The whole concept of insurance is that everybody contributes to the fund and in the event that they suffer a loss,they take from the fund. If you have a property or a risk which is worth say €100k, then you should contribute a percentage of this to the fund and in the event that it is lost, then you get the €100k. However, if you only contribute half of what you should to the fund, then you will only get half in the event of a loss. If you think about it, why should you get the full value of insurance of €100k, when you only pay a premium for cover of €50K.
Average is calculated on the basis of the sum insured( SI)divided by the Value At Risk (what it should be insured for ) multiplied by the loss. Exactly as Brendan outlined..ie
SI 50,000
VAR 100,000 X Loss 10,000 = 5,000.
All commercial policies have a condition of Average. Only a few household policies have Average, most notably FBD. However, this is changing and over the next couple of years, I would think most household policies will have a condition of Average. Be warned against having a low sum insured to save on premium. It really is a false economy. Check out for rebuilding guidelines for house insurance. Also, make sure that the sum insured on contents is realistic.