Solar String Sizing Calculator
Work out min and max panels per string for your inverter.
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, or fail to stay within the MPPT tracking window 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 and the domestic DC voltage limit.
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. On some inverters with a high maximum input voltage, the domestic limit is actually the stricter constraint, which is why this calculator checks both rather than just the inverter spec.
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.
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