How we model
How our numbers are built
The short answer
Your monthly power spend becomes annual energy; the share used in daylight sets the system size; regional yields set generation; cost bands, the Investment Boost and verified loan terms set the money. Used power is valued at 20.4 to 25.5 cents per kWh, from the national commercial average less its fixed charges up to the average, and exports at 8 cents.
The method
Eight steps from a power bill to a payback
01
Power bill to energy
Your monthly spend, ex GST, divided by 25.5 cents per kWh gives your annual use. That is MBIE's average price for commercial customers in the year to March 2026, lines and fixed charges included.
02
How much falls in daylight
Each kind of operation has a daily pattern: 30 percent of use in solar hours where power used mostly early and late, 45 percent of use in solar hours where power used across the day, 60 percent of use in solar hours where power used mostly in daylight.
03
System size
The system aims to cover 90 percent of that daytime energy at your region's yield, so it is sized to your load, not your roof. Results run from 15 to 500 kW; outside that, we say so.
04
Installed cost
A cost band per kW that falls with scale, interpolated between the sizes in the table below, so every result is a range, never a single price.
05
Investment Boost
Stated as year-one tax cash: the 20 percent immediate deduction at a 28 percent company rate, worth about 5.6 percent of the cost. A deduction, not a discount.
06
Generation and what you use
Size times regional yield gives generation. The share used on site runs from 55 percent to 80 percent by daily pattern, and never more than your daytime use; the rest is exported.
07
Saving
Power used on site is valued at 20.4 to 25.5 cents per kWh. The low end is that average less 20 percent for the fixed daily, capacity and metering charges solar does not reduce, the same rule a feasibility study applies to a bill that does not separate them; the high end is the average itself. Exports earn 8 cents, the bottom of the 7 to 17 cent range retailers pay.
08
Payback and finance
Payback is the cost after the Investment Boost divided by the saving, fast end to slow end. Where a loan offer is live and verified, the repayment is set next to the saving: we only say cashflow positive when the loan covers the whole cost and the repayment sits below the saving, and otherwise show the net monthly cost while the loan runs.
The assumptions
Every assumption, versioned
| Assumption | Value | Why |
|---|---|---|
| Average commercial price, all in | 25.5 cents per kWh | MBIE, year to March 2026; converts your spend into energy |
| Saving valued at | 20.4 to 25.5 cents per kWh | The average less 20 percent for fixed charges, up to the average |
| Saving on an irrigation connection | 13 to 18 cents per kWh | Energy and per-kWh lines charges only; the pump capacity charge stays |
| Export buy-back | 8 cents per kWh | Bottom of the 7 to 17 cent range |
| Company tax rate | 28 percent | States the Investment Boost as cash |
| System size | Low | High |
|---|---|---|
| 30 kW | $1,800 | $2,600 |
| 100 kW | $1,400 | $1,800 |
| 250 kW | $1,100 | $1,500 |
| 500 kW | $1,100 | $1,500 |
| Pattern | Share of use in solar hours | Share of generation used on site |
|---|---|---|
| Power used mostly early and late | 30 percent | 55 percent |
| Power used across the day | 45 percent | 70 percent |
| Power used mostly in daylight | 60 percent | 80 percent |
| Region | kWh per kW a year | What 100 kW makes a year |
|---|---|---|
| Marlborough | 1,390 | 139,000 kWh |
| Nelson Tasman | 1,330 | 133,000 kWh |
| Canterbury | 1,310 | 131,000 kWh |
| Central Otago | 1,220 | 122,000 kWh |
| Coastal Otago | 1,220 | 122,000 kWh |
| Southland | 1,140 | 114,000 kWh |
| West Coast | 1,120 | 112,000 kWh |
| Assumption | Value |
|---|---|
| Value of solar used on site | The charges your bills show per kWh (energy and per-kWh lines); fixed daily, capacity, demand and metering charges are left out, as solar does not reduce them |
| Bills that show only a total | The all-in rate less 20 percent for fixed charges; on an irrigation connection, 13 cents per kWh |
| Panel output decline | 0.5 percent a year (warranties claim about 0.4) |
| Operations and maintenance | $15 per kW a year |
| Inverter replacement | $150 per kW in year 13 |
| Analysis horizon | 25 years |
| Site yield | PVGIS for your exact location, derated to 90 percent for real installs |
