Extending an array: what the controller will accept

Extension is where a working installation most often gets quietly worse, and always for the same handful of reasons.

Start at the controller, not the roof

The controller has a maximum input voltage, a maximum input current and a maximum array wattage, and all three are published. The plan has to satisfy every one of them in the coldest conditions the site will see, because open-circuit voltage rises as the cells get colder.

Decide whether the new panels join or stand apart

Adding to an existing string means the new module shares a current with modules it may not match. A separate string on a second controller input, or a second controller, keeps the groups independent and is frequently the cheaper answer once a mismatch is accounted for.

Check the cable that is already there

An array that doubles doubles its current if the extension is in parallel. The existing run, its connectors and its fuse were sized for the old array, and none of them announce that they are now marginal.

Re-survey the shade

The new panel goes where there was room, which is often where there was shade. A module that spends the morning behind a vent contributes little and, in series, can cost more than it brings.

Then confirm it worked

The daily energy-in figure before and after, over comparable days. It is the only evidence that the extension did what it was bought to do, and occasionally it shows that it did not.