A simple, low-cost and energy-effective method has been developed in this work to fabricate giant graphene sheets by double microwave assisted exfoliations of expandable graphite. The graphene sheets, which are large (10–100 microns) in size and thin (1–10 nm) in thickness, manifest superior electrochemical performance for energy storage, achieving 221 F g−1 capacitance after 5000 cycles at 10 mV s−1 in a symmetric capacitor measurement and 400 mA h g−1 in a coin-type Li-ion battery after 300 cycles at 0.5 A g−1.