Installation

String Sizing

String sizing is the process of calculating how many solar panels to wire in series per string to ensure the combined voltage stays within the input voltage window of your inverter or charge controller under all temperature conditions. Getting string sizing right is one of the most critical — and most commonly botched — aspects of solar system electrical design.

Two voltage limits define the safe operating window for each string. The maximum string voltage (Voc at the coldest expected temperature) must not exceed the equipment's maximum input voltage — exceeding this limit can permanently destroy the inverter or charge controller. The minimum string voltage (Vmp at the hottest expected temperature) must remain above the equipment's minimum MPPT operating voltage — falling below this threshold prevents the equipment from starting or tracking properly.

Temperature adjustments are essential because panel voltage changes significantly with temperature. Voltage increases in cold weather and decreases in hot weather, with the rate of change specified by the panel's voltage temperature coefficient. A panel with Voc of 42V and a -0.27%/°C coefficient sees its Voc rise to approximately 46V at -10°C and drop to approximately 38V at 60°C cell temperature.

The string sizing formula: Maximum panels per string = Equipment max voltage ÷ Panel Voc at coldest temperature. Minimum panels per string = Equipment min MPPT voltage ÷ Panel Vmp at hottest temperature. The actual string length must fall between these two limits.

String sizing calculators are available from most inverter and charge controller manufacturers, and tools like PVWatts and Aurora Solar automate the temperature-adjusted calculations. For DIY installations, always verify string sizing calculations against the equipment manufacturer's specifications and use conservative temperature estimates for your location. When in doubt, err toward shorter strings — a slightly undersized string loses marginal production, while an oversized string risks equipment destruction.

Frequently Asked Questions

What happens if my string voltage is too high?

If string voltage exceeds the inverter or charge controller's maximum input rating, you risk permanent damage to the input capacitors, MOSFETs, and other components. This typically happens on the coldest winter mornings when Voc is at its peak. The equipment may fail immediately or degrade over time from voltage stress. Always calculate using the coldest expected temperature for your location.

Can I use different panels in the same string?

It is strongly discouraged. Panels in series should have matching current specifications because the lowest-current panel limits the entire string. Mismatched panels in series waste the higher-current panel's excess capacity. If you must mix panels, put different types on separate strings connected to separate MPPT inputs.