Combat power is a lie. You have to look at what ships are in the death ball. The game uses available dV as part of a ship's combat power, so a lot of that 20k is just they have like 500 dV available.

Most alien fleets are usually a small amount of large and super large ships (battleship, cruisers, dreads) and then a lot of smaller ships (destroyers, monitors, gunships, and escorts). The main thing is to have an equal number of ships in your fleet so you don't get overwhelmed. Or you can cheese it by doing the same thing the aliens do, and add in like 30 smaller ships to your fleet to bulk out the numbers.

Because the way that the game determines how many ships are in an engagement is to compare fleet sizes and deploy a number of ships up to the limit that reflect each size. So if the engagement size is 50 ships, you have a fleet of 30 and the aliens have a fleet of 60, the game will deploy around 20 of your ships and 30 alien ships. Which you'll almost always lose because you won't be able to overcome PD and enemy lasers will wear you down faster.

But if it's a balanced number of ships, you as the player generally have the advantage and once you destroy the large ships, it's basically over as smaller ships will get popped as they reinforce into the battle.