Worked example - check the Sun's power from its mass loss. The Sun converts 4.3 million tonnes of mass into energy every second. Step 1: convert to kilograms: 4.3 x 10^6 tonnes = 4.3 x 10^9 kg. Step 2: apply E = mc^2: E = (4.3 x 10^9) x (3 x 10^8)^2 = 4.3 x 10^9 x 9 x 10^16 = 3.87 x 10^26 J per second. Step 3: joules per second is watts, so P ~ 3.9 x 10^26 W - matching the measured solar luminosity of 3.828 x 10^26 W. This confirms that mass loss via fusion fully accounts for the Sun's brilliance.