Here are a few words about my own way to look at the powergraph:
1) Building power
This one is the 'true' power of your civ when you need neither growth nor research (which is for example the situation of most players in the end game, either conquering with brand new, non obsolete units, or mastering Fusion Power and willing to build SS parts at full throttle).
In that case your civ produces only gold and shields ( and the food needed, but no research = no beakers, no growth = no surplus food).
What I call Building Power (BP) is very simple:
BP = Shields available + 1/2 Cash Flow, where
Shields available = Shields produced - Shields lost to support
Cash Flow = Gold earned - Gold lost to maintain improvements
1/2 means that 1 shield is worth 2 gold (which is true for all players using incremental rushbuilding and building wonders with caravans).
Example:
My civ produces 50 shields, 10 of those are used to support units:
Shields available = 40.
My civ earns 180 gold, 60 of those are used to maintain improvements in my cities:
Cash Flow = 120 gold
Therefore BP = 40 + 1/2(120) = 100 'shields'
My civ is able to build 2 caravans/turn.
This is exactly the meaning of 'Building Power': your capacity to build caravans or vet spies or anything that pleases you (and I confess that vet spies please me).
2) Remark about research
If your equipment is the same in the field of research as it is in the field of taxes (for example Market Place and Library everywhere), then 1 gold = 1 beaker.
More generally, research reduces cash-flow (and it is very easy for anyone to measure that; if you haven't done it yet, please have a try: this will teach you a lot about how this game works; for example, look at a city with Stock Exchange, no Library and 10 Einsteins; switch those Einsteins to taxmen and let me know).
3) Growing power
In the early game, growth is obtained through food surplus.
Food surplus can be used in the city itself (new city, 2 surplus food, 10 turns = city grows to size 2) or else it can be used (combined with 'Building Power') to build settlers and expand (great fun comparing internal and external growth).
In the late game, food surplus is almost completely replaced by:
- food caravans (Commie and Fundy)
-WLT*D (Republic and Democracy)
Anyone knows how it works, so I give only one calculation, showing when a food caravan becomes theoretically better than food surplus.
Basic assumption: 1 shield = 1 food (you experience that when you move a worker from plain to forrest).
Surplus food needed to grow a city from size 4 to size 5 (granary inside the city): 5 times 5 = 25 food (surplus food much better than caravan).
Surplus food needed to grow a city from size 19 to size 20 (granary inside): 5 times 20 = 100 food (caravan much better than surplus food).
Therefore, the food caravan is preferable when city size is more than 5 (without granary), or 10 (with granary).
(La Fayette, after rereading this post)
