In most instances, increasing the size of your solar array is a sensible and fairly straightforward thing to do. Having said that, adding just one or two more panels might be more trouble than it’s worth.
Current size of solar system
The size of your current system may influence how many more solar panels you plan to add. If your solar installation stays below 3.68kW (or 16 Amps) generation with the added panels, there shouldn’t be any problem.
If you plan to generate over 3.68kW with the added panels, you’ll have to submit a G59 application form to the Distribution Network Officer (DNO). You must receive permission before installation can commence. This is to determine whether the increased size of the system can operate within the existing grid framework. Your installer can help you with the application process, but it may take several months before you get a response.
Increasing the size of your solar array usually comes with upsizing the inverter. As they’re accurately sized to the solar installation, adding more panels will probably exceed the amount of electricity your inverter can process.
Since string inverters have an average lifespan of 10 years, try to coincide the installation of your extra panels with replacing your inverter to save money. This isn’t the case with micro-inverters since they are connected to individual panels, and they have a lifespan of over 25 years.
Also, string inverters reduce the power output of all the solar panels to that of the worst performing panel. If you added new 300W panels to an installation made up of 250W panels, the string inverter would reduce the maximum output of your new panels down to 250 watts. For this reason, any panels you plan to add should have the same power rating as the original panels.
Again, this issue is avoided by getting micro-inverters, as the solar panels work independently from one another.
Feed-in Tariff (FiT)
Depending on when you installed your current solar array, it’s worth noting that the FiT rates have dropped significantly since 2011 (from about 44p to just 4p per kWh from the generation tariff). Adding more panels to your installation will not increase your old FiT rates.
Solar panels weigh between 15 and 30kgs, so you’ll need to make sure that your roof can support the added weight and space.
These issues will be covered by a MCS-certified installer. You can find a qualified installer near you here.
 Sourced from Ofgem.gov.uk
Figures correct at time of writing (October 2017).