Power siphon

From Teletraan I: The Transformers Wiki

The Power Siphon collects ambient energy and converts it into energon for transport. The Decepticons made frequent use of power siphons to collect energy from Earth.

Power Siphons are apparently capable of converting any form of energy into Energon, including thermal, solar, and even acoustic energy. They may require a certain level of available energy to properly function, which might be why Megatron didn't simply leave a few outside collecting sunlight.