Grid & off-grid

Grid export cap calculator

Utilities commonly cap how much a solar system can export relative to your electrical panel's busbar rating. Enter your exact panel and inverter specs to check against the applicable rule.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your inverter and system
The export cap applies to your inverter's continuous AC output, not your panel's DC rating. The calculator sizes the solar breaker at 125% of that output current.
A supply-side connection bypasses the panel busbar limit entirely, though it adds installation complexity and cost.
3 Your electrical panel
Printed on the panel label, usually the same as the main breaker rating but not always.
Printed on the main breaker at the top of your electrical panel.
Almost all single-family U.S. homes run on 240V split-phase service.
Check with your installer or local electrical inspector for the rule that applies in your jurisdiction.
4 Other loads on the busbar Optional
Include any other backfed sources, such as a generator interlock or a second inverter, already sharing the busbar.

How it works

Three steps to your result

Step 1

Enter your inverter's AC output rating and connection type

The cap applies to your inverter's continuous AC output rating, not your panel's DC rating. Choose whether you're using a standard load-side connection or a supply-side tap.

Step 2

Enter your panel's exact busbar and main breaker ratings

These are printed on the panel label and the main breaker itself. Settle on your planned inverter size first, since the cap check only means something against the equipment you intend to install.

Step 3

Check whether your system fits

If your system exceeds the cap, talk to your installer about a panel upgrade, a supply-side connection, or an export limiting device. The grid interconnection cost estimator can help you budget for the extra work if needed.

Understanding why export caps exist

The physical limit behind the rule

Your electrical panel's busbar, the metal strip that distributes power to all the circuit breakers, has its own maximum current rating. When solar backfeeds into the panel alongside normal household loads, the combined current needs to stay within that rating to avoid overheating the busbar. Export caps translate that physical safety limit into a maximum solar system size for a given panel, following the wiring rules in NFPA 70, the National Electrical Code. See the grid tie with battery backup comparison tool if you're also weighing whether to add storage, since an AC-coupled battery inverter can count as another source on the same busbar.

How the 120% rule works in practice

Under National Electrical Code section 705.12's commonly applied 120% rule, the sum of your main breaker rating and your solar breaker rating can't exceed 120% of the panel's busbar rating. Because a solar breaker is sized at 125% of the inverter's continuous output current, a 200A panel with a 200A busbar rating and a 200A main breaker leaves room for a solar breaker of up to 40A. That supports about 32A of continuous inverter output, or roughly 7.7 kW on a standard 240V connection, and a DC array of around 9 to 10 kW can sit behind an inverter that size at a DC to AC ratio near 1.25. The exact figure depends on your inverter and breaker specifications. Some jurisdictions apply a stricter 100% rule instead, which leaves little to no headroom on a fully rated main breaker.

When a supply-side connection makes more sense

A supply-side connection, which taps into the electrical service ahead of the main breaker rather than through the panel's busbar, avoids the busbar limit entirely. This is often the preferred path for larger systems that would otherwise trigger a costly panel upgrade, though it typically requires more involved utility coordination and, in many cases, a higher installation cost.

What to do if your planned system exceeds the cap

A panel upgrade to a higher busbar rating is the most straightforward fix on a load-side connection, though it adds cost and sometimes requires a utility service upgrade as well. A smaller inverter is another route, and the solar inverter size calculator can help you judge how much output your household needs before you settle on one. Your installer can advise which option, panel upgrade, supply-side tap, smaller inverter, or export limiting device, fits your specific panel and system size best.

How this fits into your overall interconnection

The export cap is one of several factors your utility reviews as part of interconnection approval, alongside the broader circuit capacity in your neighborhood. The U.S. Department of Energy's interconnection roadmap notes that small systems, often those under 25 or 50 kilowatts, typically qualify for a simplified review with no study required, while larger ones move to fast track or full study tracks. The grid interconnection cost estimator covers what to expect for application, equipment, and any additional study fees your system size might trigger.

Questions

Frequently asked

A grid export cap limits how much solar power a system can send back through your home's electrical panel and into the grid, based on the panel's busbar rating. It exists as a safety measure, since the panel's busbar has its own physical current limit that both your household loads and solar backfeed share.

The 120% rule, from National Electrical Code section 705.12, allows a solar interconnection breaker on a load-side connection as long as the sum of the main breaker and the solar breaker doesn't exceed 120% of the panel's busbar rating. On a 200A panel with a 200A main breaker, that leaves a 40A solar breaker, which supports roughly 7.7 kW of inverter output. It's the most common rule U.S. installers work within for standard residential interconnections, though some jurisdictions apply a stricter 100% rule.

A load-side connection adds a breaker inside your existing panel, which is subject to the busbar percentage rule. A supply-side connection taps into your electrical service ahead of the main breaker entirely, bypassing the busbar limit, though it typically costs more and requires closer utility coordination.

Your installer typically has a few options: upgrading the electrical panel to a higher busbar rating, using a supply-side connection instead, or adding an export limiting device that caps the inverter's output electronically. The grid interconnection cost estimator can help you budget for whichever path your installer recommends.

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