Awaiting expert review. This page covers rules that vary by utility and jurisdiction. It is kept out of search results until a qualified reviewer has checked it; the sources below are dated so you can verify each point.
Quick answer
Electrical Panel An electrical panel, also called the main service panel, breaker box or load center, is the metal cabinet where utility power enters a home and is split into branch circuits, each protected by a circuit breaker. For solar, it is usually where the inverter's output connects to the house.
Its busbar rating, the amperage of the copper bars inside, limits how much solar can be fed through a breaker, which is why some solar quotes include a panel upgrade.
Quick facts
The key facts about electrical panel, with sources:
- Also called
- Main service panel, breaker box, load center
- Key ratings
- Busbar rating and main breaker rating, in amps (on the panel label)
- Formula
- Main breaker + 125% of inverter output current ≤ 120% of busbar rating 1
- Breaker position
- Solar breaker at the opposite end of the busbar from the main 1
- South Carolina
- 2020 NEC in force since 1 January 2023 4
Key takeaways
- Your electrical panel is where grid power enters the home and splits into circuits.
- Two numbers on the door label matter for solar: the busbar rating and the main breaker rating.
- The NEC "120% rule" caps how much solar can feed through a breaker on the busbar.
- A 200-amp panel with a 200-amp main often caps that kind of connection near 7.6 kW AC.
- If the panel is too small, options include a derate, a supply-side connection or an upgrade.
- All panel work is licensed-electrician work under a permit.
What is inside the panel
The utility's service wires land on a main breaker at the top of the panel. A breaker is a switch that trips off when too much current flows. From there, two copper busbars run down the cabinet. Each branch breaker clips onto them.
The label inside the door lists the busbar rating and the main breaker rating in amps. On many homes both are 100, 150 or 200 amps. They are not always equal, and that difference matters for solar.
A grid-tied inverter usually connects through its own breaker on the same busbars. That breaker is a second source of current. If both the main and the solar breaker push current in, the bars could carry more than their rating with neither breaker tripping. The code's connection rules exist to stop that.
How solar power flows through the panel
- Solar panels → inverter makes AC power
- AC → solar disconnect → a breaker in the electrical panel
- Busbars → branch breakers → your lights, appliances and outlets
- Extra power → back through the main and the meter → the grid
The 120% rule in plain English
NEC 705.12(B) gives several ways to connect a power source on the house side of the main. The best-known one is the 120% rule.
It works like this. Take the main breaker rating. Add 125% of the inverter's rated output current. The total must be no more than 120% of the busbar rating. The solar breaker must also sit at the opposite end of the bus from the main.
Why opposite ends? Current from each source then flows in from a different end. It never piles up beyond the bar's limit at one point.
The 125% factor is there because inverter output counts as a continuous load, one that runs for three hours or more. The 2023 edition split section 705.12 into two parts, feeders and busbars, so the subsection number differs between editions 2.
Worked example: how much solar a panel accepts
Illustration only, using the formula above and round panel ratings. The 100 A row matches the example in NYSERDA's inspection fact sheet 1. An electrician confirms the numbers from your actual label and inverter data sheet.
| Panel | 120% of busbar | Minus main breaker | Room for 125% of solar current | Max inverter output current |
|---|---|---|---|---|
| 200 A busbar, 200 A main | 240 A | 240 − 200 | 40 A | 40 ÷ 1.25 = 32 A |
| 225 A busbar, 200 A main | 270 A | 270 − 200 | 70 A | 70 ÷ 1.25 = 56 A |
| 100 A busbar, 100 A main | 120 A | 120 − 100 | 20 A | 20 ÷ 1.25 = 16 A |
At 240 volts, 32 A of continuous inverter output is about 7.7 kW AC; 16 A is about 3.8 kW AC. That is why a 200-amp panel with a 200-amp main often caps a load-side connection near 7.6 kW of inverter capacity, and why a 100-amp panel is a common reason for an upgrade quote.
Types of panels you may hear about
| Type | What it is | Why it matters for solar |
|---|---|---|
| Main service panel | Receives utility power and holds the main breaker | The usual place solar connects |
| Meter-main combo | Meter socket and main breaker in one outdoor box | Common on newer homes; may offer a supply-side spot |
| Subpanel | Fed from a breaker in the main panel; serves a group of circuits | Solar can sometimes connect here, with its own math |
| Critical-load panel | A subpanel for the circuits a battery keeps on in an outage | Needed for many partial-home backup setups |
| Smart panel | A panel with breakers you can monitor or switch from an app | Can help manage loads with a battery; check listing and compatibility |
Where the panel shows up on a solar quote
- A line item for "MPU" (main panel upgrade), "derate" or "supply-side tap".
- The connection method on the one-line diagram in the plan set.
- The busbar and main breaker ratings the designer used. They should match your panel label.
