fourier - Essential Bandwidth of rect(t/T)

Here is a question I have been trying to solve:

Estimate the "essential bandwidth" of a rectangular pulse

$$ g(t) = \operatorname{rect}\left(\frac{t}{T}\right), $$ with $T>0$, where this "essential" bandwidth contains 90% of the rectangular pulse energy.

What I have so far is that the Fourier Transform of $\operatorname{rect}\left(\frac{t}{T}\right)$ is

$$ G(f) = \mathcal{F}\{g(t)\} = \mathcal{F}\left\{ \operatorname{rect}\left(\frac{t}{T}\right) \right\} = T \operatorname{sinc}(fT) $$

where $$\operatorname{rect}(u) \triangleq \begin{cases} 0 & \text{if } |u| > \frac{1}{2} \\ \frac{1}{2} & \text{if } |u| = \frac{1}{2} \\ 1 & \text{if } |u| < \frac{1}{2} \\ \end{cases}$$

$$\operatorname{sinc}(u) \triangleq \begin{cases} \frac{\sin(\pi u)}{\pi u} & \text{if } u \ne 0 \\ 1 & \text{if } u = 0 \\ \end{cases}$$

$$ X(f) = \mathcal{F}\{x(t)\} \triangleq \int\limits_{-\infty}^{+\infty} x(t) \ e^{-i 2 \pi f t} \ dt $$ and $$ x(t) = \int\limits_{-\infty}^{+\infty} X(f) \ e^{+i 2 \pi f t} \ df. $$

Integrating $G(f)$ over $\pm \infty$ results in $1$. Also, integrating $|g(t)|^2$ over $\pm \infty$ results in $T$. This is about where I am lost.

Any help is appreciated.