If your home is 15+ years old, your electrical panel may cap your solar before you start. Utilities limit "backfeed" by your panel's rating — the formula: (120% × service rating) − main breaker = allowable backfeed, and max system size ≈ a quarter of that in kW. A typical old 100A/100A panel allows only a 4-5kW system. Upgrading to 200A typically runs $3,000-$4,000 — cheap if your service comes in overhead, potentially $12,000+ if underground trenching is required.
Escape hatches exist: 125A upgrades without trenching, renewable meter adapters (RMA) that feed solar behind the meter, and batteries that circumvent backfeed limits. Some old panels (Zinsco, Sylvania, Stab-Lok) are fire hazards that must be replaced regardless. Here's the tell: if your home is older and a rep never mentions your electrical panel, that's a red flag the size of a billboard. Included with solar, the upgrade rides your financing.