Panel installation and wiring
How to Add More Solar Panels to an Existing UK Installation
If your electricity bills have grown after adding an EV or heat pump, your original solar array may no longer offset enough of your usage. Expanding an existing UK installation is possible, but it involves inverter checks, a fresh DNO notification, and MCS re-certification carried out by a qualified installer.
Video: “UK solar panel installation upgrade using Solaredge inverters – Smart Energy Pt 1”, a UK solar installation and smart energy advice channel.
This is MCS-notifiable work. Adding panels to a grid-connected UK solar array is not a DIY project. It requires MCS certification, updated DNO notification, and work by a qualified installer. Working near the consumer unit, on DC string cabling, or on roof-mounted fixings always needs a qualified person. The steps below help you understand the process and brief your installer.
1. Check your inverter's available capacity
A standard UK string inverter is sized for a specific peak DC input. Before ordering extra panels, check the inverter's datasheet for the maximum DC input power and the number of MPPT (maximum power point tracker) inputs. Many hybrid inverters have two MPPT channels: if one channel is free, a second string of panels can often connect without replacing the inverter.
If both MPPT channels are in use and the inverter is running near its maximum, adding more panels will cause clipping: generation that exceeds the inverter's AC output limit is simply lost. Your installer can either accept a small amount of clipping (which is usually worth it if the panels are cheap), or replace the inverter with a larger or higher-specification model.
Also check the inverter's age and condition. An inverter that is already 10 or more years old may be worth replacing rather than extending: newer hybrid inverters add battery capability and are more efficient.
2. Review your DNO connection agreement
Most UK domestic solar installations of 3.68 kW or less were connected under G98: a connect-and-notify process. Taking the total AC output above 3.68 kW on a single-phase supply moves the installation into G99 territory. Under G99, your installer must submit an application to the Distribution Network Operator (DNO) and wait for approval before the expanded system is energised. Approval typically takes 20 to 45 business days under the fast-track route.
On a three-phase supply, the G98 ceiling rises to around 11 kW. Homeowners on three-phase can add considerably more capacity before G99 becomes necessary.
If your original installation already has an export limit device or a G100 export-limiting agreement, check whether the new array output exceeds the approved export level. Your installer will confirm this before submitting the notification.
3. Choose an inverter upgrade strategy
Three routes exist for adding panels to an existing system. Your installer will recommend one based on your inverter, your budget, and whether you also want battery storage.
Route A: Use a spare MPPT input. If your existing inverter has an unused MPPT channel with spare DC input capacity, the new string of panels connects there. No inverter replacement is needed. This is the cheapest route and works well when the original system is lightly loaded.
Route B: Add a second inverter. A separate string inverter sized for the new panels runs in parallel with the original. Each inverter handles its own string and its own G-code notification. This approach adds a second unit to maintain but avoids replacing working equipment.
Route C: Replace with a hybrid inverter. The original inverter is removed and replaced with a larger hybrid model that handles the full array and also supports DC-coupled battery storage. This costs more up front but means one piece of equipment to monitor, one G-code application, and battery-readiness built in. If battery storage is on the roadmap, this route often makes more sense economically.
4. Get MCS certification for the expanded installation
Any material change to a grid-connected solar installation is notifiable under the MCS scheme. Your installer produces updated MCS documentation covering the full expanded system: total installed capacity, inverter model, and panel specifications. This new certificate replaces the original for insurance and property sale purposes.
If you have a legacy Feed-in Tariff arrangement, material alterations to your installation can affect your payment terms or export rate. The FIT closed to new entrants in 2019, but existing arrangements continue. Check with your FIT licensee and the MCS body before any work begins to understand the implications.
The new MCS certificate also determines your eligibility for 0% VAT on the expansion, as qualifying energy-saving materials. Your installer can advise on this when quoting.
5. Submit a new DNO notification
Once the expanded system is commissioned and passes inspection, your installer submits a new G98 or G99 notification covering the total system output, not just the added capacity. The notification covers the whole installation. Where G99 approval was needed, commissioning cannot happen before the DNO issues its connection agreement.
Keep the new MCS certificate and DNO confirmation alongside your original installation paperwork. These documents are needed when selling the property, adding further capacity, or applying for SEG payments after switching supplier.
Frequently Asked Questions
Can I add solar panels to an existing system myself in the UK?
Not if the system is grid-connected. Adding to a grid-connected MCS installation is notifiable work and must be carried out by an MCS-certified installer. The expanded system needs new MCS documentation and a fresh DNO notification. You can plan the upgrade, get multiple quotes, and specify what you want, but the physical work and commissioning must be done by a certified installer.
Will adding more panels affect my FIT or SEG payments?
The Feed-in Tariff closed to new applicants in 2019. If you have a legacy FIT arrangement, check with your FIT licensee before expanding: material alterations can affect your export rate. SEG payments depend on metered export, so a larger array generating more surplus should increase your SEG income, provided the export meter is correctly calibrated.
How much does adding more solar panels cost in the UK in 2026?
A typical expansion on a UK home currently costs between £2,000 and £5,000, depending on the number of panels added, whether the existing inverter needs replacing, and scaffold requirements. Replacing the inverter with a larger hybrid model adds cost but adds battery storage capability at the same time. Getting at least three quotes from MCS-certified installers is the standard approach.