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While not quite the answer you were looking for when it comes to buildcraft the best method is too generate either exactly or just below exactly what you need for basic factory operations and then have batteries of engines that are ready to be turned on when needed, I personally attach wiring to gates on the engines and place a switch on a wall close to what machine needs the power.
If you have the Power Converters mod enabled, you can always turn your MJ into EU for storage with an energy bridge, then back to MJ when you need it. Otherwise, I second SatanicSanta.