The electrical energy source includes a portable enclosure that houses at least one battery pack, at least one battery charger, at least one AC inverter and, optionally, a regeneration device that can power the battery charger. Operation of batteries, AC inverters and regeneration unit is coordinated by a control circuit. During operations under a load, the batteries supply an input to AC inverters which in turn provide voltage regulated AC power to the load. When the batteries have discharged to a predetermined level, the load is removed from the AC inverters, and a dedicated on-board inverter is used to power the regeneration unit which recharges the batteries. Once the batteries have been recharged, the AC inverters are again made available to the load.