HyreSolar tools
Screen a proposed inverter against your existing panel before you are quoted for a panel you may not need.
A main service panel upgrade is one of the largest line items that can appear in a solar quote, and one of the least examined. Whether you need one turns on arithmetic printed on your own panel label. This screens that arithmetic so you can ask an informed question — it is not a design, and nothing here is a reason to open a panel.
What this returns for the most common residential panel
On the most common residential panel — a 200 A busbar with a 200 A main — the NEC 705.12 120% rule allows a 40 A backfeed breaker, which supports an inverter of about 7.7 kW. An 11 kW inverter needs a 60 A breaker and comes up 20 A short. But derating that same main from 200 A to 150 A raises the ceiling to 17.3 kW on the identical panel — often a far cheaper route than replacing it.
Last updated . Data as of 5 September 2026.
Three numbers off your panel label and your proposal. A screening check, not a design.
Screens NEC 705.12 arithmetic only. Not a design, not a permit, not electrical advice.
A licensed electrician confirms the label, the load calculation and the enforced code edition. Never open a service panel to gather these numbers.
"Plausible fit" means the arithmetic clears, not that the job is approved. A licensed electrician still has to confirm the busbar rating on the label, the physical space for the breaker, the condition of the panel, whether the manufacturer permits a backfeed breaker in that position, and which NEC edition your jurisdiction actually enforces. This tool checks one gate of several.
"Likely needs a change" does not mean you need a new panel. It means the inverter as proposed does not fit as proposed. There are at least four ways out before replacement: derate the main breaker, use a supply-side connection, use a listed power control system, or fit a smaller inverter. A quote that jumps straight to a full service upgrade has skipped that list.
The single most useful output is the maximum inverter size. If a proposal specifies an inverter above your ceiling, either the installer is planning work you have not been told about, or the design needs revisiting. Either way it is a question worth asking before signing.
Every input below is a number you can find, not one you have to guess. This is where each one comes from.
This is the rating of the metal bar the breakers clip onto, and it is the figure the rule is about. It is not always the same as the main breaker rating.
Where to find it The printed label inside the panel door, visible without removing anything. Look for a bus or busbar rating in amps. If the label is missing or unreadable, stop and ask an electrician — do not guess and do not remove the cover.
The number stamped on the main disconnect handle. This is what the utility supply is limited to, and it is often but not always equal to the busbar rating.
Where to find it Stamped on the largest breaker in the panel, usually at the top. A common combination is a 200 A main on a 200 A busbar; another is a 200 A busbar with a 175 A main.
The AC output rating, not the DC array size. A proposal listing a 12 kW system may pair it with a 9.6 kW inverter, and it is the inverter figure the code cares about.
Where to find it The equipment page of your proposal, under inverter. If only a model number is given, the AC output is on the manufacturer datasheet.
Residential single-phase service in the United States is almost always 240 V, which is the default. Change it only if you know your service differs.
Where to find it The panel label. Leave the default unless you have a specific reason.
The tool reports the 120% rule and the sum rule separately because installers choose between them and they give different answers. The 120% rule requires the backfeed breaker at the opposite end of the busbar from the main.
Where to find it Nowhere on your paperwork — this is the calculation itself. If a proposal relies on the 120% rule, the physical position of the breaker matters and is an installation detail worth confirming.
That is the entire purpose of the output. It converts a vague worry into a specific question with numbers attached.
Where to find it Your installer, or an independent licensed electrician if you want a second view on a quoted upgrade.
Inverter continuous output current × 1.25, rounded up to the next standard overcurrent device size. This is the breaker the solar needs.
Busbar rating × 1.2, minus the main breaker rating. Valid where the backfeed breaker sits at the opposite end of the busbar from the main.
Busbar rating minus the main breaker rating. Valid in any position, and frequently zero on a panel whose main equals its busbar.
The required breaker against the better of the two allowances, plus the largest inverter that would fit.
The formula, in full
backfeed breaker = next standard size ≥ (inverter kW × 1000 ÷ volts) × 1.25. 120% allowance = busbar × 1.2 − main. Sum allowance = busbar − main. Fits if backfeed breaker ≤ the larger allowance.
A homeowner is quoted an 11 kW inverter on a house with the most common residential panel in America: a 200 A busbar with a 200 A main. The quote includes a $4,000 service panel upgrade, described as required.
Inputs
Result
20 A short — but not necessarily a new panel
As designed it does not fit: the solar needs a 60 A breaker and the panel allows 40 A. That much of the quote is honest. What the quote does not say is that derating the main breaker from 200 A to 150 A raises the allowance to 90 A, which accommodates the 60 A breaker with 30 A to spare and would support an inverter up to 17.3 kW. Whether a 150 A main is adequate depends on the house’s actual load calculation, which an electrician performs — but it is a question that should have been asked before a $4,000 line item was added.
A 7.6 kW inverter against different panels, and the 11 kW case. Every figure is computed by the calculator on this page.
| Panel and inverter | Breaker needed | 120% allows | Sum allows | Verdict | Largest inverter |
|---|---|---|---|---|---|
| Base case200 A bus, 200 A main, 7.6 kW | 40 A | 40 A | 0 A | Plausible fit | 7.7 kW |
| 11 kW inverter instead of 7.6 | 60 A | 40 A | 0 A | Likely needs a change | 7.7 kW |
| Main derated to 175 A | 40 A | 65 A | 25 A | Plausible fit | 11.5 kW |
| Main derated to 150 A | 40 A | 90 A | 50 A | Plausible fit | 17.3 kW |
| 150 A busbar, 150 A main | 40 A | 30 A | 0 A | Likely needs a change | 5.8 kW |
| 125 A busbar, 125 A main | 40 A | 25 A | 0 A | Likely needs a change | 4.8 kW |
| 100 A busbar, 100 A main | 40 A | 20 A | 0 A | Likely needs a change | 3.8 kW |
| 225 A busbar, 200 A main | 40 A | 70 A | 25 A | Plausible fit | 13.4 kW |
| 400 A busbar, 400 A main, 11 kW | 60 A | 80 A | 0 A | Plausible fit | 15.4 kW |
Read rows three and four against row two. The panel does not change; only the main breaker does. Derating a 200 A main to 150 A takes the ceiling from 7.7 kW to 17.3 kW on the same busbar — a 124% increase in allowable inverter size from a breaker swap rather than a service upgrade. Note also that the sum rule returns 0 A whenever the main equals the busbar, which is why the 120% rule and the position of the backfeed breaker matter so much on ordinary panels.
