Each adjacency gives +5%. So, for every adjacent manufacturing building, the Solar Plant gains 1 more bonus than the Mega Factory, which translates into a +5% advantage.

Since the Solar plant starts 8% behind the Mega Factory, you need to have at least 2 adjacencies for the Solar plant to be more beneficial than a Mega Factory in the same position.

Even if you only build a single manufacturing improvement on a planet, allways do the (solar/fusion/quantum) power plant (since you are only allowed one) first.

The reason being the manufacturing you invest in it in comparison to the benefit you get. In case of no adjacency mega factory vs. solar pp, you are looking at 30+45+67 = 142 manufacturing for 33% vs. 45 manufacturing for 25% manfacturing. It takes quite a bit of time for the mega fac to pay of that deficit. Once you can upgrade to fusion pp, this discussion is obsolete anyways, since that one gives 50% manufacturing and is as good as the highest tier factory at lower manufacturing cost (45+67 = 112) than a mega factory.

There might be some special cases when you have very high pop planets with very low +% manufacturing, but apart from those:

DR (if you can spare the durantium) > powerplants > factories.

Also remember the upgrades to the solar plant are much better then the factories (fusion +50%/+3 adj, quantum +100%/+5 adj).

Well, if you put a manufacturing capital on a planet you should have the DR next to it in any case anyways. But lets take a look at what I wrote above:

So you have: 40 base manufacturing x3 because of the +200% so: 120.

If you put a DR on, which adds, lets say 12 manufacturing (4 from adjacency), you get 52 x3 = 156

If you use a manufacturing capital instead, you get +200% manufacturing (total of +400% manufacturing), you have 40 x5 = 200.

This is a rough estimate which mostly ignores adjacencies, but you get the idea: for high pop low +% manufacturing bonusses, the capital can be the better improvement.

But as I said before. If you can afford a DR, you should put it on the same planet as the capital anyways.

I suppose this discussion continues here now.