A grid-tied inverter (GTI) is a special type of inverter that, in addition to converting direct current (DC) to alternating current (AC), can synchronize its output AC power with the frequency and phase of the mains power supply, allowing the output AC power to be returned to the mains. Grid-tied inverters are commonly used in applications where DC voltage sources (such as solar panels or small wind turbines) are connected to the power grid.
Inverters will print information sheets for their product lines. The wording and content of these information sheets may vary from manufacturer to manufacturer, but they generally include the following:
Rated output power: The unit can be watts or kilowatts. Some inverters may have different rated output power for different output voltages. For example, an inverter can be set to output at 240VAC or 208VAC, and the rated output power will be different for the two.
Output voltage: This refers to the grid voltage that the inverter can connect to. For small residential grid-connected inverters, the output voltage is mostly 240VAC. For commercial inverters, the voltage will be 208, 240, 277, 400, 480, or 600VAC, and it may be a three-phase output.
Peak efficiency refers to the optimal efficiency an inverter can achieve (efficiency is calculated by dividing the output AC power by the input DC power). In July 2009, most grid-connected inverters on the market achieved a peak efficiency of 94%, with some reaching as high as 96%. The energy dissipated is primarily heat. For an inverter to output its rated power, its input power must exceed the rated power. For example, a 5000W inverter with a full-power efficiency of 95% requires 5,263W of input power (rated power divided by efficiency). If an inverter can output power at several different voltages, it will also have a peak efficiency at each voltage.
CEC-weighted efficiency: This efficiency is calculated using the algorithm published by the California Energy Commission (CEC) on its GoSolar website. Unlike peak efficiency, CEC-weighted efficiency is an average efficiency, which can be used to compare the performance of the inverter during operation. If the inverter can output power at several different voltages, it will also have a CEC-weighted efficiency at each voltage.
Maximum input current: This is the maximum DC current that the inverter can use. If the current output by the system (e.g., solar cells) exceeds this limit, the inverter cannot use this current.
Maximum output current: This refers to the maximum AC current that the inverter can continuously provide. This value is generally used to determine the minimum current rating of overcurrent protection devices (fuses or circuit breakers) and the minimum current rating to disconnect related equipment. If the inverter can output at several different AC voltages, it may also have different maximum output currents at each voltage.
Peak power tracking voltage: This indicates the range of DC voltages within which the inverter can operate for maximum power point tracking (MPPT). System designers need to ensure that the system's series connection is optimized so that the resulting voltage remains within this range for most of the year. This can be challenging because voltage fluctuations and temperature variations can make this a difficult task.
Start-up voltage: Not all inverters specify this value. This value refers to the minimum DC voltage required for the inverter to start and operate. It is especially important in solar energy applications because system designers need to ensure that there are enough photovoltaic modules connected in series, and their voltage exceeds the start-up voltage. If the manufacturer does not specify this value, system designers generally use the lower limit of the peak power tracking voltage as the minimum voltage for the inverter.
International IP rating (IPxx rating): The protection rating, or IP number classification, refers to a product's ability to protect against solid foreign objects (indicated in the first digit of the protection rating) and water (indicated in the second digit of the protection rating). A higher number indicates better protection. The US NEMA enclosure types are similar in application to international protection ratings. Most inverters used outdoors are IP45 (not dustproof) or IP65 (dustproof), or in the US, NEMA 3R (no dust protection) or NEMA 4X (dustproof, protection against direct splashes, and additional corrosion protection).
