Imagine living in an apartment in Berlin with two solar panels installed on your balcony. You plug them in, and your washing machine, dryer, and electric fan start running one after another, while the electricity meter stands still this isn’t a scene from a science fiction movie, but a real scenario in Europe’s current energy transition. According to the latest “Plug-In Solar” report by SolarPower Europe, a technology known as “plug-in photovoltaic systems” is sweeping across the European continent, with Germany at its epicenter. It allows renters and low-income households to enjoy green photovoltaic power and save on their electric bills. Furthermore, VDE is set to release the world’s first technical standard for plug-in solar systems in the third quarter of 2025. According to the latest information, this standard will cap the system’s installed capacity at 960W.

Figure: SolarPower Europe

I. From Germany to All of Europe: The Wildfire-like Spread of Plug-in Systems

Germany: The Birthplace of the Technology and Market Engine

As the “Silicon Valley” of plug-in systems, the German market is experiencing explosive growth:

· Surge in Installations: In 2024, 435,000 new systems were registered, accounting for 26% of the total PV installations for the year. Considering the large number of unregistered systems, it is estimated that the actual number in operation could be as high as 4 million units, meaning one in every 20 households owns a “balcony power station.”

· Policy Deregulation: The “SolarPaket 1” law, passed in May 2024, simplified the registration process to a 10-minute online declaration, unleashing the market’s potential. Previously, the cumbersome approval process by grid operators was the main obstacle preventing users from registering.

· Economic Breakthrough: System prices have dropped to as low as 200 euros (for a basic 400W model), with the payback period shortened to 2-6 years. Some cities have even introduced “solar care packages.” For example, Bonn offers a 90% subsidy for low-income households, and Berlin provides a discount of up to 500 euros.

Annual registered number of plug-in solar systems in Germany (Figure: SolarPower Europe)

EU Market: A Testing Ground for Divergent Policies

Other countries exhibit three development models:

· Fast Followers (France, Austria, Lithuania): France’s plug-in installations reached 34MW in 2024, allowing DC-side oversizing up to 3kW (with an AC limit of 600W). Austria added 20,000 new plug-in systems annually in 2023 and 2024. Lithuania launched a subsidy program of 102 euros for 400W systems and 204 euros for 800W systems.

· Conservative Wait-and-See (Spain): Spain mandates that “plug-in” systems must be installed by a qualified electrician and connected to a device that prevents electricity from being fed into the grid. They cannot be connected to standard household outlets.

· Prohibitive (Sweden, Hungary): Sweden prohibits the use of household sockets for connection, citing “fire risk,” and only permits professional electricians to install off-grid systems.

EU Policy Framework: The 2024 Electricity Market Design Directive (EU/2024/1711) gives a “green light” to small-scale systems, recommending that member states exempt them from approval processes and allow grid feed-in. However, the specific rules are still left to each country to decide. This model of “unified principles, divergent implementation” has become the root cause of market fragmentation.

Figure: SolarPower Europe

II. DC Oversizing: Technological Dividends and Regulatory Tug-of-War

What is DC Oversizing?

The “hidden feature” of plug-in systems is that the capacity of the DC side (photovoltaic panels) can be higher than the limit of the AC side (micro inverter). For example, one could install 2kW of PV panels (DC side) with a microinverter that limits the current to 800W (AC side). This design can compensate for efficiency losses due to shading or non-ideal angles, thereby increasing power generation. With the growing adoption of energy storage systems, DC oversizing is becoming increasingly common; the largest systems have reached configurations of an 800W inverter paired with 6kW of PV modules and a 25kWh energy storage system.

A Map of “Oversizing Tolerance” in EU Countries

Key Conflict: The greater the oversizing capability, the higher the system’s cost-effectiveness. However, it may pose risks such as cable overheating and grid fluctuations; in severe cases, it can cause fires (Reference article: Is Lithium Iron Phosphate No Longer Safe? A German Residential Energy Storage Fire Sparks 5 Conjectures). Germany’s approach is a case in point: the VDE standard requires dynamic current limiting through software in the Power Conversion System (PCS) to ensure the AC side output does not exceed 800W. Spain, on the other hand, effectively kills the economic viability of oversizing by mandating the installation of a separate circuit (requiring an electrician).

Annual installation volume of small-scale energy storage systems (<5kWh) in Germany (Source: SolarPower Europe)

III. The Plug War: Schuko vs. Wieland, Who Will Prevail?

The Technical Showdown Between Two Plugs

Schuko (Type F):

· Advantages: A common European household socket; plug-and-play with zero installation barrier.
· Risks: Ordinary users might mistakenly connect multiple devices, leading to circuit overload.

Schuko plug

Wieland (dedicated connector):

· Advantages: Specifically designed for PV, prevents incorrect insertion, and includes a locking feature.
· Disadvantages: Requires professional installation by an electrician, adding 200-500 euros to the cost.

Wieland plug

EU Countries “Taking Sides” on Plugs

· Germany: Compromise After Policy Vacillation
o Before 2024: Mandatory use of Wieland sockets, leading to a large number of users installing Schuko systems “off the books.”
o After SolarPaket 1: Tacit tolerance of Schuko, but the draft VDE standard requires the addition of a mechanical protection device.

· France: The Liberal Experiment
o Allows Schuko plugs, but grid feed-in requires signing a “non-injection agreement” (a de facto ban).

· Sweden: The Outright Ban
o Deems Schuko connections illegal, permitting only Wieland plugs with professional installation, which has brought the market to a near standstill.

The Domino Effect of Installation Requirements:

In countries that use Schuko (e.g., Germany, France), users can perform DIY installations, and the market size has grown by over 200% annually. In countries that mandate Wieland (e.g., Spain, Sweden), system costs increase by 30%-50%, relegating them to a niche market.

Figure: SolarPower Europe

IV. VDE 0126 95: Q3 2025, The “D-Day” for Plug-in Solar

Why is this standard so important?

The VDE 0126 95 standard, developed by the German Association for Electrical, Electronic & Information Technologies (VDE), is the first comprehensive technical specification for plug-in systems. It covers:

· Key Safety Requirements: Anti-islanding protection, voltage/frequency response, electromagnetic compatibility (EMC).

· Performance Metrics: The currently circulated version specifies a DC-side limit of 960W which, although lower than Germany’s legal limit of 2kW, may become the de facto standard.

· Plug Specifications: Schuko sockets must be fitted with mechanical or electromechanical protection.

The final version of this standard is scheduled for release in the third quarter of 2025. The Netherlands and Austria have already indicated they will reference the VDE standard. Although the standard is not mandatory, landlords and insurance companies are likely to use it as a benchmark, which could have a significant impact on the plug-in solar system market.

Tip: Plug-in solar is not limited to balcony installations.

Plug-in solar is often referred to as “balcony solar” in the media. However, actual balcony installations currently account for less than one-third of all plug-in solar systems. More plug-in systems are installed in yards, patios, on rooftops, and on facades.