I've been looking for the optimal time to place granaries. So, I decided to do a little analysis to determine when I should place them and when I should upgrade. In order to do this, I had to make some assumptions about villager cost, since you'll have some villagers go to timber and some to food. I used a typical approach: villagers alternate going to food and wood. I then added the cost of the villager and the farm together, subtracting the farm completion bonus. All costs were considered equal and added together (which may not be true depending on your playstyle). To compare cost effectiveness, I examined the ratio of incremental income and total cost.
- The first granary (70 resources) is cheaper than your 7th farm (~76 resources)
- The second granary is cheaper than the 13th farm
- It is cheaper (by about 30 resources) to get to +150 using 10 farms and 2 granaries (plus the three towns) than it is to build out 12 farms (plus the three towns)
- At 10 farms, it's better to upgrade (+30 food) than it is to build 3 more farms
- On the third city, you shouldn't put down a granary until you've built out all 5 farms, or you've got 3 farms and have researched Agriculture (+50% to production)
- It makes sense to get SCI2 and CLASSICAL as early as possible so you can put down a granary to boost production. Going to COM2 before CLASSICAL means you either don't fully benefit from the extra commerce or you'll have to spend extra to get to +150 prior to granaries being available. This defies the current conventional wisdom, but is in line with the results I am some others have been getting. You can go early CLASSICAL and then begin the two granaries while you are upgrading COM2. The third town will max you out at +150.
- Assuming a sizable economy, priority should go to getting production upgrades as soon as possible. Boosting the production with upgrades is much more efficient than building extra farms, particularly in later ages. Only when the production bonuses are not enough to get you to the cap, then build extra farms and granaries to match them. Fully upgraded, it's better to build one farm, then a granary, before building any more farms on that town.