What is solar string sizing?

String sizing determines how many solar panels to connect in series (a 'string') to match your inverter's input voltage range. Panels connected in series add their voltages together. If your inverter accepts 200–600V DC and each panel has an open-circuit voltage of 40V, you can safely wire 5–12 panels per string (200–480V).

Why does string sizing matter?

Incorrect string sizing can damage your inverter or reduce system performance. Too few panels in a string means input voltage falls below the inverter's minimum tracking voltage, especially in cold weather. Too many panels can exceed the inverter's max voltage on cold days when panel voltages rise. Proper string sizing ensures safe, efficient operation year-round.

What is a solar string and why does sizing matter?

A solar string is a series of panels connected together feeding into one inverter input. String sizing determines the voltage and current reaching the inverter. Strings must be designed within the inverter's MPPT (Maximum Power Point Tracker) voltage range — too few panels and voltage may fall below minimum; too many and voltage can exceed the inverter's maximum input.

What is the maximum number of panels in a solar string?

Maximum panels per string depends on your inverter's maximum input voltage (typically 480–600V DC for residential) and each panel's Voc (open-circuit voltage, typically 40–50V). At 50V Voc per panel with a 480V max inverter input, you can fit approximately 9–10 panels per string. Most string inverters have 1–3 MPPT inputs allowing multiple strings.

Solar String Sizing Calculator

Determine how many solar panels to wire in series per string. String sizing must keep voltage within your inverter's MPPT window across all temperature conditions — too few panels and the inverter won't operate; too many and you risk equipment damage.

Panel Voc (V)

Panel Vmp (V)

MPPT Min (V)

MPPT Max (V)

Inverter Max Input (V)

About This Calculator

The Solar String Sizing Calculator determines how many panels to connect in series on a single string, ensuring the string voltage stays within the inverter's operating window across all temperature conditions your site experiences. String sizing is one of the most technically critical steps in solar system design — an undersized string falls below the inverter's minimum MPPT voltage on hot afternoons, causing the inverter to shut off or derate production. An oversized string exceeds the inverter's maximum input voltage on cold mornings, which can permanently damage the inverter and void its warranty. The calculation must account for temperature-corrected voltage at both extremes.

Solar panels are semiconductor devices whose voltage changes significantly with temperature. Open-circuit voltage (Voc) increases at cold temperatures — typically by about 0.3% per °C below the STC test temperature of 25°C. On a -10°C winter morning, a panel with a 25°C Voc of 40.5V might actually produce 43–44V open-circuit. Multiply by the number of panels in your string and that voltage spike must stay below the inverter's absolute maximum input voltage to avoid damage. Conversely, maximum power point voltage (Vmp) decreases at high temperatures, and a string that barely meets the MPPT minimum at 25°C may fall below it when panels heat to 60–75°C on summer afternoons. This calculator uses simplified temperature correction (approximately ±0.3%/°C for Voc and ±0.4%/°C for Vmp) as a practical estimate; a full NEC-compliant design uses manufacturer-specified temperature coefficients from the panel datasheet.

The optimal string size is the largest number of panels that keeps cold Voc below both the inverter maximum and the MPPT maximum, while also keeping hot Vmp above the MPPT minimum. When these constraints conflict or leave only a narrow range, it may indicate a mismatch between the chosen panel and inverter combination. Common residential string inverters have MPPT windows of 200–480V and maximum inputs of 600V; commercial string inverters often work at 600–1000V. Microinverters and DC power optimizers (SolarEdge) eliminate string sizing constraints entirely — each panel operates independently — which is one reason they are preferred for complex roofs.

For practical system design, always get the exact temperature coefficients (in %/°C or mV/°C) from the panel specification sheet and use those in a full NEC Article 690 calculation before submitting for permits. The NEC requires using the lowest expected ambient temperature from the ASHRAE 2% annual extreme data for your location, which is often colder than the intuitive "coldest day" estimate. Some jurisdictions also require calculations stamped by a licensed engineer. Use this tool for planning and preliminary design — your installer or engineer will confirm the final string count with site-specific data.

Calculations based on NREL solar modeling data and industry-standard assumptions, built and maintained by the independent SolarToolsOnline research team.

Estimates only — not financial, tax, or legal advice. Verify important results with a licensed solar installer or financial professional before making decisions.