Solar inverters fall into 3 categories: micro, string and central
Micro inverter – is directly connected to each individual panel and converts dc to ac. Enphase is a popular microinverter company that is also publicly traded. The inverter ratings range from 200 Watts to 365 Watts. Microinverters are mainly used on residential projects and some commercial projects. For shaded roofs due to trees or adjacent buildings, these are the best option. They are also good if you mix and match panels from different vendors with different ratings.
String inverter – A string of modules are connected in series and then connected to a string inverter. String inverter are available in a wide range of ratings from 2 kW to 12 kW and sometimes up to 30 kW and 50 kW. String inverters are mostly used on commercial and industrial projects.
Central inverters – Central inverters are used on large solar plants connecting several hundred panels. They start at ~100 kW on the low end and go up to 2 MW and beyond.
In addition, a solar PV system will require a grid-tie inverter if the system is grid-connected. A grid-tie inverter synchronizes the AC phase from the solar power plant with the utility phase, and shuts down automatically when the grid is not supplying electricity. This “anti-islanding protection” protects electrical workers who repair power lines. It also protects equipment and appliances from a fluctuating power supply.
A grid-tie inverter should be compliant with the following:
(i) the UL 1741 Standard for Safety of Inverters, Converters, Controllers and Interconnection System Equipment for Use with Distributed Energy Resources as published by Underwriters Laboratories Inc.; and
(ii) the IEEE 1547 Standard for Interconnecting Distributed Resources with Electric Power Systems.
Although these are standards in the United States, other countries accept them. Even if a country does not have equivalent national standards, an inverter that conforms to one of the above (or similar) standards ensures protection of both the utility grid and the local building grid.
Solar inverters have come a long way in terms of product maturity and if you buy an inverter from a reputed vendor at a fair market price, then you are good to go. Here are some points (not comprehensive) to consider when selecting an inverter:
- What is the kW rating of the inverter? Is it oversized or undersized compared to the number of panels? For example if you are installing a 10 kW system, then you should look at whether the inverter is ~8 kW or ~10 kW or ~12 kW. An 8 kW inverter may be good with a 10 kW solar panel system if the energy yield calculations show that the inverter will be loaded only 5% or 10% of the time. An oversized inverter is also OK if that is what you can get for a low price; it can also be helpful if you want to add more panels in the future.
- UL/IEC or equivalent certification is a must
- Efficiency should be >98%; transformer based or transformerless
- Is it an indoor unit or can it be installed outside? What is the operating temperature range? Wide range is better especially on the hot side. How is the unit cooled – forced cooling with a fan or convection cooling?
- Protection features – does the inverter have disconnect switches, arc fault interrupter, circuit breaker, fuses and other safety measures?
- How many years of warranty is provided? An inverter is expected to last 7 to 10 years. So one should expect to replace it with a new inverter at that time.
- What happens if the inverter fails during operation? Within how many hours or days can you expect the vendor to respond to the issue and fix it?
One general advice is that you should download data sheets of 3 or 4 inverters in the same size rating you are considering buying and then look at the line items and values. You will be surprised how much you can learn by comparing regardless of your prior experience or lack thereof with solar. The top inverter vendors globally are: SMA, ABB, Enphase, SolarEdge, Solectria, Schneider Electric, TMEIC, Fronius
Here is an inverter landscape slide from greentechmedia
Finally the prices of inverters and other balance of system components have been falling rapidly. According to greentechmedia the average price of an inverter in 2015 was ~20 – 25 cents per watt i.e. ~$2,500 for a 10 kW inverter.
Enphase, the leading manufacturer of microinverters projects that its costs will decline from ~$0.23 per watt at the end of 2015 to ~$0.10 per watt by 2017.
To finish the point on cost decline, here is how SolarCity’s costs have been declining. They are targeting a $2.50 fully installed cost by 2017.