Inverter

The Inverter page allows you to choose an inverter performance model and either choose an inverter from a list, or enter inverter parameters from a manufacturer's data sheet using either a weighted efficiency or a table of part-load efficiency values.

SAM can only model a photovoltaic system with a single type of inverter. Specify the number of inverters in the system on the Array page.

SAM displays the name of the active inverter model at the top of the Inverter page. Click Change to choose a different model.

SS_PVInverter-ChooseModel

You can choose from three different inverter performance models:

Inverter CEC Database calculates the system's AC output using parameters from SAM's CEC database of inverter parameters with the Sandia inverter model. To use this model, you simply choose an inverter from the list. Use this model for most analyses.

Inverter Datasheet allows you to specify the inverter's parameters using values from a manufacturer's data sheet, and calculates coefficients for the Sandia inverter model from the parameters you provide. Use this model for an inverter that is not in the CEC database.

Inverter Part Load Curve allows you to specify a table of part-load efficiency values for an inverter using data from a manufacturer's data sheet or other source. Use this model when you have the inverter's part-load efficiency data.

Each of the three inverter models calculates a DC to AC conversion efficiency, assuming that the DC power input to the inverter is equal to the derated DC output of the photovoltaic array. The inverter models limit the inverter's output to the inverter's Maximum AC Power parameter so that the inverter's output is "clipped" to this value. SAM uses the inverter Operating Ranges parameters to either size the system or display sizing suggestion messages on the Array page.

The inverter model reports several hourly simulation results on the Results page that you can use to understand how SAM models the inverter or to troubleshoot your analyses:

Gross ac output (kWh)

The inverter's AC output in kilowatt-hours before the interconnection derate factors from the Array page.

Inverter clipping loss (Wac)

The portion of the inverter's AC output not delivered to the grid during hours when the AC output exceeds the inverter's maximum AC output. During these hours, inverter's gross AC output is equal to the inverter's maximum AC output. SAM assumes that the system is designed to handle this excess electricity, but does not explicitly model the hardware required to do so.

Inverter dc input voltage (V)

The DC voltage at the inverter's input, equal to the array's DC string voltage. For systems with two or more subarrays, SAM estimates the inverter DC voltage as the average of the subarray string voltages.

Inverter efficiency (%)

The inverter's DC to AC conversion efficiency. The inverter efficiency is equal to the gross AC output divided by the net DC array output.

Inverter night time loss (Wac)

The amount of electricity consumed by the inverter at night when the array does not generate electricity. The night-time loss is equal to the value you specify for hours when the gross DC array output is zero. The night-time loss is zero for hours when the gross DC array output is greater than zero.

Inverter power consumption loss (Wdc)

The amount of electricity consumed by the inverter, not including the night-time loss. SAM estimates the hourly power consumption loss by adjusting the value you specify for Power consumption during operation based on the inverter's DC input voltage. The Inverter Part Load model assumes that the inverter's power consumption is accounted for in the part-load efficiency table that you specify, so it reports an inverter power consumption loss value of zero for all hours.

Net ac output (kWh)

The inverter's AC output, before the interconnection derate factors.

Click to expand or collapseInverter CEC Database

The Inverter CEC Database model is an implementation of the Sandia Model for Grid-Connected PV Inverters (Sandia inverter model). It is an empirically-based performance model of inverter performance that uses parameters from a database of commercially available inverters maintained by the California Energy Commission (CEC).

The Inverter CEC Database model consists of a set of equations that SAM 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 coefficients for each inverter are empirically determined from data provided by the inverter's manufacturer and either field measurements from an inverter of the same type installed in an operating photovoltaic system, or laboratory measurements. Measured values are taken using the CEC inverter test protocol.

The Sandia inverter model is described in King D et al (2007) Performance Model for Grid-Connected Photovoltaic Inverters, Sandia National Laboratories, SAND2007-5036, and on the PV Performance Modeling Collaborative website.

The CEC inverter test protocol is described in Bower W et al (Draft 2004) Performance Test Protocol for Evaluating Inverters Used in Grid-Connected Photovoltaic Systems, and on the PV Performance Modeling Collaborative website.

Note. SAM stores the list of inverters for the Inverter CEC Database model in the Sandia Inverter library. The library contains parameters for inverters in the List of Eligible Inverters per SB1 Guidelines at http://www.gosolarcalifornia.org/equipment/inverters.php. We try to keep the library as current as possible, but there may be periods when SAM's library is out of date. If the library appears to be out of date, you can check for updates by clicking Check for updates to this version on the Help menu.
 
