There are basically three formulas you need to calculate unit-on-unit combat. In the three examples below, “Power” refers to the Attacker, “Shield” and “Armour” refer to the defender.
When Power is lower than Armour (e.g. Fighters v. Cruisers), but higher than Shield.
When Power is greater than Armour (e.g. Dreadnought v. Cruiser)
When Power is less than Shield (except those Ion units that have 50 percent of their power bypass shields).