In vanilla massfarms were highly profitable, but explosive. In FA they're even more explosive and not very profitable. FA will be referred to in this article. The efficiency can be increased with the use of power generators. Mass fabricators come in two forms in Fa; T2 massfabs using up 150 energy and giving out 1 mass, and T3 fabricators using up 3500 energy and giving out 12 mass. So with the T2 ones it costs 150 energy per mass and the T3 ones it costs 291.6 energy per unit off mass. However with Adjacency bonuses this can be changed. T2 fabs dont benefit much from adjacency but T3 ones do a lot. A good example of this is a template from this guide at the bottom of the page. It is a shield surrounded by T3 power generators that also have T3 fabricators adjacent to them. Here the fabricators are more efficient than T2 ones and are protected by a shield.
A standard massfarm with T2 fabricators would be a T3 power plant surrounded by 16 T2 fabricators. This will produce 16 mass and 550 energy and pay for itself in 5 mins. There are a few drawbacks to this farm though; First off T2 massfabs only have 360 health and deal 370 damage over an area of 5 when they explode. So if one T2 fab is destroyed they will all go with it, and all that will be left is the power generator with 3800hp left, so as you can see building these is risky and they should be shielded whenever possible.
- When should I build massfarms?
You should build massfarms only when all your mexes have been upgraded to T3 and are surrounded by mass storage. Then you should start building them only if your enemy is not attacking. When he is not attacking upgrade all mexes and start with the massfarms. Another thing that is also worth noting is the UI mass fab manager mod. This mod is UI only so not everyone has to have it and it can be used in ranked games. This mod will automatically turn fabricators on and off when you have a surplus or deficiency of energy. Another time when you should build mass fabs is when you just have your first T2 pgen. You will have about +400 wasted power so build 2 T2 fabs to convert that into mass. With this UI mod you can just leave them be, no micro required. Also when you have lots of ecces power just build some T2 fabs. For every extra +150 power you have that can be converted to mass.
This mod is available on vault and is called UI mass fab manager FA.