- Any battery or critical-load subpanel, which adds its own connection.
- Panel overcurrent sizing is one of the most common critical faults NYSERDA's inspectors record on New York load-side connections 1. Check this part of the plan.
Benefits and limits of a load-side panel connection
What it does well
- Often the simplest, least costly way to connect solar.
- Uses the panel you already have when the math works.
- Keeps solar on its own clearly labeled breaker.
- An upgrade, if needed, can also make room for an EV charger or heat pump.
Where it falls short
- The busbar rating can cap system size.
- Older or crowded panels may have no free spaces.
- Adding a battery or EV charger later can change the math.
- A panel upgrade adds cost, a permit and sometimes a utility visit.
When the panel is too small: the usual options
| Option | What changes | Trade-off |
|---|---|---|
| Main breaker derate | A smaller main breaker frees room on the bus | Only works if a load calculation shows the house can live on the smaller main |
| Supply-side (line-side) connection | Solar connects ahead of the main breaker, under NEC 705.11 | Needs utility and inspector acceptance; done by an electrician |
| Smaller inverter or fewer panels | Lower output current fits the existing bus | Less production |
| Panel upgrade | A new panel with a higher busbar rating | Adds cost and a separate permit; may involve the utility |
What drives the cost of panel work
We do not publish a price for panel upgrades. Costs swing widely by home and region, and we only print figures we can source.
The drivers: whether the meter or service wires must move, the panel size, how many circuits need moving, utility fees and wait times, and permit fees. A derate or a supply-side tap usually needs less material than a full new panel, but it is not always allowed.
Tax help has changed. The federal Energy Efficient Home Improvement Credit (25C) covered qualifying panelboards of 200 amps or more, up to a per-item cap.
It expired: the IRS says it applies only to property placed in service before 31 December 2025 3. Do not count on it for new work. See cost by system size for how equipment scales.
Sources: [3]
How a panel is checked and connected for solar
- The installer photographs the panel label, breakers and meter during the site visit.
- The designer runs the 120% math, or picks another NEC method, for the planned inverter.
- If the panel is short, the electrician proposes a derate, supply-side tap or upgrade.
- The plan goes to the building department with the solar permit. A panel upgrade may need its own permit.
- If the service must be cut, the utility schedules a disconnect and reconnect.
- A licensed electrician makes the connection and labels the solar breaker.
- The inspector checks it, then the utility grants permission to operate.
Maintenance and lifespan
A panel needs little care from you. Keep 3 feet clear in front of it, keep the door shut, and keep the circuit directory up to date.
Panel life varies with brand, climate and use. There is no single number we can source. An electrician can judge yours from its make, age and condition.
If you add a big load later, such as an EV charger, ask an electrician to redo the load calculation. It can affect the room left for solar. Our EV charger installation page covers that pairing.
Warning signs and when to call an electrician
- A breaker that trips again and again, or will not reset.
- A warm panel door, a burning smell or scorch marks.
- Buzzing or crackling from the panel.
- Rust or water stains inside or around the cabinet.
- Lights that dim when a big appliance starts.
- A solar breaker that keeps tripping on sunny days.
Safety rule
Never remove the inner cover of your panel. The parts behind it stay live even with the main off. Read ratings from the door label and call a licensed electrician for anything else.
The rules that apply
The National Electrical Code (NFPA 70) sets the connection rules. Article 705 covers power sources connected to the grid. Section 705.12 covers load-side connections, the 120% rule among them. Section 705.11 covers supply-side connections.
Your state or city adopts an edition, and inspectors enforce that edition. The 2023 edition renumbered parts of 705.12, so ask which wording applies.
Your utility adds its own rules in the interconnection process, such as meter changes and who may cut the service. Read our interconnection guide for how that works.
Electrical panel vs related equipment
| Equipment | What it does | Solar role |
|---|---|---|
| Electrical panel | Splits grid power into protected circuits | Where solar usually connects |
| Utility meter | Measures power in and out | Records exports for billing |
| Solar disconnect | Isolates the solar system | Safety switch between inverter and panel |
| Rapid shutdown switch | Lowers roof wiring voltage | Firefighter safety |
| Critical-load subpanel | Feeds backed-up circuits | Used with batteries |
Common misunderstandings
- Myth A 200-amp panel can take any size of solar.
- Reality The 120% rule often caps a load-side connection near 7.6 kW AC on a 200 A bus with a 200 A main.
- Myth The main breaker rating is the busbar rating.
- Reality They can differ. The busbar rating is listed separately on the label.
- Myth Every solar job needs a new panel.
- Reality Many do not. A derate or supply-side connection can avoid one when allowed.
- Myth Solar adds to my panel's load.
- Reality Solar is a source, not a load. The issue is extra current on the busbars, not more usage.
