There are many factors to consider in upgrading a system. Panel size, inverter capacity, and space are going to be the biggest deciders.
As far as possible, stick with the same manufacturer. But since brands come and go, it isn’t unheard of to mix and match components – as long as the voltage is compatible and can be balanced it can generally work. Not the best scenario, but if options are limited as far as what’s available, it is possible. However, if the panels are too unbalanced, they will only perform as good as the lowest wattage panels.
For example: Say you had a system installed 5 years ago comprised of 200W panels and you are looking to now increase the size of the array. If you connected 330W panels to the 200W panels’ string, your 330W panels would only produce up to 200W.
Also, inverters can only handle so much power. Since your system was sized accordingly when it was designed, the inverter may be limited in the amount it can process. Sometimes inverters are installed that can handle additional panels (if the future needs were taken into account when the system was designed), so take a look at the size of the inverter you have installed already to see whether or not it can handle more power.
So, what to do?
For the example mentioned, if there is ample space, one option would be to install a separate array, complete with a separate inverter if the current inverter is too small. The power produced by both systems will be combined in your main AC power panel, or in your battery bank if you are connected to storage batteries instead of the main grid.
If you have not had a system installed yet and are just trying to plan ahead for the future, there are a few key things to make note of as far as adding solar panels:
1. Choose a manufacturer that’s been established and will be around for a while, even if it might cost a little more initially. Don’t just try to look for the cheapest deal or you may end up paying more in the long run.
2. Discuss the future with your consultant. Let them know your concerns and the possibility of wanting to add to your system later. That way, you can design a system that will accommodate future growth.
3. As soon as you know that you know your electrical needs will be increasing (i.e. you’re installing a hot tub, planning to have children, etc.), get your system expanded. The longer you wait, the more you risk not being able to upgrade with compatible components.
Note: If you had/have panels installed with microinverters, the future add-on issues will be greatly reduced. Since each panel has its own inverter in the string, adding more panels is more of a space consideration and additional wiring. Microinverters are initially a little more expensive but make up for it in cases like this.