Expanding an Existing Solar Power System: Here's How for Businesses
Want to make use of unused roof space and increase your yield? When expanding an existing PV system, EEG thresholds and tax status are key factors. The 2026 Commercial Guide.
The short answer
Anyone wishing to expand a PV system should, above all, be familiar with the EEG system consolidation provision under Section 24 of the EEG: The expansion will be subject to the currently applicable—and typically lower—feed-in tariff rate, while the existing system retains its original rate. The expansion must take place within twelve months, and capacity thresholds (100 kWp for direct sales, 30/100 kWp for tax exemption) may trigger new obligations. The expansion is particularly worthwhile financially if it increases self-consumption.
Many businesses installed a solar power system years ago and now want to expand it—either because they have unused roof space or because their electricity consumption has increased due to electric cars, heat pumps, or new processes. Expanding an existing PV system is usually technically straightforward but legally complex. This article is aimed at commercial businesses and outlines the 2026 regulations—the Photovoltaics Industry Guide provides an overview.
Ways to Expand an Existing Solar Power System
There are several options for expanding a solar power system. An expansion involves installing additional solar panels on previously unused areas: covering unused roof space or utilizing outbuildings, carports, and the building facade. New panels should be technically compatible with the existing ones, and the installation must be performed by a qualified contractor to ensure safety and maximum energy output.
If additional modules are installed, the inverter capacity must be checked—modern commercial inverters allow for a DC overload of up to about a factor of 1.5, so a new unit is not always necessary. A prerequisite is sufficient roof load-bearing capacity of at least approximately 30 kg/m². A battery storage system can also be added; we will address the retrofitting of a commercial storage system in a separate article due to the specific technical and legal considerations involved. This guide focuses on expanding module capacity.
EEG Law: The Plant Summary
The legal cornerstone of every expansion is Section 24 of the Renewable Energy Sources Act (EEG). If multiple facilities are commissioned on the same property or in the immediate vicinity within a period of twelve calendar months, they are treated as a single facility for the purpose of calculating the feed-in tariff. The determining factor here is not a fixed radius in meters, but—according to Federal Court of Justice (BGH) case law—the common grid connection point.
The "split principle" applies to feed-in tariffs: New installations receive the EEG rate in effect at the time of commissioning, while existing installations retain their original rate. Since the feed-in tariff decreases by about one percent every six months, the rate for new modules is generally lower. The current rates can be found in the 2026 EEG Feed-in Tariff Guide.
Performance Thresholds and Their Consequences
Below are the most important thresholds and their impact on your expanded photovoltaic system.
| Threshold | Triggering Obligation |
|---|---|
| 30 kWp per unit / 100 kWp total | Termination of the tax exemption under Section 3, No. 72 of the Income Tax Act (EStG) |
| 100 kWp and up | Direct Sales Requirement (Section 21b of the EEG) |
| 7 kW or more installed | Installation of a smart metering system (Section 29 of the MsbG) |
| Over 500 kW installed | Investment Certificate Required |
| 1,000 kWp and up | Requirement to Issue a Request for Proposals (Compensation Only Upon Award of Contract) |
| Source: EEG 2023 (Sections 21b, 24), Section 29 of the MsbG, Section 3, No. 72 of the EStG | |
The most important thresholds at a glance:
- 30 / 100 kWp: Above this threshold, the tax exemption under Section 3, No. 72 of the Income Tax Act (EStG) no longer applies.
- 100 kWp: Starting at this level, direct sales are mandatory—with a sales fee and a contract with a direct sales provider.
- 7 kW: Starting at this level, a smart meter is required.
- 500 kW / 1,000 kWp: Starting at this level, the system certificate and the requirement to participate in a competitive bidding process apply.
Tax Pitfalls
One often-overlooked risk is the tax exemption under Section 3, No. 72 of the German Income Tax Act (EStG). If a single system exceeds 30 kWp or the total capacity of all systems exceeds 100 kWp, the exemption is completely forfeited—no tax-exempt base amount remains. This applies prospectively from the date the threshold is exceeded, not retroactively. For businesses, this is not only a risk but also an opportunity: The article “Photovoltaics: Save on Taxes with IAB and Depreciation” explains which depreciation strategies apply.
Cost of the Expansion
The investment depends on the size and technology of the system expansion. According to Fraunhofer ISE, system costs for commercial rooftop expansions exceeding 30 kWp range from approximately 900 to 1,600 euros per kWp. If the expansion is combined with a roof renovation that was already planned, costs decrease significantly due to synergies in scaffolding and installation. The additional solar power is particularly worthwhile if it increases self-consumption and thus replaces expensive grid purchases.
