Solar PV Array Sizer & String Calculator

Professional engineering tool for sizing Photovoltaic (PV) arrays. Compliant with IEC 60364-7-712 and NEC 690. Designed to bridge the gap between Standard Test Conditions (STC) and real-world site extremes (Temperature correction). Calculates safe string lengths, inverter matching, and optimal DC/AC ratios for Residential, Commercial, and Utility-Scale projects.

Quick Load Presets:

1. PV Module Specifications (STC Data)

Power ($P_{max}$) (W)

Open Circuit Volts ($V_{oc}$)

Max Power Volts ($V_{mp}$)

Short Circuit Amps ($I_{sc}$)

Temp Coeff of $V_{oc}$ ($\beta_{Voc}$) (%/°C)

Temp Coeff of $V_{mp}$ ($\beta_{Vmp}$) (%/°C)

2. Inverter Specifications

Max Input Volts ($V_{max}$)

MPPT Min Volts ($V_{min}$)

MPPT Max Volts

Max DC Current ($I_{dc}$) (A)

Rated AC Power (kW)

3. Site Conditions

Record Low Temp ($T_{min}$) (°C)

Avg High Temp ($T_{max}$) (°C)

Mounting Type

Target DC Capacity (kW)

Sizing Results

Optimal Configuration

Parameter	Value	Check
Max Panels per String ($N_{max}$)	0	Safe
Min Panels per String ($N_{min}$)	0	MPPT
Recommended String ($N_{opt}$)	0	-
Total Strings Needed	0	-
Total DC Capacity	0.00 kW	-
DC/AC Ratio (ILR)	0.00	-

Voltage Check: Verify against cold/hot extremes.

Voltage Window Analysis

Step-by-Step Calculation

1. Temperature Coefficients: The Cold Danger

Solar panels are semiconductors. Unlike conductors, their voltage increases as temperature drops. This is the most critical safety check in PV design.

The Open Circuit Voltage ($V_{oc}$) at the site's record low temperature ($T_{min}$) must never exceed the inverter's maximum input voltage ($V_{max,inv}$). A single cold morning can destroy an entire inverter bank if this is ignored.

$$ V_{oc}(T_{min}) = V_{oc,STC} \times [1 + \frac{\beta_{Voc}}{100} \times (T_{min} - 25^\circ C)] $$

Since $\beta_{Voc}$ is negative (e.g., -0.3%/°C), a drop to -10°C increases voltage by roughly 10% compared to Standard Test Conditions (STC, 25°C).

2. Inverter Voltage Windows

Inverters have two key voltage limits:

Absolute Max ($V_{max}$): Exceeding this destroys components instantly (Safety limit).
MPPT Range ($V_{min} - V_{mppt,max}$): The range where the inverter can efficiently track the Maximum Power Point.

We must ensure that on the Hottest Day ($T_{max}$), the string voltage ($V_{mp}$) stays above the inverter's turn-off voltage ($V_{min}$). If the voltage sags too low due to heat, the inverter wakes up late and sleeps early, losing valuable production hours.

3. The DC/AC Ratio (ILR)

It is standard industry practice to oversize the DC array relative to the AC inverter rating. This is called the Inverter Loading Ratio (ILR) or DC/AC ratio.

Why Oversize? Panels rarely produce their rated STC power due to heat, dust, wiring losses, and sun angle. A ratio of 1.2 to 1.3 ensures the inverter runs at full capacity for more hours of the day.
Commercial: Often 1.25 - 1.4.
Utility: Can be 1.5+ for "block" designs to flatten the production curve.

4. Power Clipping

When the DC array produces more power than the inverter's AC rating (e.g. at high noon on a cold day), the inverter "clips" the excess power. While this looks like a loss, the gain in energy harvest during the morning and evening hours usually far outweighs the clipped energy at noon.

Economic Optimization

A higher DC/AC ratio improves the Levelized Cost of Energy (LCOE) by maximizing the utilization of the expensive AC infrastructure (Inverter, AC cables, Switchgear, Grid Connection).

5. Applicable Standards

NEC 690.7: Mandates temperature correction for maximum system voltage calculation using lowest expected ambient temperature (Record Low).
IEC 60364-7-712: Requirements for special installations or locations - Solar photovoltaic (PV) power supply systems.
IEC 62548: Photovoltaic (PV) arrays - Design requirements. Defines voltage calculation methods for safety.