How powerful was ENIAC what all could it do?

ENIAC had 20 accumulators each of which could potentially do up to 5000 additions/subtractions per second (this was its total data storage capacity as originally built). To simplify programming it had a multiplier unit that used 4 accumulators to do multiplications by repeated addition and a divider/square rooter that used 5 accumulators to do division or square roots by repeated subtraction. Programming was done by wiring different units together with cables. With optimal programming speeds of 50000 to 100000 additions/subtractions per second could be achieved in short bursts but were impossible to sustain.

Later upgrades were added including 100 words of magnetic core RAM and a limited form of stored program operation from ROM that reduced the machine to having only one programmer accessible accumulator (the other 19 now had fixed hardware uses that the programmer could not directly control) and a maximum speed of about 800 instructions per second. This reduction in performance was not considered a problem as it dramatically simplified usability and the machine was always throttled in performance by its only input/output devices: one punchcard reader and one punchcard punch each operating at 100 cards per minute (with 8 numbers punched per card).

Its first real problem (taking 2 months to run: December 1945 .. January 1946) was simulation of Edward Teller's first hydrogen bomb design called The Super. The problem used a million punchcards and showed almost immediately that this design could not work and was a dead end. On completion of the run it was obvious that a totally different approach to hydrogen bomb design was needed.

It was a very flexible computer and could do anything a modern computer can, IF the variables of the problem could fit in its limited memory space.