This is my user experience....
The Atik Horizon seeks 12v and close to the full specified 2A hence 24 watts. This is a high AMP pull for a camera, but that is perhaps to be expected given its large sensor, megapixel and cooled. The Celestron Powertank is rated at 86 Watt hours, hence in theory should supply enough power for four hours (86Wh/24W). But such claims are often overstated by 25% dependent on many factors. Begs the question, is three hours enough, also is there a better option? Here are a few thoughts....
A MaxOak K2 "50000 mAh" operating at 12v/2.5A is rated at 185 Wh so should last "up to" eight hours. Again, let's deduct 25% for other factors and you should get around six hours. My MaxOak K2 has comfortably exceeded that in test (there have been too many clouds to do much outside!). The Celestron costs £129 whilst the MaxOak K2 costs £109 . I chose the latter and it's been working fine. I have been developing a wireless/cordwrap free set up so the MaxOak K2 has undergone many days of continuous testing and has been a decent purchase despite KayoStar not being a big brand name (I bought via Amazon).
However, I am now using the MaxOak K2 at 20v/3A to power my Intel NUC (that requires up to 48W) and am instead now running the Atik Horizon using a Tracer 12v 8Ah (£116). The Tracer is rated at 96Wh so should last a little longer than a Celestron Powertank despite a worthwhile saving on price. If you are willing to spend £129 on the Celestron Powertank, your best bet may be a higher capacity Tracer. However, prices of Tracers seem to vary significantly. Look for a deal, but IMHO, the MaxOak K2 is appealling and I know of at least four other owners using this power bank with a Horizon or the similar ZWO AS1600.
ADDED
Forgot to mention, as long as the Amps exceed the requirement any of the above 12v units will do the job. However, the MaxOak K2 is limited to 2.5A at 12v. Whilst enough to power the Atik, it might not be enough 'oomph' to power your telescope as well, but you did ask the question in isolation. That is why I use a dual battery set up (plus my Evolution has an internal battery). An advantage of the Tracer and Celestron units is that they can output higher Amps and your devices will only take what they need. However, the greater the demand, the faster the battery depletion and that is why I have quoted Watt-hours. My point here is you need to holistically consider your total power requirements. A much larger Tracer might be the way forward if you plan to grow your set up and you want some future proofing.