You can export the entire Sandia inverter library as a comma-separated values (CSV) text file from SAM's library editor. To open the library editor, on the Tools menu, click Library Editor.
 
If you are an inverter manufacturer and would like to add your inverter to the list, you should contact the California Energy Commission (CEC) or Sandia National Laboratories directly. For information about the Sandia Test and Evaluation program, see http://energy.sandia.gov/?page_id=279. For a list of Sandia contacts, see http://energy.sandia.gov/?page_id=2772. For CEC contacts, see http://www.gosolarcalifornia.ca.gov/equipment/add.php.

To use the Inverter CEC Database model:

1.On the Inverter page, click Change and choose Inverter CEC Database.

2.Choose an inverter from the list of available inverters. You can type a few letters of the manufacturer or inverter name in the Search box to filter the list.

If you are modeling an inverter not included in the database and want to use the Sandia inverter model, you can use the Inverter Datasheet model with values from a manufacturer's data sheet.

Each inverter listing shows the manufacturer name, model number and AC voltage rating, and information in brackets about the organization responsible for generating the test data and the year the data was generated. "CEC" indicates that test data was generated by the California Energy Commission.

Efficiency Curve and Characteristics

When you select an inverter from the list, SAM displays an efficiency curve and the inverter's parameters for your reference. The parameter values are from the Sandia inverter library, and are values that the Sandia inverter model uses as inputs.

Note. SAM displays a few of the parameters from the CEC database on the Inverter page. If you want to see the complete set of parameters, you can do so in the library editor.

Weighted Efficiency, %

SAM calculates and displays both the CEC weighted efficiency and European weighted efficiency for your reference. It does not use these efficiency values during simulations. To calculate the efficiencies, SAM calculates the inverter's nominal efficiency at seven different power levels, and applies the set of weighting factors for the CEC and European methods of calculating the weighted efficiency.

The following list briefly describes the each parameter that SAM displays on the Inverter page. You can read more about these and all of the other Sandia inverter model input parameters in the King 2004 reference cited above. The names in brackets are the names used in the King reference.

Maximum AC power [Power ACo], Wac

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

Maximum DC power [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.

Power consumption during operation [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.

Power consumption at night [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.

Nominal AC voltage [AC Voltage], Vac

Rated output AC voltage from manufacturer specifications.

Maximum DC voltage [Vdcmax], Vdc

The inverter's maximum DC input voltage.

Maximum DC current [Idcmax], Adc

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

Minimum MPPT DC voltage [MPPT-low], Vdc

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

Nominal DC voltage [Vdco], Vdc

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

Maximum PPT DC voltage [MPPT-hi], Vdc

Manufacturer-specified maximum DC operating 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 four coefficients C0..C3 are empirically-determined coefficients that are inputs to the Sandia inverter model. Manufacturers do not provide these coefficients on inverter datasheets.

C0, 1/V

Defines the relationship between AC and DC power levels at the reference operating condition.

C1, 1/V

Defines the value of the maximum DC power level.

C2, 1/V

Defines the value of the self-consumption power level.

C3, 1/V

Defines the value of Coefficient C0.

Click to expand or collapseInverter Datasheet

The Inverter Datasheet model is an implementation of the Sandia Model for Grid-Connected PV Inverters that allows you to model an inverter by entering data from a manufacturer's data sheet.

The Inverter Datasheet model consists of a set of equations that SAM 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 coefficients that describe the inverter's performance characteristics. SAM calculates the coefficients from the manufacturer data you provide.

Note. If you have a table of part-load efficiency values for the inverter, you may want to use the Inverter Part Load Curve model instead of the Inverter Datasheet model.

To use the Inverter Datasheet model:

On the Inverter page, click Change, and choose Inverter Datasheet.

Enter input values from the manufacturer's data sheet. See below for descriptions of the inputs.

Power Ratings

Maximum AC output power

The inverter's rated maximum AC output in Watts. Manufacturers may use different names for this value, such as continuous output power, rated active power, peak output, etc.

Weighted efficiency and Manufacturer efficiency

Inverter manufacturers provide different efficiency ratings on their product data sheets. SAM can model the inverter using either a weighted efficiency or a nominal efficiency. If the manufacturer provides a weighted efficiency, you should use it rather than the nominal efficiency. The weighted efficiency more accurately reflects the inverter's performance under different operating conditions.

