The biggest difference between a defensive battle and an offensive battle is who has the missile bases, if any, that are present.
Missile bases can be a very powerful force and correct play can turn a hopeless battle into a winnable battle.
Often there is going to be multiple types of enemy ships in an attacking fleet and there is likely to be one type that is the absolute most threatening type that the enemies are fielding.
To make sure you know what that is, it is a good idea every time you see a new ship to hit "scan" in combat before you do anything else. This uses the battle scanner technology to determine enemy ship configurations. The battle scanner technology is built into every missile base you make, it is also available as a special equipment item for ships.
Note, If you only intend to use a ship for defending your own planet alongside missile bases, the battle scanner will still give you +3 initiative and increases the level of your targeting computer by 1.
The thing you want to learn how to do is to use ships to physically block the travel of enemy ships. At the start of the game, all ships can only move one square on the combat grid. Ships can move through each other, but they can never end their turn in the same space as another ship, so you can only build an effective "wall" of ships early on.
If you setup your ships correctly, you will be able to practically negate all progress a single enemy ship can make toward your planet with three ships and you will be able to massively slow its progress with two ships. This can buy you the extra shots you need from the missile base to win the combat.
One turn of difference in the firing of a missile base can be game-breaking and turn a sure loss into a sure win, so be sure to practice this.
Also, many times the combat map will include asteroid fields, or impassable spaces. These are much better than ships for blocking as long as you are able to enter the field first and situate your ships in important positions.
Note, a movement diagonally still counts as one space, so the only way to keep a ship from moving at all would be to somehow position five ships around it when it was at the side of the board or to have five or six designs on the board and at least two asteroid fields within a distance of one of the enemy ship.
Generally, slowing them down is good enough, and it is not important to completely stop them.
Missile bases fire three missiles per turn each, so their damage adds up very quickly. With Scatter Pack technology, that could go as high as about 30 missiles per turn per missile base.
Note also, you want to try as much as possible not to shoot missiles through an asteroid field. Every asteroid field the missiles travel through will cause about half of the missiles to collide with space debris and detonate. That is a major reduction in damage.
There are two more weapons that are very useful in terms of planetary defense: the repulsor beam and the warp dissipator.
The repulsor beam pushes ships away from the equipped ship. If you put two ships with this equipment on opposite sides of a planet (adjacent, not diagonally), there is no possible way for enemy ships to get close enough to drop bombs on the enemy planet. They will be required to defeat the ships first in order to proceed to attack the planet.
The warp dissipator can also be very good (the computers use it often), but it is somewhat overrated beyond the early stages of the game. This weapon will slow down an enemy ship by one maneuverability per turn.
What this means is that if the enemy ship has an even number of maneuverability, the first shot will cause the enemy to lose one move per turn in combat. Every two shots after that will cause them to lose another square per turn of movement.
If the enemy has an odd number of maneuverability, it will take two shots to slow them down one space, and another one space every second shot thereafter.
When using the warp dissipator, it is important that you use a number of different ship designs and include it on all of them, if possible. These weapons have a pretty long range, but the name of the game is still how many shots you can put down and how fast you can put them down. If you have five designs in combat with warp dissipators that is five shots per turn. If you only have one in play with a warp dissipator, that is one shot per turn.
Another important thing to note, any ship that has its maneuverability reduced to 0 cannot retreat. If you can reduce a set of 32,000 ships to speed 0 they will be sitting duck for your ranged attacks for a long time in combat.
The weapon, though, is completely useless against high maneuverability targets. If you reduce a ship from moving five grid spaces per turn to four, they can still get right up on your planet in two turns. Don't try to use this weapon much longer than about 50–100 turns after you research it, as its usefulness will be gone that quickly. Counting the time it takes to construct a fleet capable of using it in large numbers, that isn't a lot of time.
The weapon is also of relatively little value on the offense, so it is best to load it on defensive ships.
Note, if you do use it, make sure you use heavy mount lasers or missiles on your ships so you can shoot beyond range 1.
Also, the possibility exists to make "the ultimate defensive ship". To make a ship like this, you generally want to use the huge class ship and put the absolute best engines and ECM Jammers that you have researched onto it as well as the best armor as well as the repulsor beam, the warp dissipator, and the automated repair special equipment. Any extra space could be used for heavy mount weapons. Missiles are bad because of how long you want this ship to stay in combat.
The idea is that it will be nigh invincible on the battle grid and it will soak up enemy attacks and give your missile bases more time to attack the enemy.
The high maneuverability and ECM Jammers will make it very hard to land hits on the ship and the automated repair will erase most of the damage that does get on the ship.
If you get to the end of the game, you will want to get rid of the Warp Dissipator when you get High Energy Focus and get rid of the Automated Repair when you get Advanced Damage Control. If you are somehow still in a defensive position at this stage it is probably best to keep the Repulsor Beam on there because you can still use it to make bombers have a very difficult time getting to the planet. If you want to go offensive with this juggernaut, change the Repulsor Beam out for Anti-Missile Rockets, the Zyro Shield, or a Lightning Shield to maintain how hard it is to kill the ship, or change it for an Ion Stream Projector or something for more of an offensive punch.
Don't forget the Repulsor Beam because you designed it early in the game. The enemy ships usually have trouble fitting the High Energy Focus onto their ships due to the limitation of using only three equipment items. Usually they will pick three other things. If they do, all of their 1 space range weapons will be nullified by it the entire game. That is potentially a lot of weapons to make useless including a lot of very powerful ones.
The Displacement Device is another excellent equipment to put on one of these defensive ships. The inertial stabilizer and nullifier are very good too, giving an extra 20% and 40% chance to dodge hits respectively, but note that better engines with higher maneuverability classes are often better than Inertial Stabilizer since better engines also get better warp speed, for a better strategic movement.
Ships like these, especially in the middle game (after Class 5 planetary shields are up and before High Energy Focus is researched and implemented) can be quite useful for defending your planets.
Once the High Energy Focus is researched and implemented, though, usually it is best to begin decisive offensive aggression as quickly as possible. Very little defensive play occurs after this point, usually. It is even possible to go on the offensive earlier, as soon as (Heavy) Fusion Beams, Megabolt Cannons, Stingers, or Omega-V Bombs are available --- these weapons are enough to punch through Class V Planetary shields.