Power outages in New Jersey and New York are no longer a once-a-year inconvenience. Between summer heat waves that strain the grid, nor'easters that snap power lines, and an aging utility infrastructure, homeowners are rethinking how they keep the lights on. A home battery backup system — like a Tesla Powerwall or similar residential energy storage unit — has become one of the most practical upgrades a homeowner can make. But not all battery systems are created equal, and und