Why battery capacity alone does not determine how long a backup system will operate.
Battery capacity is commonly expressed in kilowatt-hours, but the usable energy may be lower than the headline figure. Backup duration also changes as the connected load changes. A 5 kWh battery does not provide the same duration when supporting lighting and internet equipment as it does when supporting heating, pumps or cooking appliances. Inverter efficiency, battery reserve, temperature, battery condition and surge events also matter. Ask for the load profile and assumptions behind every backup-time estimate.
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