Power Armor in the Fallout lore, is a set of heavy mechanized armor designed to be used by basic infantry, which was developed in the U.S. during the Sino-American war by defense contractors. The Power Armor frame itself was designed and produced by a company known as West-Tek, which later on would design and produce multiple versions of power armor. Power Armor was built from a multitude of conditions that plagued the U.S. during the war with the Chinese. Heavy fighting in Anchorage, and still having troubles with liberating the oil pipelines in Alaska, the U.S. was looking for answers to two issues; how to protect troops to fight more effectively, and what source of fuel can be used without reliance on oil. West-Tek was then able to design a frame which could hold extremely heavy pieces of armor that would be impossible for regular infantry to use reasonably with their own body strength. The frame was powered by a back mounted TX-28 micro-fusion pack, able to generate 60,000 Watts of electricity. This fuel was also known as a “Fusion Core” later on by individuals in the pre-war era and post war era.
In January of 2067, West-Tek introduced the first set of power armor, designated T-45. It was sent to the frontline in Anchorage to assist the troops and allow them to halt the Chinese from advancing into Alaska. Originally to be a stop gap type armor, the T-45 was still used widely in the U.S. Army throughout the Sino-American war, which lasted for 10 years in total. The new armor proved to be effective in defensive roles against enemy assaults and small attacks on enemy positions, but otherwise was not very effective. This was evident through where the U.S. attempted attacking the Chinese with units of power armor supported by infantry and mechanized units, the T-45 through which was simply not strong enough to allow for effective assaults to be conducted. However, the next set of power armor to come along was the T-51, which was considered and was easily a superior set of armor, which protected the user inside more effectively than the T-45. In Assaults, the T-51 was able to achieve the results that the T-45 failed to accomplish. And unlike the T-45, the T-51 was also able to deflect shots from lazer weapons and other energy weapons due to a coating of poly laminate composite and a layer of micron silver.
The next designs of power armor that came afterwards were known as the T-60, T-65, and X-01, with the T-60 seeing the most involvement of pre-war America, compared to the T-65 and X-01 power armor. These last three sets all were designed and built before the Great War had occurred, which was the event where the world descended into nuclear fallout. Contrary to popular belief, the X-01 power armor was designed and tested before the events of the Great War, and a few prototypes existed. Although the first few iterations were made by the Enclave, the shadow government of the United States, who then later started producing the X-01 after the Great War had ended the world.
Wanted to note also: T-60, T-65 and X01 were never part of the lore before Fallout 4 and 76. Although the X01 is basically a new vision of Bethesda’s idea of the ‘Advanced Power Armor’ made by the Enclave. Its my firm belief they took that set of power armor and turned it into the X01 in the new Fallout 4 engine and general aesthetics of the lore.