Ranked. A proposal can change any of these without saying anything untrue, so these are the inputs to check first.
The whole game. A 200 A busbar with a 200 A main has 40 A of 120% headroom; the same busbar with a 175 A main has 65 A. Panels are frequently supplied with a main equal to the bus, which is the least generous configuration possible.
The cheapest lever available, and the one most often skipped in a quote. It depends entirely on the house’s calculated load — an electrician performs that calculation and it is not something to estimate from a bill.
Not the array DC size. A 12 kW array on a 9.6 kW inverter is a 9.6 kW problem for this rule, which is one reason inverters are commonly undersized relative to arrays.
The rule has moved subsection across code cycles — 705.12(D)(2) in 2014, 705.12(B)(2)(3)(b) in 2017, 705.12(B)(3)(2) in 2020, 705.12(B)(2) in 2023 — and the available alternatives changed with it. Listed power control systems, for instance, arrived with the 2020 cycle.
The dwelling load calculation, physical breaker space, panel condition and age, manufacturer listing for backfeed position, busbar de-rating for other sources, batteries, and any supply-side option. Each of those can independently decide the answer and none of them can be screened from three numbers.
They are frequently equal and sometimes not, and the rule is about the busbar. Using the main rating for both will usually understate your headroom. Read the label rather than inferring.
A quote headlined "12 kW" may carry a 9.6 kW inverter. The code cares about what the inverter can push into the busbar, not what the panels can produce on a cold clear day.
It is the single most valuable question this tool enables. A main breaker swap and a service upgrade are different orders of cost, and the scenario table shows the derate frequently solves the problem outright.
Clearing the arithmetic is one gate. Breaker space, panel condition, manufacturer listing and the local inspector are others, and any one of them can still require work.
The busbar rating is on the label inside the door and is readable without removing the dead front. Do not remove the cover. Bus bars are live even with the main off in most residential panels, and this is the point at which a screening tool stops and an electrician starts.
An EV charger, a heat pump or a battery all compete for the same headroom. If any of those are plausible within a few years, size the conversation for them now rather than solving for solar alone. See solar and EV charging together.
NEC 705.12 limits the total current that can feed a panelboard busbar from the utility and a solar system together. Where the solar backfeed breaker is placed at the opposite end of the busbar from the main, that total may reach 120% of the busbar rating. On a 200 A busbar with a 200 A main, that permits a 40 A solar breaker — which supports an inverter of roughly 7.7 kW.
Both happen, and this tool exists to tell them apart. If the arithmetic says you are short, the constraint is real. What is often missing from the quote is the list of alternatives — derating the main, a supply-side connection, a listed power control system, or a smaller inverter — any of which may cost far less than a service upgrade. Ask which were considered and why they were rejected, in writing.
The 120% allowance is the busbar rating times 1.2 minus the main. Lowering the main raises the allowance directly. Going from a 200 A main to a 150 A main on a 200 A busbar takes the allowance from 40 A to 90 A, and the largest supportable inverter from 7.7 kW to 17.3 kW. Whether your house can run on a 150 A main is decided by a load calculation an electrician performs — it is not something to assume.
Because it is busbar minus main, and on a great many residential panels the main equals the busbar. That is exactly why the 120% rule exists and why the physical position of the backfeed breaker matters: on a 200/200 panel the sum rule offers nothing and the 120% rule offers 40 A.
The inverter AC output. Arrays are routinely oversized relative to their inverters, so a proposal headlined 12 kW DC might carry a 9.6 kW inverter, and it is the inverter that determines the backfeed current. If your proposal only gives a model number, the AC output is on the manufacturer datasheet.
You can read it, and you should not go looking for it. The rating is printed on the label inside the panel door and is visible without removing anything. Do not remove the dead front cover to find it — busbars remain energised with the main switched off in most residential panels. If the label is missing or illegible, that is a question for an electrician, not a reason to open the panel.
Yes, and this tool does not model it. A battery is an additional power source with its own interconnection requirements, and it frequently arrives with a critical load subpanel that restructures the whole question. Treat a passing result here as applying to the array alone.
The rule exists across cycles but has moved subsection and gained alternatives over time — 705.12(D)(2) in 2014, 705.12(B)(2)(3)(b) in 2017, 705.12(B)(3)(2) in 2020, 705.12(B)(2) in 2023. Listed power control systems became available with the 2020 cycle, so a 2014-edition jurisdiction has fewer routes. The arithmetic in this tool is stable across those editions; the options when it fails are not.
Possibly both. This screens one rule from three numbers. The installer may be responding to breaker space, panel condition, an obsolete panel type, a manufacturer listing that forbids a backfeed breaker in the required position, or a load calculation you have not seen. Ask them which it is — a specific answer is easy for a competent installer to give and hard to invent.
No. An EV charger is a load and this rule governs sources. The two do compete for the same panel, and a house planning both should be assessed for both at once, but the arithmetic is different. Our solar and EV charging piece covers where they interact.
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