Considering solar? Read on to understand the basics of electrical services, and the four reasons that you might need to have your home or business electrical infrastructure replaced or upgraded before you have solar installed.
Power to You
Each home connected to the electric grid has wires that come from the electric company. These are called service conductors. Service conductors can be overhead (through the air) or underground. They connect to electrical equipment on or near your home or business. We call that equipment an electrical service.
Overhead service conductors at our office/warehouse. 
A portion of our electrical service.
Although each building’s electrical service may be a bit different, they will always have a utility meter, a main service disconnect, and a way to send energy to multiple electrical loads in the building. The utility meter measures energy from the grid, and any energy sent back to the grid from solar.
A typical residential 200A overhead electrical service with solar connected. 
A diagram of the service shown to the left.
A service disconnect allows you to shut off the electricity from the utility before it enters your building. In most homes, a main circuit breaker is the service disconnect. In a few cases, the service disconnect is a separate piece of equipment with a big handle and fuses inside. The main breaker or fused disconnect also protects your home from hazards. If there is a big short inside the building, the fuses or circuit breaker will automatically open, turning off electricity to the home.
A main breaker acting as service disconnect. 
A fused disconnect acting as a service disconnect.
Beyond the service disconnect, you’ll need a way to send electricity to each of the circuits in your home or business. Because the electricity is sent on small wires compared to the service conductors, you’ll need circuit breakers to make sure those wires don’t melt. This also allows you to turn off individual circuits in your home. Your electrical load center will have spots that circuit breakers snap into. They connect to the electricity from the utility at metal busbars in the middle, and the wires connect to the other side of the circuit breaker.
Circuit breakers in a electrical load center.
You can see the two busbars at the lower right.
Now you know the basics of home or business electrical infrastructure, how does this relate to installing solar?
Connecting Solar
Let’s imagine: Our crew of installers have come to your home. They’ve placed solar panels on your roof, and connected them to microinverters to change the DC electricity from the panels to AC electricity that can be used in your home. They’ve connected those microinverters to wires that run down to near your electrical service, which are brought together in a solar combiner panel. And they’ve wired the output of the combiner through a solar production meter, and to a solar disconnect. But what next?
We now need to connect the solar to your electrical service. For most homes, this is as simple as placing a double-pole, 240-volt circuit breaker at the bottom of your electrical panel. When the solar produces energy, that energy goes to the loads in your building as needed. If there’s “extra” energy, it goes back up through your service disconnect, through your utility electric meter, and to the utility. There are limits, however, on how big that circuit breaker can be—we will talk details in a later section.
A 200A all-in-one electrical service, typical for homes in southern Arizona, with a solar breaker installed at the lower left.
Reason One: Replacement for Safety
Most homes have an electrical service that’s in good shape. It’s of good quality, has been installed well, and isn’t broken or overly worn out. But sometimes, when we evaluate a home for solar, we find a problem.
Some electrical panels are simply dangerous. Either through poor manufacturing or poor design, they are unsafe, and could cause fires. These include Zinsco and Federal Pacific Electric (FPE) electrical panels. Inspectapedia has good information on Zinsco, and J. Aronstein of http://fpe-info.org/ has a detailed paper on the hazards and history of FPE panels. Installing solar in these poor-quality panels isn’t more dangerous than using the panel for your current electrical loads, but it is always our strong recommendation that these panels be replaced. Depending on the specifics of what needs to be done and our schedule, we can either refer you to a trusted electrician to replace unsafe equipment, or we can sometimes replace it as part of the solar installation process.
A Federal Pacific Electric Panel. 
The inside of a FPE panel.
Other safety issues are more site-specific. For example, perhaps a busbar has been damaged or is loose. Or maybe someone previously installed circuit breakers or other equipment in an unsafe way. Although our evaluation for solar shouldn’t be considered a full inspection of your electrical infrastructure, we’ll do our best to alert you to any potential problems.
Reason Two: Upgrade for Solar Electric System Size
For solar breaker connections in electrical load centers, the National Electrical Code (NEC) has a few rules. There are limits to the electrical “size” of the breakers, and they are required to be placed on the opposite end of the busbars from the main breaker or fused disconnect.
