There are two type of solar system basically in the on-Grid methodology.
First, there are traditional string systems whereby the panels provide power to a single inverter below at house level using DC voltages. These systems are limited in upgradeability and must be a consideration. NZ5033 states since August 2013 by regulation any upgrades to a system must be Panels like for like exact electrically identical. So as time goes by the likelihood of manufacturer keeping the same model number panel is reduced eliminating upgrade paths. Also typically you would not invest in a larger inverter because of this in an attempt to upgrade the system as you lose efficiency and output with a larger DC string inverter planning to upgrade later. Just to find NZ5033 regulation prohibits the upgrade due to panel changes. If there is a failure the entire system is down until the inverter is returned from the manufacturer from being repaired or replaced. Also makes it difficult to actually monitor the individual panel output warranty due to system panels being measured as a whole instead of individually as in Micro inverters.
Second, Is what is called a micro inverter system, they are newer technology and each panels has its own inverter and converts the DC current to AC current directly on the roof which the house uses as its power standard 230v AC. This makes them safer, and they incur less cable loss as there is virtually no DC cable length. They also negate the regulation as each panel has its own inverter and therefor once the initial system is in place it is easily upgradeable in the future and provide better efficiency as shading does not affect the whole system but rather each panel independently.
Also Micro inverters can be replaced on the same day if they have a failure and only one panel is affected by a failure as well. However they are slightly more expensive. Enphase Micro inverters also will have battery modules c that easily store the excess power you would export back to the grid and make it available to use nightly to further reduce power bill charges. They also allow more precise measurement of 25 panel output warranty due to each panel having its own inverter.
The next decisional factor is to design a system to maximise return on the investment while limiting risk of over exporting back to the grid since there is no contracts here in New Zealand guaranteeing feed in tariffs, they can be changed by the retailers at their discretion on no notice. So you want to find a happy medium where you have a system large enough to cover a good portion of the bill to insulate from price increase which are about 5% per year, and also limit export as much as possible and keep the return on investment as high as possible. Below are the summaries for your options available.
Things to measure, warranty, failure rate, efficiency, transformer or no transformer (optimum is transformer less as transformers do have the potential to fail over time), connectivity for monitoring, and convection cooling is preferred over ball bearing fans as again fans can fail.
Warranty: Manufacturer’s warranty coverage period.
Efficiency: We go by Euro efficiency not peak efficiency. This is often misquoted as peak efficiency. This is how well the inverter converts energy.
Power trackers: either 1 or 2 or called Mppt’s the number of Mppt’s determines how many directions the panels can face, and how high the total system voltage is. Single Mppt’s inverters must face all panels in same direction and all be connected together and all panels affect each other. 2 Mppt’s are better.
Cooling: Convection or fan (ball bearing) – Convection is non-moving heat sink cooling and preferred over ball bearing fans as ball bearings tend to cease up over time and cause heat shut down.
Transformer: either yes or No (transformer less) transformers are known to fail over time and are moving electrical parts, whereas transformer less is solid state and non-moving and less prone to failure. Transformer less is preferred.
IP rating: This is dust and moisture rating. First digit either 5 or 6 is for dust, 5 means dust will enter,6 means dust will NOT enter. Second number either 5,6,7,8 means water entry. 5 means water can enter and must be indoors, 6 means sealed no water entry, 7 means submersible to 1 meter, and 8 means submersible to 3 meters. Link to IP ratings http://www.dsmt.com/resources/ip-rating-chart
Start-up voltage: the lower this number means it takes less sun on the panels to start up the inverter and produce power and runs longer in the evening before shutting down. Lower is better.
Things to measure, warranty first as it’s the longest warranty of anything on the system in particular the output warranty which guarantees a certain output over the 25 year period. Panels are rated in tiers, 1,2,3 tier 1 means company is large volume selling, bankable and profitable, automated manufacturing, spends heavily in R&D, making panels over 5 years. Recommend using only Tier 1 panels. This is highest importance.
Watt: This is the upfront wattage or sizing of the panels. There are different sizes but normally you would try to stay within the mass produced sizing of the time in order to maximise cost per watt. Currently 250w to 265w are the most common and best priced per watt.
Output wattage: This is the actual output of the panel. This is what you actually receive to the inverter per panel after loses in efficiency.
Efficiency: This is for a comparison of the quality of the silicone in the panel and most tier 1 panels will be very close here using top grade silicone. The lower the efficiency the worse cost per watt the panel will incur and also will degrade faster over time which is usually reflected in their warranty.
Power output tolerance: This is maybe the most important and least talked about measurement. This relates to power fluctuations. A negative first figure (-5 was in older panels) means you go into a brownout type power production which is bad for inverters and can cause failures. Most new panels do not go negative anymore. The second figure means how high it can go in tolerance and most are (+5) which means the power being provided to the inverter can fluctuate up to 5%. Again power fluctuations are hard on inverters and shorten lifespan. The smaller the + number the better and tighter the power with less fluctuation which is better for inverters. +3 is better than +5 by almost half. This is a Subtle measurement but important.
Warranty: This is twofold. The material warranty which covers materials side within the panels and second, which are the output guarantee and the longest part of the warranty. This number guarantees a certain output over time or the panels gets replace under warranty. Two type of warranty staggered (1 year 90%, year 2 to 10 90%, year 11 to 25 80%) this is older and most newer panels use a linear warranty which starts at 97% or 97.5 and degrades .06 or .07 % per year over the 25 year period. This is a better warranty providing 8% approximately better output guarantee over the 25 year period.