The short answer: Sunlight has zero rest mass, but its energy carries a mass equivalent through E = mc^2. For example, 1 kilowatt-hour of sunlight corresponds to only about 4 x 10^-14 kilograms (a few hundred billionths of an ounce).
Sunlight weight by type
Light is not ordinary matter and has no rest mass, yet it carries energy and momentum. That energy translates into an extremely small mass equivalent.
| Sunlight amount (example) | Approximate mass equivalent |
|---|---|
| 1 joule of light | about 1.11 x 10^-17 kg |
| 1 kilowatt-hour of sunlight | about 4 x 10^-14 kg |
| Sunlight on 1 m2 for an hour (~1 kWh) | about 4 x 10^-14 kg |
| Total sunlight hitting Earth per second | a few kilograms |
What affects sunlight weight
- Energy delivered. More light energy means a larger mass equivalent.
- The c^2 factor. Dividing by the speed of light squared makes the mass tiny.
- Intensity. Brighter sunlight delivers more energy per second.
- Area and time. Larger areas over longer times collect more total energy.
- No rest mass. Light itself has zero rest mass; only its energy counts.
- Momentum. Light carries momentum, which lets it push solar sails despite being massless.
How sunlight weight compares
The mass equivalent of all the sunlight striking Earth in one second is only a few kilograms, lighter than a house cat, even though that light powers nearly all life on the planet.
Frequently asked questions
If light is massless, how can it weigh anything?
Light has no rest mass, but its energy carries a mass equivalent through E = mc^2. The value is extremely small for ordinary amounts of light.
How can sunlight push a solar sail if it is weightless?
Light carries momentum even without rest mass, so it can transfer a tiny push to objects it strikes, which solar sails use for propulsion.
Does sunlight add weight to Earth?
The mass equivalent of incoming sunlight is tiny, and Earth re-radiates most energy back into space, so there is no meaningful net gain in weight.
