E=mc² shows that a tiny amount of mass contains enormous energy. 1 kg of matter = 9 × 10^16 joules.
1 gram of uranium-235 completely fissioned releases ~24 MWh — enough to power a US home for 2+ years.
E = 1 × (3×10⁸)² = 9 × 10^16 joules = 25 billion kWh. But practical nuclear conversion is ~0.1%.