Provided is a backup power system for providing power to a load when a primary lighting system is disabled. The backup power system includes an energy source configured to supply an amount of power to the backup power system, and a charger connectable to the energy source and a power source of the primary lighting system. The backup power system additionally includes a controller configured to measure a current value of the primary lighting system and determine a backup current value corresponding to an amount of backup current that is a fractional amount of the determined current value. The backup power system include a current source configured to provide the determined backup current.