What size solar system do I need?

Start with a full year of electricity use

Your last bill is not enough. Canterbury demand changes sharply between a mild January and a cold July, especially in an all-electric home. Add the kWh from 12 months of bills or download the annual total from your retailer. EECA puts average New Zealand household use at about 7,000 kWh a year, but treats that only as context. [1]

The annual total tells an installer the scale of the job. It does not show how much solar you can use directly. For that, you need the daily pattern: how much power is used between morning and late afternoon, and how that changes by season.

kW and kWh describe different things

A 5 kW array does not produce 5 kW all day. Output rises and falls with sun angle, cloud, temperature, shade and season. This is why a quote should include estimated monthly or annual generation in kWh, along with the assumptions behind it.

Three official examples show why household context matters

EECA publishes three home profiles. They are worked examples rather than universal packages. The large example is particularly useful for Canterbury: it models a five-person rural Christchurch household that is rarely home during the day, uses 12,000 kWh a year and has an EV. [1]

The table also exposes a common mistake: panel count alone is not a system size. Modern panels come in different wattages. Compare the total DC panel capacity, inverter capacity and expected generation.

Your daytime load decides how much solar is valuable

Solar first supplies whatever the house is using. Surplus goes to a battery, hot-water control or the grid, depending on the design. Electricity you use directly normally avoids a retail purchase; exported electricity earns the retailer's buy-back rate. Those two values are rarely the same.

Ask the installer to split forecast generation into three buckets: direct household use, battery charging and grid export. If every kWh is valued at the full retail rate, the savings forecast is overstated. Our Canterbury buy-back guide explains how to compare the whole power plan.

The usable roof can set the maximum before the bill does

A clean north-facing plane is straightforward, but it is not the only workable roof. East and west arrays can spread generation into the morning and afternoon. That may suit a household better than concentrating every panel around midday. Shade, hips, valleys, vents, chimneys, setbacks, roof condition and safe access all reduce the area that can actually be used.

Ask for a panel layout, not just a total panel count. It should show each roof face, panel orientation and any shade assumption. If the roof is older, compare the cost of reroofing first against removing and reinstalling panels later. For long-run metal roofs, use our Colorsteel and metal-roof solar guide.

A bigger array on the wrong roof face is not automatically a better design. The generation profile has to meet the household's load and the roof has to remain serviceable.

Panel capacity and inverter capacity do not have to match

EECA says around 20% more panel capacity than inverter capacity tends to give good value for many New Zealand homes. Panels seldom deliver their full nameplate output, so a 5.5 kW array on a 5 kW inverter can capture more energy in weaker light while clipping a small amount at the strongest peaks. [2]

This is not permission to use any combination. The inverter has voltage, current and total DC input limits, and the design must comply with its instructions and the network connection requirements. Ask the installer to state the DC-to-AC ratio and explain expected clipping.

Size for credible future loads, not vague possibilities

An EV, second heat pump, electric hot-water cylinder, induction cooking, spa pool or extra household member can materially change annual use. Put likely changes on a timeline and estimate them separately. “We might buy an EV one day” is not enough reason to double a system without modelling when and how the car will charge.

An EV parked away from home on weekdays may charge mostly at night. More panels will not directly serve that load unless the vehicle can charge during daylight or energy is stored. A smart charger can direct surplus solar to the car, but the charger, vehicle availability and minimum charging power all matter.

A battery does not fix an oversized array

Battery capacity is measured in kWh; battery output is measured in kW. The first tells you roughly how much energy can be stored, while the second affects how many appliances it can support at once. EECA lists common home battery bands of roughly 5–10 kWh, 10–15 kWh and 20+ kWh, and warns that a small solar system may not fully use a large battery. [2]

Model the solar-only system first. Then show what the battery changes: self-consumption, imports, exports, backup circuits, capital cost and payback. If resilience is the goal, confirm which circuits run during an outage and whether solar can keep charging the battery while the grid is down. See the full battery-or-no-battery guide.

Installed capacity and export capacity are different

From 11 May 2026, the Electricity Authority introduced a 10 kW default static export limit for qualifying streamlined small-scale connections. A distributor can apply a lower limit where a network study shows it is needed, or offer dynamic or flexible export control. The Code also says distributors must not set a nameplate generation limit for Part 1 or Part 1A applications. [3]

That means a 10 kW export setting is not automatically a 10 kW panel cap. A home can consume solar behind the meter while export is controlled, subject to the approved design. Orion says local capability still matters and assesses export increases against network conditions. [4]

“Add more panels later” needs an actual design path

Expansion can be possible, but it is not automatic. A string inverter may have spare input capacity, yet future panels still need compatible electrical characteristics and suitable roof space. Microinverters can make panel-by-panel additions easier, although each return visit adds labour and network paperwork may need updating. Another complete inverter-and-array system is also possible in some homes.

Ask what is genuinely reserved: inverter headroom, switchboard capacity, cable route, roof area, battery compatibility and monitoring integration. Product ranges change. A promise that the same panel will be available in five years is weak unless the design can tolerate a different module.

What a properly sized quote should show

If two quotes recommend different sizes, ask both installers to show the assumptions above. The better answer is not necessarily the larger system. It is the design that explains what each extra panel produces, how that energy will be used and what return it adds after export rates and installation cost.