The Inverter Sizing feature validates electrical compatibility between your selected inverter and PV module library components before generating layouts or running yield simulations.
When to use inverter sizing
- Project Setup: Verify component compatibility and determine optimal string sizes early by specifying your project location to automatically adapt calculations to local climatic conditions
- Preliminary Design: Evaluate secondary module or inverter alternatives to assess their impact on system capacity, yield, and Levelized Cost of Energy (LCOE).
- Detailed Design: Re-evaluate MPPT connections and voltage bounds when modifying module counts or string configurations prior to construction.
How it works
- Head over to the Electrical Design tab in the ribbon.
- Choose your PV module and inverter from the dropdown menus. Click Manage Database to add any missing models.
- If you select a site, its configuration settings appear automatically. If not, open Layout Generation Settings and set the Project location.
Note: The location sets the minimum and maximum temperatures according to historical meteorological data from the PVGIS database. You may also manually modify these temperatures if you have a reliable alternative source.
- Check the recommended string size.
Note: Sizes are under “recommended range”. You can manually enter min. and max., but they may go beyond the inverter's range. Consult the manufacturer to confirm.
- Review the recommended string limit, or manually set your own to adjust the inverter's DC/AC ratio.
- Click Apply to lock in your inverter sizing configuration.
Technical Calculations & Formulas
The software computes string length limits using the following equations:
Worst-case maximum module voltage at open-circuit
The maximum open-circuit voltage \(V_\text{oc}^\text{max}\) is calculated by adjusting the Standard Test Conditions (STC) open-circuit voltage \(V_\text{oc}^\text{STC}\) for the minimum operating temperature \(T_\text{low}\) using the temperature coefficient \(\beta_{V_\text{oc}}\), plus a thermal voltage term for rear-side irradiance gain under Bifacial Stress Irradiance (BSI) conditions, accounting for the module’s bifaciality \(\varphi\), cell count \(N_\text{s,c}\), and diode ideality factor \(n\).
\[ V_\text{oc}^\text{max} = V_\text{oc}^\text{STC} \left[ 1 + \beta_{V_\text{oc}} (T_\text{low} - T_\text{STC}) \right] + N_\text{s,c} \; \frac{n \; k \; T_\text{low}}{q} \; \ln\left(\frac{G_\text{front, BSI} + \varphi \; G_\text{back, BSI}}{G_\text{STC}}\right) \]
Worst-case minimum module minimum voltage at maximum power point
The minimum maximum-power-point voltage \(V_\text{mp}^\text{min}\) is calculated by derating the STC voltage \(V_\text{mp}^\text{STC}\) for maximum operating temperature \(T_\text{high}\) via the temperature coefficient \(\beta_{V_\text{mp}}\).
\[ V_\text{mp}^\text{min} = V_\text{mp}^\text{STC} \;\left[ 1 + \beta_{V_\text{mp}} \; (T_\text{high} - T_\text{STC}) \right] \]
String length bounds
The number of series-connected modules per string \(N_\text{series}\) is bounded by rounding up the ratio of the inverter's minimum MPPT voltage \(V_\text{mppt,min}\) to \(V_\text{mp}^\text{min}\), and rounding down the ratio of the maximum inverter input voltage \(V_\text{inv,max}\) to \(V_\text{oc}^\text{max}\).
\[ \left\lceil \frac{V_\text{mppt,min}}{V_\text{mp}^\text{min}} \right\rceil \le N_\text{series} \le \left\lfloor {\frac{V_\text{inv,max}}{V_\text{oc}^\text{max}}} \right\rfloor \]
DC/AC ratio
This is also known as the inverter loading ratio (ILR), generally defined as R:
\[ R = \frac{P_\text{DC}}{P_\text{AC}} \]
where \(P_\text{DC}\) is the total peak DC capacity of the PV array connected to the inverter at STC expressed in kWp; and \(P_\text{AC}\) is the nominal output power of the inverter expressed in kVA.
PV arrays are typically paired with smaller inverters to maximize efficiency—a practice known as inverter undersizing (or DC oversizing). This creates a DC/AC ratio greater than 1.0. For instance, in northern Europe, string inverters are often sized at about 1.2, so for a 6kWp PV array, the inverter's AC output is limited to 5kW.
In PVcase Ground Mount, the DC/AC Ratio Matrix displays recommended string sizes and inverter combinations:
- Recommended Range: Displays your calculated optimal parameters graphically.
- Green Indicators: Signal that your selected values fall entirely within recommended operating limits.
If you exceed PVcase's recommendations and overload the inverter, the matrix indicators will turn orange, and an advisory message—such as a warning that your string count exceeds calculated limits—will appear in the Inverter Sizing window.
To evaluate the impact of the DC/AC ratio, conduct a yield study with tools like PVcase Yield or PVsyst to accurately determine how inverter sizing affects PV system performance.
Generating with the Inverter sizing
Smart stringing will be updated to match your chosen string size range. Now you can confidently string the site, knowing that the inverter will be capable of connecting to all the strings you are generating.
In the Electrical Devices panel, inverter settings such as size, power, and the number of strings per inverter will be configured. To disable automatic inverter sizing and enter values manually, select Off.
Finally, the power values will be used in cable sizing calculations. As usual, you can edit these values at any time.