UL Solutions Sets Safety Plug-in Solar

UL Solutions, an Illinois-based testing and certification company, has released UL 3700, a safety standard for plug-in photovoltaic (PIPV) systems, published earlier this year. The new standard addresses shock, fire and overload hazards that arise when solar panels are connected to residential outlets.

Why Plug-in Solar Is Different

Plug-in photovoltaic systems mount panels on a balcony, connect them to an inverter and plug them into a household outlet. For renters and apartment owners who cannot install rooftop solar, the appeal is clear. But that simple connection changes how a household outlet is used. PIPV plugs look like those used by everyday appliances, but their function is fundamentally different fixtures. When a PIPV system is disconnected, the panel can still produce electricity if it is absorbing light, which the inverter converts into AC power. If the system can export that power through the plug, disconnecting the plug does not necessarily mean that the plug blades are safe to touch. In the absence of other safety protective circuits, hazardous voltage can be present on the plug blades, a shock risk that the typical 120V AC supply in US residences presents.

Safety Assessment and UL 3700

UL Solutions used hazard-based safety engineering to assess shock, fire and injury risks. The findings fed into the development of UL 3700, published earlier this year. The standard requires additional protection to shut down power export before a consumer can contact hazardous voltage at the plug. Existing inverter shutdowns, designed to protect utility workers, are too slow to address this shock risk. PIPV systems therefore need an additional safety function built into the system.

Interaction with Existing Protection

Residential circuits rely on overcurrent protective devices such as circuit breakers to guard wiring and equipment. A conventional load draws current through the breaker, allowing it to detect and interrupt excessive current. A PIPV system connected downstream of that breaker injects current that the breaker cannot see, potentially overloading parts of the circuit without detection. The most likely consequence is a fire rather than an immediate electrical failure. Gradual degradation in connections or insulation could cause issues over weeks or months. To mitigate this, wiring and equipment must be rated for the combined utility and PIPV current, or power control systems must monitor the circuit and intervene when current reaches unsafe levels. Boyce distinguishes these from conventional energy-management systems, which focus on consumption or cost. A power control system can perform a safety function, shutting down or reducing loads to prevent overload.

Regulatory Gap

Even a product that meets UL 3700 still faces a regulatory question in the US. The 2026 National Electrical Code (NEC) does not specifically address PIPV systems. The NEC is a model code that becomes enforceable when adopted by a state or local jurisdiction, potentially with amendments. Enforcement is then carried out by authorities. This omission creates uncertainty about where PIPV systems can be installed.

UL Solutions also examined the interaction between PIPV systems and ground-fault circuit-interrupters (GFCIs). Backfeeding a GFCI can damage the device so that it no longer provides protection while still supplying power, creating a dangerous failure mode that may appear normal. In its December 2025 white paper, UL Solutions translated hazard mitigation ideas into UL 3700 requirements. The standard does not dictate a single architecture; manufacturers may innovate alternative approaches that meet the same safety outcomes. Boyce notes that none of the products tested complied with UL 3700, highlighting safety issues in the current market. The technology can be made safe with existing engineering if safety is built into the design from the start stats.