Battery energy-storage products are marketed with kilowatt-hour figures. Backup duration depends on much more than nameplate capacity. Continuous power output (kilowatts, not kilowatt-hours) limits which loads can run simultaneously. Starting-surge requirements for motors—refrigerators, well pumps, air conditioners—can exceed a battery's rated surge capacity. Reserve settings prevent deep discharge that shortens battery life. Solar recharge during an outage depends on panel output, which varies with season, weather, shading, and whether the inverter is configured to operate without a grid reference. NREL's REopt tool models resilience scenarios for distributed energy systems, allowing users to test how different storage sizes, solar configurations, and load profiles affect outage coverage duration. DOE's Solar and Resilience Basics page explains the relationship between solar-plus-storage system design and grid-independence capability. The IEA's Batteries and Secure Energy Transitions report places household storage in its broader energy-security context. A practical framework for evaluating a storage system requires: usable capacity (not nameplate), maximum continuous and surge power, the specific loads designated as critical, minimum state of charge under the inverter's programming, whether islanding transfer equipment is installed, and the expected solar contribution during a winter outage. "Whole-home backup" in marketing material typically means the system can power a whole home at low load for a period—not that it will run central air conditioning and an electric water heater simultaneously for several days. Editorial caution: Do not infer backup duration from nameplate capacity. State usable capacity, power limits, and load assumptions in any worked example.