If you choose Weighted efficiency, you can use the weighted efficiency calculated with either the European or CEC method. The European method is best for locations with lower solar resource where the inverter operates more often at lower load levels. The CEC method is best for sunnier locations where the inverter operates at higher load levels. See Inverter Efficiency Values for more details.

If you choose Nominal efficiency, you can use either a peak efficiency or another efficiency value from the data sheet that represents the inverter's efficiency at a single load level. You should also specify a value for Power consumption during operation to improve the accuracy of the model at low power levels.

Maximum DC input power

SAM calculates and displays the equivalent rated DC input capacity based on the maximum AC output power and efficiency value that you specify (either weighted or nominal):

Maximum DC Input Power (Wdc) = Maximum AC Output Power (Wac) ÷ Efficiency (%) × 100%

SAM uses the maximum DC input power value to size the array when you choose Specify desired array size on the Array page, and to display sizing messages when you choose Specify modules and inverters.

Operating Ranges

SAM uses the operating range variables to help you size the system on the Array page.

Nominal AC operating voltage

The inverter's nominal AC output voltage.

Maximum DC voltage

The inverter's maximum input DC voltage.

Maximum DC current

The inverter's maximum input DC current.

Minimum MPPT DC voltage

The inverter's minimum DC operating voltage.

Nominal DC voltage

The inverter's nominal DC operating voltage.

Maximum MPPT DC voltage

The inverter's maximum DC operating voltage.

Losses

The two loss variables account for electricity consumed by inverter components during operation and to keep the inverter in standby mode at night when the array is not generating electricity.

When you specify the inverter's efficiency using a weighted efficiency, you only need to specify a value for the night-time power consumption because the weighting factors account for the power consumption during operation.

SAM displays a suggested value for each loss variable, which is based on an analysis of the loss parameters for the inverters in the SAM 2013.1.5 CEC library, and should be a reasonable approximation for inverters currently available on the market. If the manufacturer does not provide values for the inverter's power consumption, you can use the suggested value. (You must type the value in the input box, SAM does not automatically assign the suggested value to the variable.)

Power consumption during operation

Electricity consumed by the inverter during the day when the photovoltaic array is generating power. SAM disables this variable when you specify a weighted efficiency.

SAM calculates the suggested value using the following equation:

Suggested Value for Consumption during Operation (Wdc) = 0.8% × Maximum AC Output Power (Wac)

Power consumption at night

Electricity consumed by the inverter during the night when the photovoltaic array is not generating power. This value is sometimes also called tare loss or standby loss.

SAM calculates the suggested value using the following equation:

Suggested Value for Consumption at Night (Wac) = 0.25% × Maximum AC Output Power (Wac)

Click to expand or collapseInverter Part Load Curve

The Inverter Part Load Curve model allows you to model an inverter by entering part-load efficiency and other data from a manufacturer's data sheet. Unlike the CEC Database and Inverter Datasheet inverter models, this model is not based on the Sandia inverter model. Instead, it determines the inverter's hourly conversion efficiency based on the part-load efficiency data points and night-time loss values you provide.

Note. If you do not have a table of part-load efficiency values for the inverter, you may want to use the Inverter Datasheet model instead of the Part Load Curve model.

To use the Inverter Part Load Curve model:

1.On the Inverter page, click Change, and choose Inverter Part Load Curve.

2.Type a value for the Maximum AC output power, and choose CEC efficiency or European efficiency.

3.Type values for the Operating Ranges input variables and for Power consumption at night loss.

4.Type values in the part-load efficiency table and for the operating range input variables.

See below for descriptions of the variables, and more detailed instructions for working with the part-load efficency table.

Power Ratings

Maximum AC output power

The inverter's rated maximum AC output in Watts. Manufacturers may use different names for this value, such as continuous output power, rated active power, peak output, etc.

CEC efficiency and European efficiency

Specify the inverter's weighted efficiency calculated with either the European or CEC method. The European method is best for locations with lower solar resource where the inverter operates more often at lower load levels. The CEC method is best for sunnier locations where the inverter operates at higher load levels. See Inverter Efficiency Values for more details.

Maximum DC input power

SAM calculates and displays the equivalent rated DC input capacity based on the maximum AC output power and efficiency value that you specify (either weighted or nominal):

Maximum DC Input Power (Wdc) = Maximum AC Output Power (Wac) ÷ Efficiency (%) × 100%

SAM uses the maximum DC input power value to size the array when you choose Specify desired array size on the Array page, and to display sizing messages when you choose Specify modules and inverters.

