Just a quick question, is it worth while building up the fusion reactor? Or is the cost to deuterium not worth it?

With a few caveats, yes fusion reactors are worthwhile. With energy tech at level 20, my Fusion Reactor level 16 produces as much energy as my Solar Plant 32.

The important thing is that in order to be worthwhile, your energy tech needs to be reasonably high. I haven't run the numbers, but the formula is in the game text for Fusion Reactor. I started with an energy tech of 12 which is the requirement for other research, and even then (if I remember correctly) it was beneficial for me to use fusion reactors in order to supplement level 30 or higher Solar Plants.

The point is that it is a trade-off. You are sacrificing a relatively small amount of hourly production of deuterium in order to increase power production without having to build the next highest level of solar plant. At higher levels, solar plants begin to become almost prohibitively expensive for the amount of power they produce, but fusion reactors provide an indestructible (unlike solar sats) alternative.

If you are just starting out, I recommend building solar plants primarily. Fusion reactors provide a better return on investment at higher levels and higher levels of energy tech.

OK sweet, sounds logical. What level of energy tech would you recommend a minimum before trying to supplement with fusion reactors?

Also to save starting another topic, can anyone help me understand the benefit of robotics factories other than lower construction time? cos wouldn't it then be pointless get a high leveled on as it will use up fields that could be better used with a higher level resource field? Correct me if I'm completely out of left field here but I don't quite understand the tactics of everything yet.

I usually stop building my robot factory once it gets to lvl 10.