String inverter, microinverters or optimisers?
How to choose a string inverter, power optimisers or microinverters for solar panels on a New Zealand roof, based on shade, faces and later battery plans.
The inverter is the part that turns panel direct current into alternating current the house can use. On a straightforward north-facing metal roof with little shade, a string inverter is the usual tool and the lower hardware price. Microinverters and power optimisers cost more and earn that cost when shade or several roof faces would otherwise drag a whole string down.
Buy the problem you have. A brochure that puts microinverters on every house is selling the upgrade, not reading the roof. Ask the designer to say, in one paragraph, why this roof needs the dearer option or why it does not.
String inverters
Panels are wired in series into one or more strings. The inverter sits on a wall, usually near the board. If one panel in a string is shaded, soiled, or failed, that string's output falls. On a clear single face this is rarely the deciding issue. On a roof with a chimney shadow across one corner for two hours, it can be.
String inverters are a single box to replace later. Warranties are often shorter than the panel warranty. Ask for the years, and whether the quote includes an extended warranty. A hybrid string inverter can be the path to a battery later. Say if that is even a possibility, so you are not replacing the inverter in year three.
Monitoring on a basic string inverter may show the whole array rather than each panel. That is enough when the roof is simple. It is thin when you need to see which module is down.
Optimisers
A power optimiser is a small unit on each panel, or on shaded ones, that conditions that panel's output before a central inverter. Shade on one module does less harm to the others. You still have one inverter on the wall, with one warranty and one replacement job.
Optimisers are a middle price. They suit a mostly clear roof with one known shade problem, or a split face where you want panel-level data without a microinverter on every module. Ask which panels get an optimiser. A quote that adds them to every module on a clear roof should explain why.
Microinverters
A microinverter sits under each panel and produces alternating current on the roof. Shade on one panel stays with that panel. Different faces and tilts are easier to mix. Failure looks different: one small unit can drop out while the rest keep working, and you may not notice without panel-level monitoring.
Hardware cost is higher. Roof work to replace a failed unit later means access and, on some houses, scaffold again. Weigh that against the cost of replacing one central inverter. Neither path is free in year twelve.
Microinverters still have to be accepted by the lines company in the distributed generation application. A brand the network will not list is not a bargain. Ask which model is being lodged.
How to match the roof
Use the roof you have, not a default.
| Roof | Usually enough | Spend more when |
|---|---|---|
| One clear north metal face | String inverter | You already know you want panel-level data or a specific warranty |
| Two faces, little shade | String inverter with two strings, or optimisers | The faces are very different in pitch or shade |
| Chimney, tree, or dormer shade | Optimisers or microinverters | The shade covers panels for hours, not minutes |
| Battery likely later | Hybrid-ready string inverter | The battery brand locks you to a different inverter now |
Shade, yield and price
A tree or a second storey can cost more energy than the gap between two panel brands. Photographs in the morning and mid-afternoon tell the designer more than a product sheet. The roof guide covers shade and orientation: https://homesolarinstallation.co.nz/blog/solar-roof-types-nz/
Put the inverter choice on the same comparison sheet as kilowatts and price. Two quotes at the same kilowatts are not the same quote if one includes microinverters and the other does not. EECA's installed bands, about $8,500, $11,500 and $20,000 before a battery, are standard-install context. Electronics above a plain string inverter sit on top of that. See https://homesolarinstallation.co.nz/blog/solar-panel-cost-nz/
Annual yield should say whether shade was modelled. A perfect north-roof number on a shaded section is not a forecast.
Batteries and export gear
If a battery is a real plan, the inverter decision is part of it. Some string inverters are hybrid and can take a battery later. Some cannot. Microinverter systems need a battery approach that matches that architecture. Ask for the path in writing before you rule storage out forever or buy it now.
EECA's view is that solar without storage usually returns better. The battery guide is the decision rule: https://homesolarinstallation.co.nz/blog/solar-battery-storage-nz/
Export limits are enforced by the inverter. Whichever type you buy, it has to be set to the cap in the approval. The export guide: https://homesolarinstallation.co.nz/blog/solar-export-limits-nz/
What to ask
Keep the questions short enough to send.
- Which model, and is it on the lines company's accepted list?
- What shade on this roof made you choose this type?
- Warranty years, and who I call if the installer has closed
- Panel-level monitoring or array-level only?
- Can a battery be added later without replacing this inverter?
Roofs across the north and the Shore
Inverter choice follows the roof in Algies Bay, Arkles Bay, Army Bay, Big Omaha, Coatesville, Dairy Flat, Dome Forest, Dome Valley, Glorit, Gulf Harbour, Hatfields Beach, Helensville, Huapai, Kaukapakapa, Kawau Island, Kumeu, Leigh, Matakana, Muriwai, Omaha, Orewa, Puhoi, Riverhead, Snells Beach, South Head, Waimauku, Warkworth, Wellsford, Bayswater, Belmont, Campbells Bay, Castor Bay, Devonport, Forrest Hill, Hauraki, Milford, Narrow Neck, Stanley Point, Sunnynook and Takapuna. A simple long-run roof in Dairy Flat, Kumeu, Huapai or Waimauku often suits a string inverter. Tight two-storey roofs in Devonport, Stanley Point, Narrow Neck and Bayswater pick up shade from neighbouring houses and may justify optimisers. Coastal salt at Muriwai, Leigh, Gulf Harbour, Orewa and on Kawau Island is a reason to ask where the inverter is mounted and whether the hardware is rated for that air, not a reason to buy the dearest option by default.
Home Solar Installation NZ can introduce a local partner. The form is free and there is no obligation. Ask them to price the string design and the shaded design separately if both are plausible. Public cost context is on EECA's page: https://www.eeca.govt.nz/for-homes/solar-for-homes/solar-costs-and-savings/