Grid Connection, Registration, and Metering Plan
Registration in the Market Master Data Registry
Every expansion must be registered in the market master data registry. The expansion is registered as a new unit with its own commissioning date—not as a capacity update to the existing facility. The new registration must be completed within one month of commissioning. Failure to register may result in fines of up to 50,000 euros and reduce the value to be recorded to zero.
Meter Replacement
A separate meter is not required by law—a shared metering system with allocation based on the power ratio is permitted. In practice, however, notifying the grid operator of the expansion often triggers a meter replacement in order to clearly distinguish between the old and new rates.
Grid connection capacity
An expedited connection process lasting eight weeks applies only to systems up to 100 kWp and only if the existing grid connection capacity is sufficient. For larger expansions, it is important to determine early on whether the grid connection needs to be upgraded—this is often the time-critical path in the project.
Expand or build a new one?
Whether an expansion is more cost-effective than repowering or building a new system depends on the age of the existing system. An expansion is a good option if there is available space and the existing system is operating properly. Repowering—replacing old modules with higher-efficiency ones—is usually only worthwhile toward the end of the subsidy period or when the inverters are due for replacement; the payback period is typically 6 to 12 years. For details, see the article “Photovoltaic Repowering: Modernizing Existing Systems.”
The expansion makes economic sense above all when it increases self-consumption. Simply increasing capacity quickly reaches a saturation point: the additional solar power is generated at times when the business cannot use it itself. Aligning electricity consumption with the generation profile—for example, through charging infrastructure for electric cars, heat pumps, or rescheduled processes—is therefore often the real key to reducing electricity costs.
Have an expansion planned
Are you considering expanding your commercial facility and want to accurately calculate EEG thresholds, tax implications, and grid connection costs? We plan the facility expansion as a comprehensive package—from system consolidation to the metering concept.
Frequently Asked Questions (FAQ)
Will my existing system be eligible for a new rate if I expand it?
No. Only the new addition is eligible for the current, generally lower EEG rate. The existing system retains its original feed-in tariff—even with a shared meter, thanks to the pro rata allocation under Section 24(3) of the EEG.
What does the 12-month rule mean?
If an existing facility and an expansion are commissioned at the same location within twelve calendar months, they are considered a single facility under the EEG. This is crucial for capacity thresholds such as the 100-kWp direct-sales requirement.
Do I need a new meter?
Not required by law, but often the case in practice. A shared metering system with allocation based on power ratio is permitted; in practice, however, notifying the utility of the expansion often results in a meter replacement.
Will I lose the tax exemption under Section 3, No. 72 of the Income Tax Act (EStG)?
Yes, in full and without a base amount, as soon as a unit exceeds 30 kWp or the total capacity exceeds 100 kWp. The provision takes effect as of the date the threshold is exceeded; it does not apply retroactively to previous years. A tax advisor should verify whether this applies to your specific situation.
How much does it cost to expand a solar power system?
For commercial rooftop expansions exceeding 30 kWp, the cost is approximately 900 to 1,600 euros per kWp (Fraunhofer ISE). The exact costs depend on the type of roof, accessibility, and the effort required for grid connection; combining the project with a roof renovation reduces these costs.
Does the power connection need to be adjusted?
In practice, this is almost always the case. Grid operators treat expansions legally as new installations requiring their own connection application. An expedited eight-week procedure applies only for projects up to 100 kWp and when there is sufficient existing capacity.
Conclusion: Expansion is a matter of thresholds
Expanding an existing solar power system in 2026 is primarily a matter of thresholds: As soon as the combined capacity exceeds 30 or 100 kWp, the feed-in tariff, tax status, and marketing requirements change. For commercial businesses, an expansion is economically viable if it increases self-consumption.
For those who want to take the next step: Guide to the Photovoltaic Industry. The article “Photovoltaics: Save on Taxes” explains the tax incentives, while “PV Roof Systems for Commercial Use” presents additional options for roof utilization.
References
- Section 24 of the EEG (Aggregation of Installations)
- Section 21b of the EEG (Direct Marketing)
- EEG|KWKG Clearing House: Summary of Facilities (FAQ 24)
- Federal Ministry of Finance: FAQ on the Tax Exemption for Photovoltaic Systems (Section 3, No. 72 of the Income Tax Act)
- Rödl & Partner: Expanding or Repowering Photovoltaic Systems
- Fraunhofer ISE: Recent Facts About Photovoltaics in Germany
Related Articles: Photovoltaic Repowering · EEG Feed-in Tariff 2026
