The Truth About Oversizing Your Solar PV System
If you’re considering a larger solar PV system for your home in the South East, you might be thinking:
If I can fit more panels on the roof, why not maximise it?
It’s a logical assumption. More panels should mean more energy, more savings and faster payback – right?
Not always.
When it comes to solar panels, battery storage and inverter design, bigger is not automatically better. In fact, oversizing a system without proper design can reduce financial returns and create grid compliance issues.
At SolarTherm UK, we design systems that are engineered – not just installed. Here’s what homeowners in the South East need to understand before going large.
Understanding Export Limits in the UK
Most domestic properties in the UK fall under the G98 grid connection rule, which limits export to 3.68kW per phase without prior approval. That means even if your system produces 6kW or 8kW, you may only be allowed to export 3.68kW to the grid.
Anything above must either be consumed in the home, stored in battery storage or be limited by the inverter. If export is capped and you don’t have sufficient daytime usage or battery capacity, excess generation is simply curtailed – effectively wasted.
This is where oversizing becomes inefficient.
DNO Constraints in the UK
Your local Distribution Network Operator (DNO) controls how much power your property can export. In the South East, common DNOs include:
- UK Power Networks
- Southern Electric Power Distribution
Some areas are already constrained due to a higher solar uptake. In these cases export may be restricted below 3.68 kW or you may require a G99 application, which can delay your installation as these can take up to 12 weeks for approval. Export limitations can be imposed even on larger systems.
Installing an oversized system without confirming grid capacity can lead to delays or enforced export limits that impact your ROI. A properly designed system considers DNO approval before installation – not after.
Inverter Caps: The Hidden Bottleneck
Your inverter determines how much usable AC power your system can deliver. It’s common practice to slightly oversize the DC array (panels) relative to the inverter. This improves efficiency in the UK’s variable climate.
However, excessive oversizing causes:
- Frequent clipping (lost generation during peak sun)
- Reduced long term yield efficiency
- No meaningful financial gain
If you install 8kW of panels on a 5kW inverter, you’re not getting an 8kW system in practice. System design must align panel capacity with inverter output and grid permissions.
Battery Storage Alignment: The Critical Factor
Oversizing your solar panels without correctly sizing solar battery storage creates imbalance. For example, 7kW solar array, 5kWh battery and 3.68kW export cap. On a sunny spring day, your battery could fill by late morning. After that, household usage may not absorb production, export is capped and generation is curtailed.
In contrast, a well balanced system might be 6kW solar array, 10kWh battery and smart export configuration. This increases self-consumption and improves payback. The key metric is self-consumption rate, not just installed capacity.
Future Proofing for EV Charging
Where larger systems can make sense is when planning for:
- Electric vehicles
- Heat pumps
- Home electrification
If you intend to install an EV charger within 1-3 years, designing a larger solar PV system today may be sensible, but only if:
- DNO approval supports it
- Battery capacity scales accordingly
- Export limitations are understood
- Roof orientation supports additional generation
Future proofing must be deliberate, not speculative.
Financial Reality: Diminishing ROI
In the South East, a well sized 5-6kW system often delivers stronger percentage ROI than an 8-10kW system operating under export restrictions.
Why?
- Export payments are lower than retail electricity savings
- Excess energy has reduced value
- Larger systems have higher upfront costs
- Curtailment reduces usable output
Optimisation beats maximisation.
The Design First Approach
At SolarTherm UK, we don’t believe in a one size fits all approach to solar design. Our dedicated, MCS accredited installers use historical energy usage, local DNO capacity knowledge and lifestyle patterns to design a solar PV system that is built for long term returns and reliability. We assess your roof orientation, pitch and look for any shading issues that may prevent your system from performing optimally.
Our approach ensures your system is compliant, efficient, financially optimised and future ready.
Not simply “as big as possible”.
The Bottom Line
Bigger solar systems are not automatically better. The best performing systems in the South East are those that are correctly sized, grid compliant, battery aligned and designed for real consumption patterns.
If you’re considering solar panels or battery storage and want a system engineered for maximum return – not just maximum capacity – contact SolarTherm UK today for a free, no obligation quote or design, tailored to your property, usage and future energy needs. No hard sell, just honest, expert advice.
A strategic system will outperform an oversized one every single time.
Your home. Your energy. Your future.
