How powerful is Kakashi without the Sharingan

When does Kakashi first use his Mangekyou Sharingan?

Kakashi's first known use of the Mangekyo Sharigan was against Deidara. However, I think your question is really why he hasn't used it before. There could be a number of reasons why he didn't use it or wasn't shown how he used it in the past.

Kakashi woke the Mangekyo Sharingan when Rin was killed. So he didn't keep his promise to Obito. Hence, this was an emotionally painful moment for him, connected to both Rin and Obito. Maybe he hasn't wanted to use it since then so as not to relive those emotions.

As mentioned in Lexter's answer, another reason is that as a non-Uchiha he simply didn't have enough stamina to use it effectively in combat. Even after the time jump, he could only use it twice a day and had to rest for about a week after using it. Before the time jump, he was in charge of genin-level students, and he couldn't risk getting knocked out and putting his students in danger.

Plus, before the time jump, he just couldn't fight bad guys he was forced to use it against. He only fought against Zabuza and Haku during the Land of Waves arc and against unnamed shinobi from Sand Village and Sound Village during the Konoha Arc invasion. If Orochimaru were to fight him, Kakashi would likely be pressured into using it, but Orochimaru fought the third Hokage instead.

The loss to Uchiha Itachi may have forced Kakashi due to the usual Shounen / Naruto logic: "If I want to protect my Nakama, I have to get stronger."

There might also be some authoring issues as to why Kakashi couldn't be shown earlier with it. Sasuke was a protagonist at the time, and his main goal in life was to kill Itachi. Hence, it was necessary to introduce Itachi as an overpowering villain. Ensuring that Mangekyo Sharingan is mentioned for the first time when Itachi is introduced would maximize the audience's perception of how strong and evil Itachi is.

The other reason is that it wasn't in the author's plan for Kakashi to wake the Mangekyo Sharingan at the beginning of the series and offer an alternate explanation for how he woke him up during the time jump period. However, when he started addressing the hanging threads while he was unwinding the story, he decided to include them in Obito's backstory.