At some point you'll have to decide if you want to make a game that is about attacking and stealing resources or are you going to make a game that caters to care-bears.
Yeah, I know what you mean...
The audience of that game is more vicious than average so I will go for stealing resources part. But, I want to save as many carebears as I can

I know I can not save them all, it is not preferable game style for them, but still, I'm not gonna sacriface them without a fight

I used a system like that before and what ends up happening is players will hide themselves within the formula. Decreasing / Increasing attack strength allows players to manipulate the system to there advantage.
There are tricks to avoid this:
- players can not untrain army
- not power is compared by score (which can consist of non combat power assets too, for exaple you get reputation after successful attack which actaully is bad for you since it bumps your score and without any power increase)
I was thinking about these solutions:
- each player has cummulative resources stolen variable, if you attack someone that has stolens less than you you get penalty.
But wouldn't it lead to a stagnant gameplay? Maybe add sopmething like "and the total stolen is reduced by 1% each hour"?
If you want to go the care-bear route you could put the player in protection once he/she has lost x% of there resources in total for that day.
The problem is I do not know what daily production is. (you convert turns to resources, so it could be 0 or zillion...)
I cant believe I am suggesting this

something Like Evony's storage, basically when you where attacked anything that did not fit in your storage amount was available to be stolen, given that the attackers army could actually carry the resources.
Yes... but how exactly balance how much should be allowed to put there? Sounds tricky...
In Trivan/Ebony production is based on buildings so if you have farm level X then you should be using storage level X to protect resources for Y hours. In my game the production is based on population and is very irregular (triggered by player's action, not time).