The size limits say that if breakers connected to a busbar are connected to a source of energy (like the grid or solar), the total rating in amps of the breakers can’t be more than 120% of the busbar rating. A typical home in Tucson has a 200A electrical service, with an all-in-one electrical panel that includes a spot for a utility meter, a 200A main circuit breaker, and 200A busbars that fit up to 40 circuit breakers.
In that case, we can have two sources of electricity—the utility grid and the solar. We can have 200A X 1.2 = 240A of breakers that have sources of energy connected. We already have a 200A main breaker connected to the grid, so we can install a solar breaker of 40A or smaller.
For most homes, a 40A breaker is plenty big enough for the desired solar electric system size. But for larger homes, that might not be big enough for the solar electric system needed to meet the homeowner’s needs. For the equipment we’re quoting as of this writing, we can fit a solar electric system of about 8,450 Watts DC or 7,540 Watts AC on a 40A breaker. If you want a bigger system, we’ll have to make changes to your electrical service. Check out the table below to see how different main breaker ratings and busbar ratings affect solar electric system size:
There’s one other way to connect a solar electric system to the grid—a supply-side connection. This connection is made between the utility meter and the service disconnect, and includes a new solar service disconnect. This can make sense for large homes, and is often used to connect solar electric systems for businesses. Siemens also makes a series of residential electrical panels that use this method, allowing you to connect up to 60A of solar to a dedicated connection point.
A Siemens solar-ready load center with a dedicated connection point.
Reason Three: Adding a Subpanel for Breaker Space
Your electrical panel is designed to supply a certain number of circuits in your building. Most modern residential load centers can supply a total of 40 circuits, but older panels may supply 30, 24, or even just a dozen circuits. Sometimes, this means that there’s just no space to add the two-pole breaker required for a solar electric system.
Fortunately, there’s a solution that’s easier than replacing the whole panel. We can add a subpanel to make more breaker spaces. A subpanel is installed very close to the existing main electrical load center. Circuit breakers for 2-4 electrical loads are removed from the main panel. That space is used to install a double-pole circuit breaker (often 40-80A), which feeds wires that supply electricity to the subpanel. The wires for the 2-4 electrical loads are extended into the subpanel, and connected to new circuit breakers in the subpanel. Depending on the exact configuration, the solar electric system breaker may be placed either in the main panel, or in the subpanel.
A diagram of a subpanel installed to make breaker space.
Replacement Reason Four: Additions for Energy Storage
Energy storage is becoming a strong consideration for solar customers. Although the economics don’t pencil out for consumers in southern Arizona, some folks want to have the independence and energy backup that energy storage brings. Because energy storage systems can’t usually handle all electrical loads in the home without becoming excessively expensive, we have to move electrical loads into a dedicated electrical panel. We might move lights, plugs, and maybe a window AC unit to this dedicated panel, leaving standard air conditioners, electric stoves, electric dryers, and electric heaters in the main panel. When the grid goes out, the loads in the dedicated panel will be energized by the energy storage system, while the loads in the main panel won’t have electricity.
For this configuration, it’s likely that the main electrical panel won’t need to be upgraded, but adding a dedicated loads panel and moving the electrical wiring into that panel can be quite a task, often requiring a lot of work, as well as being more invasive. For example, it may require removal and replacement of drywall to gain access to wiring.
A solar electric system with a dedicated electrical panel for generator backup.
Understand Your Options
We know this is a lot of information, and it’s rather complex, but I hope this has given you some insight into the reasons a solar installer might recommend changes to your electrical infrastructure.
The bottom line is we want you to be informed, and we’re more than happy to walk you through the specific options for your home or business. The good news is that most homeowners don’t need an electric panel replacement before going solar.
Before spending money on electrical upgrades as part of your solar electric system installation, make sure your solar installer can clearly explain why they are needed, detail the capabilities of the electrical service after the upgrade, and tell you what specific equipment that will be used.
Have questions? Please ask them below. We’d love to hear your thoughts.
