Net Metering (NEM) — still the rule in most states — lets you sell excess solar to the grid at retail rate. Produce more than you use across the year and your bill hits ~$0. Simple, forgiving, oversizing was smart.
Net Billing — California since 2023 — cut export credits by ~75%. Selling to the grid is now a bad trade for most hours of the year, which means the winning design changed completely: size solar to your daytime usage, add a battery to store your own power for nighttime, and minimize grid interaction. The twist: during August-September peak hours, SDG&E pays $1-$3.50/kWh for exports — so a well-designed battery system cashes in exactly when the grid is desperate. This is why systems designed after 2023 must be designed with software and your actual usage data (Green Button data), not napkin math. It's also why so many post-2023 systems from careless companies underperform.