The 20% rule for solar is shorthand for the 120% rule in the National Electrical Code. It limits how much solar can connect to your home’s main electrical panel through a standard breaker. On a typical 200 amp panel, that usually leaves room for a 40 amp solar breaker. It doesn’t limit your home to 20% solar power.
If you’re getting solar quotes in Tucson and an installer mentions your panel, this rule is usually why. At Solar Pros AZ, we check your electrical panel as part of every solar evaluation, so you know upfront whether your system will fit your current panel or needs a different approach. Our guide to solar panel considerations in Arizona covers the panel alongside the other factors we look at.
Why is it called the 20% rule?
Your electrical panel has a metal bar inside called the busbar, which distributes electricity to all your breakers. Every busbar has a rating for how much current it can safely carry.
Normally, electricity flows into the busbar from one direction, through your main breaker from the utility. Solar adds a second source feeding in from the other side. The 120% rule allows the combined sources to add up to 120% of the busbar’s rating, as long as certain conditions are met. That extra 20% of headroom is where the nickname comes from.
How does the 120% rule work on a 200 amp panel?
Here’s the calculation for a common Tucson setup, a 200 amp busbar with a 200 amp main breaker:
Step 1: 200 amp busbar × 120% = 240 amps allowed
Step 2: 240 amps − 200 amp main breaker = 40 amps left for the solar breaker
Step 3: Solar inverters are treated as a continuous load, so the code uses 125% of their output. A 40 amp breaker supports an inverter output of up to 32 amps (40 ÷ 1.25).
Step 4: At 240 volts, 32 amps works out to about 7.68 kW of AC inverter output.
There’s one more condition that often gets left out. For the 120% rule to apply, the solar breaker has to sit at the opposite end of the busbar from the main breaker. That placement keeps current from piling up in one section of the bar. If your panel layout doesn’t allow it, a different method is needed.
Does the 20% rule mean you can only install a small solar system?
No. The 120% rule applies to one specific way of connecting solar, a backfed breaker in your main panel. If your system needs more capacity than that allows, there are several common solutions.
Your panel may already have extra room. Some panels have a busbar rated higher than the main breaker, such as a 225 amp busbar with a 200 amp main. That combination leaves room for a 70 amp solar breaker instead of 40.
The main breaker can sometimes be downsized. Swapping a 200 amp main breaker for a 175 amp breaker opens up 65 amps for solar. This only works if a load calculation shows your home doesn’t need the full 200 amps, which can be a tough fit for Tucson homes with large AC systems.
A supply-side connection skips the busbar entirely. Solar can connect between the utility meter and the main breaker, so the 120% rule doesn’t apply. This method has its own code and utility requirements.
A panel or service upgrade adds capacity. Replacing the panel with one that has a larger busbar, or upgrading the electrical service, gives the system more room. This is often the right call for older homes with 100 or 125 amp panels.
Power control systems can manage the current. Newer code editions allow listed equipment that electronically limits how much current flows to the busbar, which can let a larger system connect without a panel upgrade.
The right option depends on your panel, your system size and your home’s electrical needs.
Can you install solar without upgrading a 200 amp panel?
Often, yes. Many Tucson homes with a 200 amp panel can connect a typical residential system without an upgrade, especially when the busbar rating and panel layout work in their favor. Older panels, panels with no open breaker spaces, or homes planning a larger system are more likely to need changes.
An evaluation of your actual panel is the only way to know for sure. Two panels with the same main breaker rating can have very different busbar ratings and layouts inside.
Does the 20% rule apply to solar batteries?
It can. Many battery systems also send power back through your panel, so they have to be accounted for alongside the solar. Some battery setups use power control equipment that manages how much current reaches the busbar, which changes the math. That’s why solar and battery storage should be designed together rather than adding a battery later without reviewing the panel.
What electrical code applies to solar in Tucson?
The City of Tucson moved to the 2023 National Electrical Code for permit applications received on or after January 1, 2026. Arizona leaves code adoption to each city and county, so homes in surrounding areas may be under a different code edition depending on who issues the permit.
On top of local permits, solar systems in TEP’s service area also have to meet TEP’s interconnection requirements before the system can be turned on. As your installer, we handle the permitting, inspections and TEP interconnection paperwork as part of the project.
Is the 20% rule related to solar tax credits?
No. The 20% rule is an electrical safety requirement and has nothing to do with incentives. It’s worth mentioning because many older articles still describe the 30% federal residential solar tax credit, which ended for systems installed after December 31, 2025.
The main incentive for Arizona homeowners today is the Arizona Credit for Solar Energy Devices, worth 25% of system cost up to $1,000 for homeowners who own their system. Talk with a tax professional about how it applies to your situation.
Frequently Asked Questions
Is the 20% rule a law?
It comes from the National Electrical Code, which cities and counties adopt as their local building code. Once adopted, it’s a requirement your installation has to meet to pass inspection.
Does a 200 amp panel limit solar to 40 amps?
Only when the busbar and main breaker are both rated at 200 amps and the system connects through a standard backfed breaker. A higher busbar rating, a downsized main breaker or a supply-side connection can all allow more.
How do I find my panel’s busbar rating?
It’s usually printed on the label inside the panel door. Main breaker and busbar ratings are often different, so check for both. Your installer will confirm this during the evaluation, and we recommend leaving panel work to a licensed professional.
How much does a panel upgrade for solar cost?
It varies widely depending on whether you need a new panel, a service upgrade or a utility meter change. Your solar quote should spell out any electrical work needed so there are no surprises later.
Is Your Electrical Panel Ready for Solar?
The 20% rule sounds technical, but the takeaway is simple: your panel needs to safely handle power coming from both TEP and your solar system. Solar Pros AZ is a licensed, bonded and insured Tucson solar company, and every free evaluation includes a look at your electrical panel so the design fits your home from the start. Call 520-549-6200 or request your free evaluation. You can also learn more about our residential solar installations.