String Inverters

String Inverters for Solar

A string inverter converts DC power from series-connected solar panels into grid-synchronized AC through centralized conversion. Unlike microinverters, string inverters handle 3-50 kW through 1-4 independent MPPT channels, delivering 97.5-98.8% efficiency with significantly lower system costs—typically $0.12-0.18/watt versus $0.28-0.34/watt for distributed alternatives in residential applications.

String Inverter Architecture: What Makes the Difference

Modern string inverters process multiple strings through independent MPPT channels, each functioning as a dedicated DC-DC converter. A dual-MPPT 10 kW unit simultaneously optimizes east and west-facing arrays experiencing different irradiance profiles, or compensates for partial shading on one roof plane while maintaining full output from another. This represents the core advantage: centralized efficiency with substring-level optimization when properly configured.

Transformerless vs Transformer-Based Topology

Transformerless designs dominate residential markets, eliminating the 60 Hz isolation transformer to improve efficiency and reduce weight. They require active ground fault monitoring but work seamlessly with crystalline silicon modules (95% of current installations).

When String Inverters Make Economic Sense

String inverters optimize value for installations with minimal shading, uniform orientation, and systems above 4 kW. The economics are compelling:

Under partial shading, inverters experience proportional losses due to series connection. However, if shade reduces annual production by less than 15%, lower string inverter cost typically outweighs production differences over 25 years. Strategic string design—placing shaded panels on separate MPPT channels—often proves more economical than distributed electronics.

String Sizing Methodology: Voltage and Current Limits

Voltage Window Calculations

String sizing ensures voltage remains within MPPT range across all temperatures while never exceeding maximum input voltage. Solar modules exhibit temperature-dependent behavior (typically -0.29% to -0.35%/°C).

Sizing Process:

Current Capacity and Paralleling

MPPT channels specify maximum input current (18-25A residential, 30-40A commercial). Using 400W modules (11.8A Imp, 12.7A Isc):

Optimal DC-to-AC Ratios

Modern sizing employs 1.15-1.30 DC-to-AC ratios, accepting brief clipping during peak conditions. Modules rarely achieve rated output in the field (realistic peak: 90-95% of nameplate), making aggressive ratios economically optimal.

Economic analysis: A 10 kW inverter handling 12 kW array (1.20 ratio) loses 100-300 kWh annually ($12-45 value) but saves $800-1,200 versus a 12 kW inverter—20 year payback on lost production.

Leading Manufacturer Comparison