Net Power
I've found this as the net power calculation:
1000 per acre of land (built or Wilderness)
1/20 (0.05) per mana in pool
1/50 (0.02) per current population
1/5000 (.0002) per geld
1000 per spell level
1000 per lesser item (includes Minor Indulgence and Magical Compass)
95000 per unique (from AM, needs confirmation in TR),
- except Holy Grail gives 0 (or 1000 like a lesser item?)
10000 per ally
0 per god's favour
100000 per battle hero (100k)
100000 per non-battle hero (100k)
+ army power (number * power rank summed for each stack)