In this landscape it means the control logic layer of a PV string inverter, not the power hardware itself: MPPT tracking algorithms, grid synchronization routines, anti-islanding detection and the broader power-control loop coordinating DC/DC and DC/AC stages. The search used here specifically ties string inverter terminology to those control functions, which is why the set is 27 families rather than the much larger universe of general string-inverter hardware patents. Filings that only describe converter topology without a control claim generally fall outside this narrower definition.

Filings peaked at 8 in 2017 and had fallen to 2 by the 2022 midpoint, and the trend continues lower toward the 2026 data cut-off. Part of that decline is real: the core control architecture appears to have been substantially claimed early, reducing the incentive for repeat filings on the same mechanisms. But part of it is a reporting artifact — publication typically lags filing by about 18 months, so the last one to two years in any patent trend are always undercounted and should not be read as zero activity.

US9397504B1, covering a hybrid inverter power control system spanning PV string, battery, grid and back-up loads, carries the highest citation count in this corpus at 25, well ahead of the next tier of records in the mid-teens. A high citation count signals that later applicants and examiners repeatedly reference its framework, which makes it a natural first stop for any freedom-to-operate check on hybrid PV-battery control architectures. It does not, on its own, mean every adjacent claim is blocked — narrower control variants may still be open.

The IPC composition shows H02S, the subclass specific to photovoltaic power generation, appearing in only 2 of 27 records despite the topic being explicitly PV-focused, alongside thin coverage in H02B switchgear-adjacent claims. Sub-areas like constant power generation mode switching, DC bus ripple damping under partial shading, and anti-islanding logic for mixed battery-PV backup configurations show limited direct claim density relative to core MPPT and grid-sync filings. Any of these merits a dedicated prior-art check rather than an assumption that the space is open, since thin IPC representation can also reflect classification habits rather than true absence of art.

The ranking is concentrated among a small number of assignees drawn from China, the US and Europe, consistent with a total pool of only 27 families rather than hundreds of scattered filers. Recent-year momentum data shows every tracked assignee at 0 filings in the latest year, which given the publication lag most likely reflects filings still working through the pipeline rather than a genuine industry-wide stop. Receiving-office data points to the US and EPO as the leading filing venues, with China, Canada and India each contributing a smaller share.