| Emissions avoided | 0.074 kg CO2e per kWh used on site; exports are not counted |
| Assumption | Value |
|---|---|
| Share of output the pump uses | 10 percent where pumping runs mostly early and late; 25 percent where pumping runs across the day; 75 percent where pumping runs mostly in daylight |
| Pump and motor efficiency | 65 percent wire to water |
| Worked pivot | 100 ha, 400 mm a season, 5 mm a day over 22 hours, 70 m head |
| Season | September to April, peaking in December and January |
| Assumption | Value |
|---|---|
| Installed cost | $600 to $1,000 per kWh of usable storage, 100 to 500 kWh systems |
| Round-trip efficiency | 85 percent |
| Duration | 2 hours at full power |
| Capacity fade | To 70 percent over 10 years; value earned on the average |
| Payback test | Inside 10 years, a common warranty term |
| Surplus solar stored | 200 days a year |
| Demand charge cut | 50 percent to 80 percent of the battery's power over a control period; 25 percent to 75 percent at an anytime peak |
Sources
Where the numbers come from
- Regional yields: PVGIS satellite irradiance from the European Commission’s Joint Research Centre (version 5.3, ERA5 data) for representative towns in each region, re-derived on 2 October 2026 and derated 10 percent for real commercial installs. The feasibility study fetches the same data for your exact site.
- Cost bands: 2025 and 2026 working ranges for commercial installs in New Zealand, cross-checked against EECA’s commercial solar guidance of $1,500 to $2,000 per kW (October 2024).
- Power prices: MBIE’s sales-based electricity prices (commercial customers paid 25.5 cents a kWh ex GST on average in the year to March 2026, lines and fixed charges included; see commercial electricity prices) and retailer buy-back rates as at mid 2026.
- Investment Boost: Inland Revenue’s rules for new business assets from 22 May 2025. Read the Investment Boost guide.
- Loan offers: checked every day against the bank’s own page; an offer disappears from the site the moment its terms change, or if it cannot be verified.
- Emissions: the Ministry for the Environment’s factor for purchased grid electricity.
- Fixed charges on a bill that shows only a total: no published figure exists, so we built bills of 6,000 to 40,000 kWh a month on the 1 April 2026 lines schedules of Orion, Network Tasman, The Power Company and MainPower, with an assumed retail daily charge. Fixed charges came to 5 to 19 percent; a study takes off 20 percent, above all of them.
- Irrigation: each network’s irrigation charges from its schedule for 1 April 2026 (EA Networks, Orion, MainPower), contract energy as reported by Ruralco, the dedicated connection rules in Orion’s and EA Networks’ pricing policies, and Orion’s irrigation load profile. Read the irrigation guide.
- Batteries: no commercial price benchmark is published in New Zealand, so the cost band is a working range below home-battery pricing ($1,219 per kWh installed including GST, MySolarQuotes survey of 63 installs, September 2026); efficiency is the floor of EECA’s solar product specification; lines charges come from each network’s schedule for 1 April 2026, listed on the battery page.
The limits
What the calculator cannot know
The calculator works from one number, so it cannot see your roof’s pitch, direction or shading, your tariff structure (fixed, demand and time-of-use charges), the shape of your load hour by hour, an export limit your lines company might set, or site costs such as a switchboard upgrade or trenching. Nor does it know your tax position.
A feasibility study replaces each of those with your own data: twelve months of bills, PVGIS for your exact location and your roof from satellite imagery. A certified installer then inspects the site before the installation is priced.
Keeping it current
How the model stays honest
The assumptions carry a version, currently 2026-10-v7, and the worked examples and tables on the site state the version they were modelled with. Policy values such as the Investment Boost and loan terms live in a separate settings table with expiry dates, so a lapsed offer leaves the site without anyone touching the code.
The model is built and maintained by Michael Wilkins. If you think a number is wrong, tell us; we would rather fix it than defend it.
See the model run on your numbers.
Thirty seconds in the calculator, or a feasibility study built from a year of your bills.