What is the mass equivalent of a year of power?
Einstein's famous equation says mass and energy are two names for one thing, and the exchange rate is enormous: E = mc². Power sources release fractions of their fuel's rest-mass energy as heat and light. Burning releases about a ten-billionth; Fusion releases under one percent. Accretion onto a spinning black hole can radiate around a tenth of the incoming gas's rest-mass energy, which makes it an efficient furnace. The full ladder, from fire to a black hole, is on the full mass-energy page.
Guess first The Vigil runs on one petawatt in its first years. What mass equivalent corresponds to that energy over a year?
351,000 kilograms. That is a few hundred tonnes, less than a loaded freight train. It is the mass equivalent of the emitted energy, not the fuel the plant consumes. At this power, the energy's equivalent mass is small.
The same arithmetic explains the seed that crossed from the Solar System to the Vigil's black hole and its star. It weighed 1,000 kg and cruised at 0.1 of the speed of light. Its motion carried 4.5 × 1017 joules, equivalent to the rest-mass energy of only 5.0 kg. A tonne of probe moving at a tenth of light speed carries that much kinetic energy, the same energy as 5 kilograms of mass at rest.
The energy stored in mass is large compared with the energy needed for the seed's flight. How the seed got its push is on the page about leaving Sol on light, and how a black hole releases a share of infalling mass-energy is on Feeding a black hole.