SM On-Grid Design Tool

Matching Power Values for an Array and an Inverter:

All inverters are rated by their maximum continuous power output, which is measured in watts or kilowatts. This value is the AC power output.
Inverters actually limit their power output, so you can use this value to figure out the maximum power input coming into the inverter from the PV array.

When you look at PV module ratings, you find that the power output ratings are based on standard test conditions (STC) where the cell temperature is 25 degree Celsius and the intensity of the sun is equal to 1,000 W/m2.
PV modules rarely operate in these conditions due to the motion of the sun across the sky and increased PV operating temperatures.

• The power input coming into the inverter is typically considerably less than the rated values.
• Both the inverter and the array (along with all the other equipment in a grid-direct system) have efficiency losses that must be taken into account.

You can quickly estimate all of these system inefficiencies by adopting the industry standard that any PV system will operate at approximately 80 percent of overall system efficiency.

VIP: It’s recommended to oversize modules power by (10~20%) more than Inverter Rated Power.

The inverter’s DC voltage window:

Inverter’s voltage window consists of a maximum input voltage that you must stay under if you want to avoid damaging the inverter and a minimum voltage you must stay above to keep the inverter operating at the array’s maximum Power Point. As a PV system designer, you need to identify these values from a spec sheet and make sure the array can operate within this window throughout the year. If you allow the array to move outside this window, you risk damaging the inverter or shutting down the system for the day.

Calculating the modules’ maximum DC voltage contribution:

As the temperature decreases, a PV module’s voltage actually increases (voltage and temperature therefore have an inverse relationship).

To make sure an array’s voltage never exceeds the inverter’s input value, you need to adjust the modules’ STC open circuit voltage value (Voc) for cold temperatures; this value is reported on spec sheets.

Applying the adjusted open circuit voltage to come up with the right number of modules in a string

Figuring out the modules’ minimum DC voltage contribution:

On the other end of the voltage window is the minimum power point tracking voltage. If the voltage from the array ever drops below this value — usually due to high heat conditions that occur on the sunniest days (as in the days system owners expect their arrays to produce the maximum amount of energy possible) — the inverter won’t be able to continue operating and will shut down. Although this scenario won’t damage the inverter, it may damage your reputation. If your client sees his inverter shut down on a bright, sunny day because the voltage isn’t high enough.

But increasing Number of Modules per string- by 1~2 Modules- than minimum number of modules calculated, is the best choice due to degradation in Module`s Power(voltage) over its long life. So that to assure you always fit inside the voltage window.

Picking the temperature to use in your calculation:

The high temperature Record: By designing with the record high temperature in mind, you ensure that the PV array will operate in all conditions.

Accounting for mounting in your calculation:

To estimate a PV module’s temperature at the hottest time of the year, you should add one of the following temperatures to the ambient temperature:

• 35 degrees Celsius for arrays with fewer than 15 cm between the backs of the modules and the mounting surface
• 30 degrees Celsius for arrays with more than 15 cm between the backs of the array and the mounting surface
• 25 degrees Celsius for arrays that are mounted on top of a pole

By adding these values to the ambient temperature, you can arrive at a best estimate for your array’s operating temperature — the temperature you expect to see at the module level if you were to measure the temperature in the middle of the summer.

Getting you Ready to Match an Inverter to an Array

1. Enter your project name, & you can add your customer details to save your project to view later.
2. Choose an On-Grid inverter using one of these two ways:
   • Choose from a list of different existing brands & Models
   • Add your own Brand & Model of On-Grid Inverters.
3. Choose a PV Module using one of these two ways:
   • Choose from a list of different existing brands & Models
   • Add your own Brand & Model of PV Modules.
   Note: You can use different Module for different MPPT’s.
4. Enter your inputs:
   • Number of Modules Per string & Number of strings/MPPT.
   • Panel Max. Temp (C).
   • Panel Min. Temp. (C).
5. A result sheet will appear with the following data:
   • PV Generator Nominal Output Per MPPT (Wp).
   • Minimum MPPT Voltage at Maximum Temperature.
   • Maximum MPPT Voltage at Minimum Temperature.
   • Maximum open circuit Voltage of PV generator at Minimum Temperature.
   • Maximum short circuit Current of PV generator at Maximum Temperature.
   • Total PV generator Output (Wp).
   • Each result of above is compared with inverters parameters, to show if your selection gives you the right values, it gives you GOOD, else it gives a Warning.
     - The right values are: :
     • PV Generator Nominal Output Per MPPT (Wp) <= Maximum Input Per MPPT of Inverter (W).
     • Minimum MPPT Voltage at Maximum Temperature >= Minimum MPPT Voltage of Inverter.
     • Maximum MPPT Voltage at Minimum Temperature <= Maximum MPPT voltage of inverter.
     • Maximum open circuit Voltage of PV generator at Minimum Temperature <= Maximum input voltage, open circuit of inverter.
     • Maximum short circuit Current of PV generator at Maximum Temperature <= Maximum short circuit current per MPPT.
     • Total PV generator Output(Wp) <= Maximum DC input power of Inverter (W)