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The Horizon Quantum of Area

  Several independent semiclassical arguments converge on $$\boxed{\Delta A=8\pi l_{\mathrm{Pl}}^2}, \qquad l_{\mathrm{Pl}}^2=\frac{G\hbar}{c^3}$$ I got interested in this reading the 2018 paper  Bekenstein, I, and the quantum of black-hole surface area . Even though I disagree with Hod, its a great paper.  Honourable mentions also go to:   The Physical Interpretation of the Spectrum of Black Hole Quasinormal Modes , 2008  The Off-Shell Black Hole , 1994     On Black Hole Spectroscopy via Adiabatic Invariance , 2012 An Analytical Computation of Asymptotic Schwarzschild Quasinormal Frequencies , 2003  Asymptotic Black Hole Quasinormal Frequencies , 2003  1. Horizon Action Near any nonextremal horizon, imaginary time turns the normal geometry into a plane: $$ds_E^2\simeq d\rho^2+\rho^2d\Theta^2+\cdots.$$ Smoothness requires one horizon cycle: $$\oint d\Theta=2\pi.$$ The gravitational action identifies the momentum conjugate to $\The...
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Is there a Maximum Power Limit in General Relativity?

Yes!
Also, it is not the 'Planck Power' (despite what you might have read in Misner, Thorne and Wheeler, P.980).  The existence of black hole horizons implies a maximum luminosity (power) limit in General Relativity. Not even gravitational waves can escape a black hole. Consider an (almost) black hole made of light (this is called a Kugelblitz) sphere of radius
\begin{equation}
\notag
R \geq \frac{2Gp}{c^3}
\end{equation}  which is filled with photons with a total mass-energy of momentum $p$ times speed of light $c$ \begin{equation}
\notag
E=p \ c
\end{equation} the shortest time which the entire sphere can release its energy is its light-crossing time: 
\begin{equation}
\notag
t=R/c
\end{equation}  with average power (luminosity) 
$$
P = \frac{E}{t} \leq \frac{p \ c^2}{R}$$
So $$P_{max}=\frac{c^5}{2G}\approx 1.8\times10^{52} \ W$$  This is maximum power in GR, for a compact, casually connected emitter. 
 
i.e. is not already enclosed by a horizon, releases an energy E from a region of radius R, and does so on no less than the regions light-crossing time. 
 
You might be tempted to call this half a 'Planck Power' but there is no $\hslash$ in this expression, it is purely classical. This is why you won't see an equation for 'Planck Power' in wiki 
Source: Cardoso (2018)
 
OK, so what if there is a maximum power limit in GR? Well, for one it means that even in classical GR, a lab-scale Kugelblitz cannot be formed. From this paper, to overcome Schwinger dissipation (which is quantum) to form one with a 1m radius would require a power of  around $P_{req}=10^{84}\ W$. This is greater than our derived maximum power! 
 
To get an energy density low enough to avoid triggering the vacuum breakdown (Schwinger effect) to build a Kugelblitz that doesn't blow itself up via quantum effects, you would need to construct a sphere of light larger than our Sun. Getting  $P_{max}$ watts of power coherently focused across a region 2 million kilometres wide is...effectively impossible.

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