Homeowners hear “you have two spaces left” and assume solar will plug in later. Inspectors and utilities do not grade the panel that way. They grade busbar rating, main-breaker size, and how much generation can land on that bus without overheating it.
Key Takeaways
- Spare slots are physical space. The 120% rule is thermal and code capacity.
- A solar-ready panel documents bus rating, main size, grounding, and a reserved backfeed location.
- A typical 200-amp upgrade that includes that planning is $3,500–$7,500.
- AJ Long Electric also installs the residential array. A typical 8–12 kW rooftop plans at $19,000–$34,000 ($2.40–$2.85/W).
What the 120% rule is doing
For a common load-side connection, the adopted code can limit the relationship among the busbar rating, main overcurrent device, generation breaker, and breaker location. A supply-side connection is a separate interconnection method with its own service-equipment, conductor, disconnect, listing, utility, and code review; the load-side busbar calculation is not applied to it as though the two methods were the same. AJ Long Electric documents the proposed method and runs the applicable check before equipment is ordered.
Need a Panel Upgrade?
Outdated or overloaded electrical panels can present safety and capacity concerns. Our team evaluates 200-amp upgrades throughout Northern Virginia and confirms scheduling after reviewing the property. Call (571) 444-6886 to get started.
When a panel replacement belongs in the solar-ready scope
A crowded 100-amp or 150-amp panel can fail the applicable load-side calculation even with empty slots. A replacement can document bus rating, main-device rating, breaker location, grounding, and reserved physical space, but it does not predetermine approval of a later array or a different interconnection method. EV and battery inputs belong in the present design, and any later solar proposal still needs project-specific code, equipment, permit, and utility review.
Start at solar-ready panel upgrades.
