Short answer
Extending is possible if the inverter can handle the extra power and the strings fit within its voltage range. If it is at its limit, a second inverter is added or it has to be replaced. If the total goes above 3.7 kW and you are on single-phase, three-phase becomes necessary.
Who this article is for Anyone who already has panels and wants more. Assessing the inverter and the connection is a job for the installer.
The inverter first
The inverter has two limits:
These two decide whether you can extend without a second inverter.
Power. It states the maximum it can handle on the DC side. A little oversizing is allowed: panels rarely deliver their full power, and manufacturers usually allow an overshoot of ten to twenty per cent. More than that and it clips, so you throw away yield on sunny days.
Voltage range per string. Each string must stay within a window, even in frost, because that is when the voltage is highest. Adding panels to an existing string can exceed that window.
Panels of a different type
In one string the panels are in series: the weakest determines the current of the whole string. If you mix a new 430 watt panel with old 300 watt ones, you drag the whole string down.
Solutions:
- the new panels on their own string with a free input on the inverter
- optimisers per panel, so each panel keeps its own operating point
- a second small inverter for the new string
What suits best depends on your roof and on the inverter that is installed.
The roof and the mounting
Adding panels often means: the remaining spots, and there is a reason they are still free. Watch out for shade from a chimney, a dormer or a tree. One panel that is in the shade every afternoon costs more than it yields if it is in the same string.
The connection in the meter cupboard
If the total grows above 3.7 kW and you are on single-phase, three-phase is required and so is an application to the grid operator. If you stay on single-phase, bear in mind that the chance of switching off due to high grid voltage increases. See inverter cuts out.
The circuit in the consumer unit must also suit the new power. Often the existing circuit can stay, but the breaker must be checked. What is involved in the consumer unit is explained in solar panels and the consumer unit.
And check the old installation at the same time
If I am working on the installation anyway, I check the existing DC side. Panels that have been there for eight years have connectors that may be weathered, and that is exactly where it goes wrong. See solar panels and fire safety.
From my own work
A customer had ten panels and wanted six more. The inverter could just handle the power, but the new panels were of a different type. Mixing them on one string would drag down the yield of the whole string. It became a second string on the inverter's free input. The same money, but without the loss.
When to call an electrician
- You want to add panels to an existing installation
- The new panels are of a different type or power
- The total goes above 3.7 kW and you have single-phase
I check what the inverter can handle before any panels are ordered. Request a quote.
Frequently asked questions
Can I put new panels with old ones in the same string?
Preferably not. In a string the weakest panel determines the whole string, so you drag down the yield of the old panels.
How much can I overload my inverter?
Manufacturers usually allow ten to twenty per cent on the DC side. More than that and it clips on sunny days.
Do I need to register the extension again?
Yes, the registered power changes. I take care of that at handover.
Further reading
A dynamic contract and a home battery: is it worth it
With a dynamic contract the battery can charge when electricity is cheap. That works, provided you let it be controlled automatically.
The net metering scheme is ending: what changes for my panels
Your panels keep generating just as much. What changes is the value of the electricity you feed back.
Feed-in charges: why you pay for your own electricity
More and more suppliers charge for feeding your solar power back into the grid. Your bill shows what that costs you.
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