Here is a formula that you can use to figure out how many fighters a sector can handle:

High Overview

Sector Fighter Support =
(SEP) / Energy per fighter


Formula Breakdown =
(Sector Energy Production aka SEP) / Energy Used per fighter

Sector Energy Production (SEP) =
(Planet Energy Production 1 aka PEP) + (PEP 2) + (PEP 3) + ...

Planet Energy Production (PEP) =
Production % dedicated to energy as a decimal x Colonists / Colonists to make 1 energy

Quick Example =

{.15 x 100,000,000 / 500} = 30000 = PEP1
{.10 x 200,000,000 / 500} = 40000 = PEP2

PEP1 + PEP2 = 70000 = SEP

SEP / .06 = 1,166,666 fighters supported

(Devs -This could be disclosed somewhere on the planet info page and people could just add up each planet and come up with sector defense support - This is needed because dignitaries can affect the output of energy)

Detailed Examples:

Energy Used Per Fighter = 0.06
Colonists need to make 1 energy = 500
Planet Production = 2 minutes
Sector Defense Degrades = 6 minutes (Not sure if this is when fighters consume energy or ONLY when it will degrade the defenses if the energy is not present -> My calculations assume it only affects degradation. Otherwise you would multiply final number by 3 (3 two minute ticks of planet energy production).


Sector Energy Production is:

100% (1.00) energy production
100,000,000 colonists on each planet
500 colonists to make 1 energy
5 Planets in the sector
NO PRODUCTION DIGNITARIES

So -

1.00 x 100,000,000 / 500 = 200,000 energy per turn

200,000 / 0.06 = 3,333,333 million fighters sustained by one 100% production planet.

3,333,333 fighters x 5 planets = 16,666,666 million fighters


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Real World example #1:

20% (.20) energy production on each planet
3 planets in the sector
150,000,000 population on each planet
500 people to make 1 energy
0.06 fighters to 1 unit of energy
NO PRODUCTION DIGNITARIES


.20 x 150,000,000 x 3 / 500 = 180,000 / 0.06 = 3,000,000 fighters can be supported in this sector


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Real World example #2:

Don't panic! It easier than it looks.

10% energy production on planet 1 with 200,000,000 colonists
15% energy production on planet 2 and 3 with 150,000,000 colonists
20% energy production on planet 4 and 5 with 100,000,000 colonists
500 people to make 1 energy
0.06 fighters to 1 unit of energy
NO PRODUCTION DIGNITARIES

Planet 1 Production =
.10 x 200,000,000 / 500 / .06 = 666,666 fighters

Planet 2 & 3 Production =
.15 x 150,000,000 x 2 / 500 / .06 = 1,500,000 fighters

Planet 3 & 4 Production =
.20 x 100,000,000 x 2 / 500 / .06 = 1,333,332 fighters

3,499,998 fighters can be supported in this sector


Conclussions:

Remember that this is a basic formula that DOES NOT take into account production enhancing dignitaries.

Sector Defenses are useful in the early part of a game but quickly become outmoded in the middle to late game when the fighters and torps that a ship can carry outnumber the sector defense.