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. Array pricing is quoted per project.
What the 120% rule is doing
On many supply-side and load-side interconnection methods, the sum of the main breaker and the backfeed breaker is limited relative to the busbar rating — often described as 120% of the bus rating for a common load-side connection. The exact method depends on the listed equipment, the adopted code edition, and how the inverter is connected. AJ Long Electric runs that check on the existing can before anyone orders an inverter.
Need a Panel Upgrade?
Outdated or overloaded electrical panels are a safety risk. Our team specializes in 200-amp upgrades throughout Northern Virginia, with same-day panel assessments available. Call (571) 444-6886 to get started.
Why a 200-amp swap is often the real solar-ready step
A crowded 100-amp or 150-amp panel can fail the math even with empty slots. Replacing the enclosure and leaving a documented backfeed space is the work that actually survives a later interconnection review. EV charger and battery loads belong in the same calculation so the reserved space is still there after those circuits land.
Start at solar-ready panel upgrades.
