Expanding Your Solar Array

We are often asked: Can we expand our system over time?

Yes, you can.  By installing a solar array in several phases (usually two, but sometimes more) a buyer can enjoy the benefits of solar now for a lower upfront cost, and then expand the system as funds become available later.

Some homeowners and businesses look to expand their systems over time for different reasons.  Some people want to take advantage of current incentives (Read more about Washington State incentives for solar energy here.)  Other people decide to break up their installations in hopes to install the next generation of equipment in the future.  There is a third group of people that want to expand their system as their energy needs grow (growing family, new EV, etc.)

No matter the reasons, if you are thinking about installing your system in more than one phase, you need to consider the following:

  1. Current incentives may not exist in the future or at a reduced rate;
  2. The cost to install a second phase may actually increase overall costs due to paying for additional trip fees and altering the initial installation to accommodate the new design;
  3. Solar technology may change in such a way that the newer phase may not match aesthetically or electrically with the first phases of the system.

You’ve thought it through and, despite the hurdles above, installing in multiple phases still seems like the best route for you and your family.  First and foremost, let us, your solar integrator; know your plan up front so we can plan accordingly.  We are going to take the following factors into consideration when designing the second phase of your system:

  1. Available roof size and shading;
  2. Capacity of your current electrical infrastructure to accept more input;
  3. Budget;
  4. Availability of matching products.

Available roof size and shading:

First (and obviously?) you are going to need additional space on your roof to install more solar panels.  Trees may have grown or gone away since your first phase, so as your installer we may need to do additional shade analysis.

Electrical Infrastructure

Your jurisdiction has the final say on how you can connect your system to the grid.  We work in several utility jurisdictions (Seattle City Light, Puget Sound Energy, Snohomish PUD,  TPU, and a few others) and cities that all have different rules as to how we can interconnect.

The most common interconnection point is in your main service panel.  The National Electric Code (NEC) has rules as to how much solar is allowed to back feed your panel.  If you’ve got more solar installed than your service panel can handle, then we can look at doing a “line-side tap” (also known as a “supply-side tap”).  We will need to plan for the ultimate capacity of your roof to know whether or make their connections in the service panel or with a line-side tap.


Installing the second phase can be more expensive than maximizing the available space from the get-go.  The reason is that you’ll incur another round of trip fees, pricing may have changed, and incentives may no longer be available.  If you’re lucky, equipment prices have dropped enough to cover the additional costs.  If the first phase was poorly planned, or technology has radically changed, there may be additional fees to change or rearrange the solar balance of system components.


There are a thousand ways to skin The Solar Cat.  You can use AC Modules, String Inverters, Micro-Inverters, Battery Based Inverters, Power Optimizers, big strings, small strings, and on and on.  Your roof size, shading, electrical infrastructure, and budget will all affect which components are used together.   The timeline of your expansion could also change your design.

The first expansion we ever installed was 3 years after the original installation.  The original modules were no longer available; in fact, nothing in the same physical size was available let alone volts, amps, and watts.  In this case, we were lucky enough to get something very close so the aesthetics did not change much.  But if our client had waited much longer, he might not have been so lucky.

Two ways I approach thinking about an expanding a PV array.

First and easiest is to think of your different phases as two totally separate installations.  In the end you’d have two arrays, maybe two (or more) different inverters, and the output would be combined before getting to the service panel.  In this scenario I might oversize some of the conduit and other balance of system components to be able to accept the next phase to save some cost and time later.

The second approach would be to rely on micro-inverter technology.  In this set up it can be easier to have a several smaller expansions as well as to be able to better handle changes in module and inverter technology.

The approach that I would not take (again) is to oversize a string inverter hoping to add additional panels to it later.  It can be extremely difficult if not impossible to find modules that match the electrical characteristics of your original installation closely enough to be able to interact correctly with the old array.  While solar does not follow Moore’s Law, the technology is changing rapidly enough to mean that last year’s module or inverter might have changed just enough to complicate your installation.

This is by no means an exhaustive study of what it takes to expand your solar array over time.  It is my hope that I have armed you with enough information to be able to have a competent conversation with a professional solar integrator if you plan to add to your array later.