In South Carolina, Georgia and Virginia
South Carolina enforces the 2020 NEC statewide, effective 1 January 2023 4.
The Building Codes Council has moved to adopt the 2023 NEC, which renumbers parts of 705.12; a 2026 joint resolution (S.1077, approving Regulation Document 5418) would approve it.
As of 16 April 2026 it was back in the Senate Labor, Commerce and Industry Committee, and it takes effect only when the Governor approves it 5.
Panel work for a solar connection is wiring work: a residential builder, a licensed residential electrician, or a mechanical contractor with the electrical classification 6. In Georgia and Virginia, ask your local building department which edition applies.
When it matters to you, and what to do next
Read your panel door label before you ask for quotes. Write down the busbar and main breaker ratings. Share them with each installer.
If one quote includes a panel upgrade and another does not, ask both how they connect. One may be using a supply-side tap or a derate. Neither answer is wrong by itself, but both should show the math.
Run your numbers through the tools below, then size the system with solar system sizing in mind.
Check your panel
- Panel upgrade checkRuns the 120% math on your busbar and main ratings
- Solar system size calculatorHow many kW you need before you check the panel
- Solar system upgradesAdding capacity or storage to an existing panel
Questions about electrical panel
Do I need a panel upgrade for solar?
Only if your busbar cannot take the solar current under the code and no cheaper option works. Alternatives include a supply-side connection or a derated main breaker. Many 200-amp panels accept a typical home system without an upgrade. Your installer should show the 120% math, or another NEC method, before quoting an upgrade.
Is a 100-amp panel enough for solar?
Usually only for a small system. On a 100-amp bus with a 100-amp main, the 120% rule leaves room for about 16 amps of inverter output, roughly 3.8 kW AC. Larger systems usually need a supply-side connection, a derate or a new panel. An electrician confirms what fits from your label and inverter data sheet.
Where do I find my busbar rating?
It is on the label inside the panel door, often listed as the bus or panel rating in amps. It can differ from the main breaker rating, which is printed on the main breaker handle. Do not remove the inner cover to look. If the label is missing, an electrician can identify the panel model.
What is the difference between a main panel and a subpanel?
The main panel receives utility power and holds the main breaker. A subpanel is fed from a breaker in the main panel and serves a group of circuits, such as a garage or a battery's backed-up loads. Solar can connect to either, but each has its own busbar math.
Can I install the solar breaker myself?
No, connecting a power source to a panel is licensed electrical work. It needs a permit and an inspection before the utility lets the system run. In South Carolina, wiring must be done by a residential builder, licensed residential electrician or contractor with the electrical classification.
What is a supply-side connection?
It is a connection made ahead of the main breaker, between the meter and the main. It sidesteps the busbar limit, so it can allow a larger system without a new panel. NEC 705.11 governs it. It needs utility and inspector acceptance, and it is done by a licensed electrician.
What does derating the main breaker mean?
Derating means swapping the main breaker for a smaller one, such as 200 amps down to 175. That frees room on the busbar under the 120% rule. It only works if a load calculation shows the house can run on the smaller main without nuisance trips. An electrician does the calculation.
Is there still a tax credit for a panel upgrade?
Not under 25C for new work. The IRS says the Energy Efficient Home Improvement Credit, which covered qualifying 200-amp-plus panelboards, expired. It applies only to property placed in service before 31 December 2025. State or utility programs may differ. Ask a tax professional about your own case.
Does a battery change my panel needs?
It can, because a battery is another source feeding the panel. It may need its own breaker, a critical-load subpanel or a different connection method. The installer reruns the busbar math with the battery inverter included. Plan both together if you think you will add storage later.
Sources
- NYSERDA NY-Sun Quality and Market Standards, The 120% Rule fact sheet (QMS-SUN-120-fs-1-v1, March 2025), retrieved .
- IAEI Magazine, 2023 National Electrical Code and photovoltaic power systems, retrieved .
- IRS, Energy Efficient Home Improvement Credit, retrieved .
- IAEI Magazine, Code updates: South Carolina adopts the 2020 NEC, effective 1 January 2023, retrieved .
- South Carolina General Assembly, S.1077 (2025–2026), joint resolution approving Regulation Document 5418 (2023 NEC); recommitted to Senate committee 16 Apr 2026, retrieved .
- SC LLR, Residential Specialty Contractor registration (Solar Panel Installer) application package, retrieved .
Expert review
Written by the HyreSolar Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: The 120% formula and breaker-placement condition are from NYSERDA's inspection fact sheet on NEC 705.12(B); the worked example applies that formula to round ratings and nothing else. SC edition and licensing are from IAEI and the HyreSolar SC fact pack. Awaiting review by a licensed electrician (G-REV).
Suggest a correction. We fix errors and say what changed.