As we know, the basic function of the inverter is to convert DC power to AC power because most of our electrical needs are for AC.
The inverter is connected directly to either the power source (solar PV array or wind turbine) or the charge controller, depending on whether backup storage batteries are used. Also, some manufacturers offer a single unit containing a charge controller and an inverter.
Inverter Specifications
Specifications provide the values of operating parameters for a given inverter. Common specifications are discussed below. Some or all of the specifications usually appear on the inverter data sheet.
Maximum AC output power
This is the maximum power the inverter can supply to a load on a steady basis at a specified output voltage. The value is expressed in watts or kilowatts.
Peak output power
This is also known as the surge power; it is the maximum power that an inverter can supply for a short time. For example, some appliances with electric motors require a much higher power on start-up than when they are running on a continuous basis. Common examples are refrigerators, air-conditioning units, and pumps.
AC output voltage
This value indicates to which utility voltages the inverter can connect. For inverters designed for residential use, the output voltage is 120 V or 240 V at 60 Hz for North America. It is 230 V at 50 Hz for many other countries.
Peak Efficiency
The peak efficiency is the highest efficiency that the inverter can achieve.
Most grid-tie inverters have peak efficiencies above 90%. The energy lost during inversion is, for the most part, converted into heat. It’s important to note what this means: In order for an inverter to put out the rated amount of power, it will need to have a power input that exceeds the output. For example, an inverter with a rated output power of 5,000 W and a peak efficiency of 95% requires an input power of 5,263 W to operate at full power.
California Energy Commission weighted efficiency
This value is established by the California Energy Commission (CEC). This value is an average efficiency and is a better representation of the inverter’s operating profile than is the peak efficiency.
Maximum Input Current
This is the maximum direct current that the inverter can utilize.
If a solar array or wind turbine produces a current that exceeds this maximum input current, the excess current is not used by the inverter.
Maximum Output Current
This is the maximum continuous AC that the inverter supplies.
This value is typically used to determine the minimum current rating of the protection devices (breakers and fuses) and disconnects required for the output circuit.
Peak Power Tracking Voltage
This is the DC voltage range in which the inverter’s maximum power point tracker operates.
Start Voltage
This value is the minimum DC voltage required for the inverter to turn on and begin operation.
This is particularly important for solar applications because the solar module or modules must be capable of producing the voltage. If this value is not provided by the manufacturer, the lower end of the peak power tracking voltage range can be used as the inverter’s minimum voltage.
NEMA Rating
This rating indicates the level of protection the inverter has against water intrusion. Most outdoor inverters are rated as National Electrical Manufacturers Association (NEMA) 3 for most weather conditions.
Total Harmonic Distortion
The total harmonic distortion (THD) is an indication of the purity, or the harmonic content, of the sinusoidal output of an inverter.
Most filtered sine waves still contain some harmonics that distort the waveform to a minor degree.
Miscellaneous Features
Inverter features vary from one model to another and from one manufacturer to another. Common features found on many inverters are as follows.
Weatherproof Enclosure
Most inverters, especially grid-tie inverters, are designed to be installed outdoors and have weatherproof enclosures.
AC/DC Disconnects
Some inverters have built-in ac/dc disconnects for safety and to facilitate removing the inverter if it needs to be serviced.
Ground Fault Protection
The National Electric Code (NEC) requires that roof-mounted solar electric systems must be grounded. Most inverters have built-in ground fault protection.
Maximum Power Point Tracking
Tracking the peak power point of a solar panel array is important for maximizing energy obtained from a PV module or array. If a system does not have a charge controller that performs this function, the inverter is connected directly to the PV source and requires MPPT.
Transfer Switch
A transfer switch is also known as a transfer relay. Grid-tie inverters usually feature a built-in load transfer switch for backup emergency power applications.
As long as utility power reaches the inverter’s AC input side, the transfer switch passes the AC grid power directly through the inverter to the load.
If the utility grid power is interrupted, the transfer relay automatically switches to the battery backup input to the inverter.
Generator Start Switch
Some inverters are available with a separate generator start switch and include a second AC input for an AC generator that is used for backup.
The generator can be programmed to start when the utility grid fails or when a low battery charge is detected.
In some applications, where the inverter has a built-in battery charger, the generator operates long enough to recharge the batteries. If there is no battery backup, the generator is used as the only power source until the grid is operating again.
Display Panel
A remote display panel option is available for many inverters to indicate the system status. This feature is particularly useful if the inverter and battery bank are located in an area that is difficult to access. A standard interface allows data to be sent to a remote site.
Inverter Data Sheet
A data sheet for a typical inverter is shown in Figure 1.
Figure 1: Typical Inverter Data Sheet
Solar Module Power Calculation Example
Determine the power that a solar module array must provide to achieve maximum power from the SPR-3300x inverter specified in the datasheet in Figure 1.
Solution
Because
POUT (efficiency)(PIN)
PIN = POUT/efficiency
Using peak efficiency, the input power to the inverter must be
PIN=POUT/Peak Efficiency=3,300 W/0.953=3,463 W
Using the CEC efficiency, the input power to the inverter must be
PIN=POUT/CEC Efficiency=3,300 W/0.945=3,492 W
Inverter Classes
Inverters can be classed according to their power output. The following information is not set in stone, but it gives you an idea of the classifications and general power ranges associated with them.
These ranges may vary from one manufacturer to another. Inverters may also be found with output power specifications falling between each of the ranges listed.
Small residential inverters
Small residential inverters are in the 1,800 W to 2,500 W range, with single-phase power.
Large residential inverters
Large residential inverters are in the 3,000 W to 6,000 W range, with single-phase power.
Small commercial inverters
Small commercial inverters are in the 13 kW to 15 kW range and can include three-phase power.
Large commercial inverters
Large commercial inverters are in the 60 kW to 100 kW range. Inverters can be combined to provide up to or above 1 MW (1,000 kW) of three-phase power.
Review Questions
What determines the required input power to an inverter so that it achieves a specified output power?
What is CEC weighted efficiency?
What does THD stand for?
What indicates the level of protection against water intrusion for an inverter?
Answers
The peak or weighted efficiency
The average efficiency of an inverter
The total harmonic distortion, which is an indication of the purity, or the harmonic content, of the sinusoidal output of an inverter
The NEMA rating