Operating Ranges

SAM uses the operating range variables to help you size the system on the Array page.

Nominal AC operating voltage

The inverter's nominal AC output voltage.

Maximum DC voltage

The inverter's maximum input DC voltage.

Maximum DC current

The inverter's maximum input DC current.

Minimum MPPT DC voltage

The inverter's minimum DC operating voltage.

Nominal DC voltage

The inverter's nominal DC operating voltage.

Maximum MPPT DC voltage

The inverter's maximum DC operating voltage.

Losses

The two loss variables account for electricity consumed by inverter components to keep the inverter in standby mode at night when the array is not generating electricity.

Power consumption at night

Electricity consumed by the inverter during the night when the photovoltaic array is not generating power. This value is sometimes also called tare loss or standby loss.

Part Load Efficiency

SAM uses the part-load efficiency table you specify to determine the inverter's efficiency during simulations. You can either type values in the table by hand, import values to the table from a properly formatted text file, or paste data to the table from your computer's clipboard.

SAM uses linear interpolation to calculate efficiency values for output power levels between the points in the table. If you specify only a single row, SAM assumes that the inverter's efficiency is constant over its full output power range.

Tips for working with the part-load efficiency table:

To clear the table, change Rows to 1, and then change it to the number of rows in your data set.

Double click a cell to select it.

Use the Tab and Shift-Tab keys to move between columns.

Use the Enter key to move down a column.

If you type a non-numeric character, SAM replaces the character with a zero.

To specify the part-load efficiency curve using the table:

1.Under Rows, type the number of data points you want to include in the table. You must specify at least one row of values in the table.

2.For each row in the table, type a value for output power as a percentage of the inverter's rated capacity, and a DC to AC conversion efficiency value as a percentage.

SAM displays the part-load efficiency curve in the plot area as you type values in the table.

To import part-load efficiency data from a text file:

1.Prepare a text file of comma-separated values. The file should have one line for each output-efficiency value pair separated by a comma with no header rows. For example:

0,0

10,96.1

20,97.55

30,97.87

...

The output percentages should increase from the first row to the last, but not necessarily in equal increments.

You can also export the efficiency data from the default flat plate photovoltaic case to see an example of what the file should look like.

2.Click Import.

SAM populates the part-load efficiency table with data from the file.

To paste part-load efficiency data from your computer's clipboard:

1.Prepare a spreadsheet file or text file with one row for each output-efficiency pair, and output and efficiency values in separate columns or separated by a tab.

2.In your spreadsheet program or text editor, select the two columns containing the data. Do not include column headings or other labels or data.

3.In SAM, on the Inverter page under Part Load Efficiency, click Paste.

SAM populates the part-load efficiency table with data from the clipboard.

Click to expand or collapseWeighted and Manufacturer Efficiency Values

When you use either the Inverter Datasheet model or the Inverter Part Load Curve model, you must provide SAM with an efficiency value that determines the invereter's maximum DC input power that SAM uses for sizing the photovoltaic array.

Inverter manufacturers often show several efficiency values on an inverter's datasheet. Weighted efficiency values are more accurate representations of an inverter's efficiency over a range of output levels than an efficiency value at a single operating point.

Many inverter data sheets will show two versions of the weighted efficiency value: The CEC weighted efficiency, or the European weighted efficiency. The table below shows the weighting factors used to determine both versions of the weighted efficiency. In general, you should use the CEC weighted efficiency to model a system in a sunny location, and you should use the European weighted efficiency for less sunny locations. The following equation shows how the weighted efficiency is calculated, where ηweighted is the weighted efficiency value, F1..F7 are shown in the table below, and η5, η10... are the inverter part-load efficiencies at 5%, 10%... of maximum AC output:

ηweighted = F1 × η5 + F2 × η10 + F3 × η20 + F4 × η30 + F5 × η50 + F6 × η75 + F7 × η100

Weighting Factors for CEC and European Weighted Inverter Efficiencies

Percent of Inverter Maximum AC Output

Factor

CEC Weighting Factor

European Weighting Factor

5

F1

0.00

0.03

10

F2

0.04

0.06

20

F3

0.05

0.13

30

F4

0.12

0.10

50

F5

0.21

0.48

75

F6

0.53

0.00

100

F7

0.05

2.00