest breakdown of the zero-upfront-cost solar model and what happened to SolarZero before signing anything.

The short version: ownership almost always beats subscription over 25 years, even when you factor in financing costs. The maths only flips if you can't access finance and won't be in the house for more than 7-8 years.

The Question Behind the Question: "Is It Worth It?"

For a fuller treatment of the worth-it question, including the cases where solar genuinely doesn't pay (heavily shaded roofs, very low power users, renters), see our companion piece: Are Solar Panels Worth It in NZ?

The honest summary is: solar suits the majority but not all NZ homeowners. If your roof gets reasonable sun, you use 5,000+ kWh per year, you'll be in the home for at least 8 years, and you can access either savings or green finance, then yes, Plan B beats Plan A by a comfortable margin. Outside those conditions, the maths gets tighter and you should be more cautious.

Frequently Asked Questions

Do I have to disconnect from the grid if I install solar?

No. Nearly all NZ residential solar systems are grid-tied. You stay connected, use the grid as your overnight and bad-weather backup, and export your surplus during the day. Full off-grid setups are an entirely different (and much more expensive) proposition.

How much can I save per year with a typical solar system in NZ?

For an average household with a 6-7 kW system, annual savings (combining bill reduction and export income) usually land between $1,500 and $2,500 in year one, with the figure growing as retail power prices rise. The exact number depends on your usage pattern, region, retailer, and whether you're on a dynamic tariff.

What happens to my power bill in a power cut?

Standard grid-tied solar shuts down during a grid outage for safety (so it doesn't electrocute linesmen working on the lines). If keeping the lights on during outages matters to you, you'd need either a hybrid inverter with backup capability plus a battery, or a small dedicated backup circuit. Most households decide outages are too rare to justify the extra cost.

Are buy-back rates going up or down?

Generally up over the last few years, with a wider spread between retailers. Octopus Energy NZ and Ecotricity have led on dynamic and competitive rates. Always check live rates rather than relying on year-old advice, and consider how much you actually export before optimising for buy-back rate.

What about when my panels degrade? Does the maths still work?

Quality tier-one panels typically degrade about 0.4-0.5% per year, meaning a 25-year-old panel still produces around 87-90% of its original output. This is factored into the 25-year maths above and doesn't materially change payback period.

Should I wait for solar to get more affordable?

Probably not. Per-watt installed costs have been broadly flat for the last 3-4 years in NZ because labour, scaffolding, electrical work, and compliance are now bigger components than the panels themselves. Meanwhile retail power prices keep climbing, so every year you wait is a year of Plan A spending you can't recover. The exception is if you genuinely can't afford it yet; low-interest green finance can fix that.

Does the maths work for a small household using only 4,000 kWh per year?

It works, but the margins are thinner. A smaller household should consider a smaller system (3-4 kW) to match actual self-consumption rather than chasing maximum export. Payback typically stretches to 10-13 years for low-use households.

What's the single biggest factor in whether solar beats the grid for my house?

Daytime self-consumption. The more of your own generation you use directly (heat pump, hot water, EV, fridge, freezer, working from home), the better the maths gets. If you can shift even one big appliance into daylight hours, you've improved your ROI without spending another dollar.

Where to Go From Here

If you've made it this far, you've got more honest grasp of the maths than 90% of homeowners who sign solar contracts. The next sensible step is to run the numbers on your roof, your power use, and your region, because the Patel example is illustrative, not prescriptive.

Three practical next moves:

• Read the full cost, finance, and ROI pillar to see all the variables that move the needle.
• Check whether you qualify for low-interest green finance through our Green Finance Qualifier Tool, because this single factor can change Plan B from "cash drain for 8 years" to "cash positive from month one".
• Get three competitive quotes from vetted installers so you're comparing apples to apples on hardware, warranty, and price.

Plan A isn't a "safe" option. It's a 25-year, six-figure subscription that quietly increases every year. Plan B has a real cost, real risks, and a real break-even, but for most NZ homes the maths is genuinely on its side.