Checklist for Choosing an Inverter

The following are the key parameters you should consider and evaluate while choosing an inverter

  • Rated Power Output
  • Maximum PV input power
  • Efficiency
  • Operating temperature
  • Frequency Output
  • Maximum Open circuit voltage

Key Parameters to Consider While Selecting a Solar Inverter

Specification What to look for Other notes Location where it is specified
Rated Output Power Ensure that the rated output power of inverter supports the power of the solar panels. For instance, for a solar panel power of 3 kW, make sure that the rated output power on the inverter specifies at least this much. For example, a 4 kW inverter works well with a 3 kW panel, but vice versa is not feasible. On the inverter
Max PV Input Power Inverter power must be matched to the panel array power. Make sure this is neither too high nor too low, when compared to the total power from the panel array. For example, for a 4 kW panel, a 5 kW inverter could be optimal, but having it much higher than 5 kW is not. Datasheet
Efficiency Choose inverters with efficiencies >95% for smaller kW scale inverters (less than 10 kW) and >98% for inverters above 20 kW. Acceptable benchmark efficiency

<3 kW: 90%

3-10 kW: 94%

>20 kW: 98%

Operating temperature The temperature range must be wide, wider the better. Look for a temperature range of -25°C to 60°C For colder climates, even a range of -25°C-45°C will be acceptable Datasheet
Frequency output Frequency variation must be as little as possible. Look for frequency output in the range (Grid Frequency ± 0.2)Hz In the US, the frequency output range should be (60 ± 0.2)Hz, for most other regions, it is (50± 0.2)Hz. On the inverter
Maximum open circuit voltage(VOC) VOC of solar panel must be less than that for inverters. If VOCP Is the open circuit voltage for panels and VOCI is the open circuit voltage for the inverter, then make sure, VOCI > VOCP Check for the data on open circuit voltages on the panels and inverters respectively and do the comparison. Datasheet

Details of Parameters
Rated Power Output

Rated power output gives the maximum output power in watts of the inverter. DC power from the solar panels is converted to grid/appliance-compatible AC power. The inverter power rating signifies the total wattage of loads it can support.

Maximum PV Input Power (PIN)
The power generated from the string of solar panels which is given to the inverter is called Maximum PV input power.
Maximum PV input power must never be exceeded by the power output from the combined panels.
PIN (of inverter)>POUT (of panels)
Else the inverter runs inefficiently. In other words, the inverter rating must be matched to the panels properly.

Efficiency of the inverter signifies the percentage of DC power from the solar panels that is converted to AC power. It is usually the primary consideration for selecting an inverter. Higher the efficiency, lower the losses associated with the inverter.The inverter must have an efficiency of > 95 % at full load.

Operating Temperature

An operating temperature is the temperature at which an electrical or mechanical device operates. Inverters are programmed to purposefully reduce its power output if they sense overheating to protect sensitive electronics from extreme temperatures. Normally the operating temperature of the Inverter is in the range of -25 to 40°C.
The temperature of the inverter should not exceed the operating temperature range. A wide operating range is advantageous for the inverters so that its performance is not compromised even in extreme hot or cold conditions.

Frequency Output

The accepted operating frequency of power supply is 50 Hz in most countries except the U.S where it is 60 Hz. The frequency of the inverter output must be in the range of 49.7Hz to 50.3Hz or 59.7Hz to 60 Hz according to the region.The variations in the frequency output of the inverter must not be too large and must be in the range specified above.

Maximum Open Circuit Voltage

Open-circuit voltage (abbreviated as OCV or VOC) is the voltage between the terminals of the inverter when there is no external load connected.The PV array’s maximum open circuit voltage must always be less than the inverter’s limit or damage may occur.
Other Parameters for Inverter Selection

These parameters need not be evaluated as critically as key parameters but you might want to check the datasheet for the following parameters also.

PV Start Voltage

PV Start Voltage gives information about when the inverter will begin to operate. In the morning, when the sun comes up, the PV panels begin to output power, but inverters require a minimum voltage before they start outputting their own power into the grid. PV Start Voltage is important since it relates to the overall efficiency of a system. PV panel’s output voltage must be higher than the inverter’s start-up voltage so as to maximize the system performance.

Voltage Output

The accepted voltage level for most countries is 240 V. The inverter output voltage should comply to the standard voltage level and has to be within 228V to 252 V.For U.S, the accepted voltage level is 110V.The inverter output voltage needs to be within 98 V to 122V.The output voltage should be in the range as mentioned above in order for it to be grid or appliance compatible.

Type of Solar Inverter
Depending on the requirement- central, string or micro-inverter can be opted.

  • Central inverters are usually suited for large scale solar power plants.
  • String inverters are usually employed for rooftop power plant applications as seen now.
  • Micro inverters are the latest development in the inverter technology and brings with it added advantages of optimised performance of the system at the expense of significant added costs.

Maximum DC Input Voltage

This is the maximum DC voltage that can be applied to the inverter. The inverter configuration is set for a maximum input voltage. Exceeding this value can damage the inverter.

Total Harmonic Distortion + Noise

Total harmonic distortion plus noise (THD+N) is a measure of how much the output of the inverter varies from the standard AC output waveform.The THD levels must remain below 5% at all load levels.