The famous equation . . . E = m c² . . . doesn't say anything about where the mass comes from.
The total conversion of 1 kg of ANY mass into energy yields
(1kg) · (c²) Joules of energy.
E = (1 kg) · (c²) = (1 kg) · (299,792,458 m/s)²
<em>E = 8.9876 x 10¹⁶ Joules</em>
To put this in easily understood terms, it's the amount of energy required to keep a 100-watt light bulb shining for 10,402,259,010 days.
(That's about 28.5 million years, at the current length of days and years.)