The Inverter page allows you to choose an inverter performance model from two options, and displays the characteristics of the of the inverter.

On the Inverter page, you specify the characteristics of a single inverter. To enter the number of inverters in the system, use the input variables on the Array page. Solar Advisor calculates the AC output of the total number of inverters in the system based on the DC output of the photovoltaic array calculated by the module performance model.

The current version of Solar Advisor includes two inverter performance models, the Sandia Performance Model for Grid-Connected PV Inverters model and the single-point efficiency model. The Sandia model allows you to choose an inverter from a list of commercially-available inverters. The single-point efficiency model allows you to model inverters that are not in the list.

To specify an inverter model:

For the Single Point Efficiency Model, enter inverter characteristics.

The Sandia Performance Model for Grid-Connected PV Inverters is an empirically-based performance model that uses parameters from a database of commercially available inverters maintained by Sandia National Laboratory. The parameters are based on manufacturer specifications and laboratory measurements for a range of inverter types.

The Sandia inverter model is described in King D et al, 2007. Performance Model for Grid-Connected Photovoltaic Inverters. Sandia National Laboratories. SAND2007-5036. http://www.prod.sandia.gov/cgi-bin/techlib/access-control.pl/2007/075036.pdf

To use the Sandia inverter model:

If you are modeling an inverter not included in the database and want to use the Sandia model, you can try to find an inverter with similar characteristics to your inverter's specifications.

The Sandia model consists of a set of equations that Solar Advisor uses to calculate the inverter's hourly AC output based on the DC input (equivalent to the derated output of the photovoltaic array) and a set of empirically-determined coefficients that describe the inverter's performance characteristics. The equations involve a set of coefficients that have been empirically determined based on data from manufacturer specification sheets and either field measurements from inverters installed in operating systems, or laboratory measurements using the California Energy Commission (CEC) test protocol.

The CEC inverter test protocol is described in Bower W et al, 2004. Performance Test Protocol for Evaluating Inverters Used in Grid-Connected Photovoltaic Systems. http://bewengineering.com/docs/index.htm

Because Solar Advisor does not track voltage levels in the system, it assumes that for each hour of the simulation, the inverter operates at the photovoltaic array's maximum power point voltage, given the solar resource data in the weather file for that hour.

Inverter Characteristics

When you select an inverter from the Sandia database on the Inverter page, Solar Advisor displays the inverter's parameters for your reference. You can see the complete set of parameters in the Inverter library by using Solar Advisor's library editor.

The Sandia inverter library includes parameters for modules that have been tested by Sandia National Laboratory. Manufacturers wishing to add their inverters to the Sandia database should contact Sandia National Laboratory directly. Because the parameters involve data from field and test measurements, it is not possible to generate parameters based only on manufacturer specifications.

The following table describes the parameters in the Sandia inverter library, which are explained in more detail in the King 2004 reference cited above.

AC Voltage (Vac)

Rated output AC voltage from manufacturer specifications.

Power ACo (Wac)

Maximum output AC power at reference or nominal operating conditions. Available from manufacturer specifications.

Power DCo (Wdc)

Input DC power level at which the inverter's output is equal to the maximum AC power level. Available from manufacturer specifications.

PowerSo (Wdc)

DC power required for the inverter to start converting DC electricity to AC. Also called the inverter's self-consumption power. Sometimes available from manufacturer specifications, and not to be confused with the nighttime AC power consumption.

PowerNTare (Wac)

AC power consumed by the inverter at night to operate voltage sensing circuitry when the photovoltaic array is not generating power. Available from manufacturer specifications.

Vdcmax (Vdc)

The maximum DC voltage input, typically at or near the photovoltaic array's maximum power point voltage.

Idcmax (Adc)

The maximum DC voltage input, typically at or near the photovoltaic array's maximum power point current.

Coefficient 0 (1/V)

Empirically-determined coefficient that defines the relationship between AC and DC power levels at the reference operating condition.

Coefficient 1 (1/V)

Empirically-determined coefficient that defines the value of the maximum DC power level.

Coefficient 2 (1/V)

Empirically-determined coefficient that defines the value of the self-consumption power level.

Coefficient 3 (1/V)

Empirically-determined coefficient that defines the value of Coefficient 0.

MPPT-low (Vdc)

Manufacturer-specified minimum DC voltage, as described in CEC test protocol (see reference above).

Vdco (Vdc)

The average of MPPT-low and MPPT-high, as described in the CEC test protocol (see reference above).

MPPT-hi (Vdc)

Manufacturer-specified maximum DC voltage, as described in CEC test protocol (see reference above). The test protocol specifies that the inverter's maximum DC voltage should not exceed 80% of the array's maximum allowable open circuit voltage.

The inverter single-point efficiency model calculates the inverter's AC output by multiplying the DC input (equivalent to the array's derated DC output) by a fixed DC-to-AC conversion efficiency that you specify on the Inverter page. Unlike the Sandia inverter model, the single-point efficiency model assumes that the inverter's efficiency does not vary under different operating conditions.

To use the single-point efficiency inverter performance model:

On the Inverter page, choose Single Point Efficiency Inverter.

Note that manufacturer specifications may include both a peak efficiency, which is the inverter's maximum efficiency; and a CEC weighted efficiency value, which is an average value that better represents the efficiency over a range of operating conditions.

Solar Advisor calculates the inverter's rated DC power input using the following equation:

Where,