Solar String Sizing & DC:AC Ratio Calculator
Panels per string for your inverter, then the array layout and DC:AC ratio.
Domestic covers houses, apartments and attached garages. Commercial/industrial covers everything else. Limits per AS/NZS 5033:2021 Clause 3.1.
Typical example values to get started quickly - not a real datasheet. Selecting a preset fills in the fields below, which stay fully editable so you can correct them against your own module's actual datasheet.
Leave blank to reuse the Voc coefficient above - many datasheets only publish one. If your module's datasheet lists a separate Vmp (or Pmax) coefficient, use that instead for a more accurate minimum string length.
Typical planning range for the nearest main centre - not an official temperature record. Always use your own site's actual expected extremes for a real job; the fields below stay fully editable.
Not a standards figure - an engineering estimate of how much hotter the panel's cells run than the surrounding air under full sun. 25°C is a common default for a typical roof mount; lower for ground-mounted/well-ventilated arrays, higher for flush-mounted panels on a dark roof.
Array layout and inverter limits
Optional. Lay out the strings on each MPPT input to check the DC:AC ratio, the inverter's input limits, and each string's length against the range above.
Modules
Bifacial modules with light reaching the back: enter the Isc at BNPI from the datasheet instead (AS/NZS 5033 Appendix J).
Inverter (from its datasheet)
Where inputs have different limits, use the lowest. For more than one inverter, work each one out on its own.
For guidance only. These calculators are planning aids for licensed electrical workers and inspectors. All results must be independently verified against the current editions of AS/NZS 3000:2018, AS/NZS 3008.1.2, and other applicable standards before being relied on for any installation. CertMate accepts no responsibility or liability for any work carried out, decisions made, or certification issued in reliance on these results. The certifying electrical worker remains solely responsible for compliance with the Electricity (Safety) Regulations 2010 and all applicable New Zealand law.
Two temperatures, two different limits
String sizing is really a search for the range of string lengths that stays safe on the coldest morning of the year and still functions properly on the hottest afternoon. Panel voltage isn't fixed - it moves with temperature, and NZ's climate swings enough across a year that both ends of that range matter.
A string sized only for a mild spring day can push the inverter's input voltage over its limit on a frosty morning, sit above its MPPT tracking window on that same cold morning, or fail to stay within the MPPT window's minimum once the roof heats up in summer. This calculator applies both temperature corrections and gives you the range that's actually safe year-round, checked against your inverter's rating, its MPPT window at both ends, and the domestic or commercial DC voltage limit - whichever installation type you select.
Then check the array layout
Once you know the string length range, lay the strings out on the inverter's MPPT inputs. The layout section works out the DC:AC ratio and checks the array's power against the inverter's maximum DC input, each input's operating current against what the inverter will take, and each input's short-circuit current against the inverter's rating - the one AS/NZS 5033 makes a requirement (Clause 4.5.1.1). Each input's modules per string is checked against the range above too.
It deliberately doesn't estimate yield or clipping losses: those depend on the site, orientation, shading and weather, and belong in a proper system design.
Worked examples
Standard residential install, moderate climate
A typical monocrystalline panel on a residential roof in a moderate NZ climate, checked against a common mid-range string inverter. This is the baseline case most installs fall into, and a good one to sanity-check your usual go-to string length against.
Alpine or high-country site, cold winter mornings
The same panel and inverter, but installed somewhere that regularly sees sub-zero mornings in winter. Colder minimum temperatures push cold Voc higher, which can force a shorter maximum string length than the same system would need at a milder site.
Hot, north-facing roof, high summer temperatures
A north-facing roof that runs hot in summer, where panel surface temperatures well above ambient are realistic. Higher maximum operating temperatures push hot Vmp down further, which can raise the minimum string length needed to stay within the inverter's MPPT window.
Frequently asked questions
Why does cold weather matter more than hot weather for string sizing?
Voc rises as temperature drops, and it's the coldest expected temperature that produces the highest open-circuit voltage your string will ever see - typically a frosty winter morning before sunrise, not a hot summer day. That worst-case cold Voc is what has to stay under the inverter's maximum input voltage.
Why does hot weather set the minimum string length instead?
Vmp falls as temperature rises, so the hottest expected operating temperature produces the lowest operating voltage. If the string is too short, that hot-weather Vmp can drop below the inverter's MPPT window minimum, and the inverter loses its ability to track the panel's maximum power point properly.
What's the domestic DC voltage limit and does it ever bind before the inverter rating does?
New Zealand sets a maximum DC voltage for domestic solar installations under AS/NZS 5033, separate from whatever the inverter itself is rated to accept, and a higher figure for commercial and industrial installations - this calculator lets you choose which applies. On some inverters with a high maximum input voltage, that installation-type limit is actually the stricter constraint, which is why this calculator checks it alongside the inverter spec rather than instead of it.
Where do I get the temperature coefficient of Voc?
It's on the panel's datasheet, usually expressed as a percentage per degree Celsius, and it's always negative since Voc rises as temperature falls (a negative coefficient run through a negative temperature difference gives a positive correction). Different panel models vary meaningfully here, so use the actual datasheet value rather than a rule of thumb.
Why does the calculator also check the MPPT window maximum, not just the minimum?
Cold weather raises operating voltage as well as open-circuit voltage, so a string that's safely under the inverter's absolute maximum input rating on a frosty morning can still sit above the MPPT tracker's own usable voltage window. That doesn't damage the inverter, but it does mean the tracker can't do its job properly, so this calculator checks the string's cold-weather operating voltage against both limits.
Is there a maximum DC:AC ratio for solar in New Zealand?
AS/NZS 5033 and AS/NZS 4777.1 don't set one. The limit that applies is the inverter maker's - the maximum DC or PV input power on the datasheet - along with its current and voltage limits on each input.
What's the difference between the inverter's maximum input current and its short-circuit rating?
The maximum input current is the most the inverter will use from an MPPT input - more than that and the inverter typically limits the current it takes, losing some power at peak (check its datasheet). The short-circuit rating is a safety limit: AS/NZS 5033 requires it to be at least the circuit's current rating, 1.25 times the module short-circuit current for each string on that input.
What's the cell temperature rise field for?
A panel's cells run hotter than the surrounding air once the sun is on them, so using ambient temperature alone understates how far the operating voltage actually sags on a hot day. This isn't a standards requirement - it's an adjustable engineering estimate, defaulting to a common rooftop-mount assumption, that you can lower for a well-ventilated or ground-mounted array or raise for a flush-mounted panel on a dark roof.
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