Dealing with contrractors I come across this all the time. There are some things you need to be aware of:
Force majeur is the group of exceptions that Jimmy mentioned. This usually includes flood, fire, acts of God, acts of war, and things beyond the supplier's reasonable control. This is pretty standard. If your panel shop isn't sharp enough to include these exceptions in the agreement they may be too stupid to do your work anyway.
Liquidated damages is the best basis for any penalty. The term usually refers to the cash equivalent of the damages to you (or your customer passed back to you) for lost revenue, increased operating expense, whatever. You and the panel shop can agree to anything, but if there isn't a reasonable basis for the amount selected it is harder to make it stand up in court.
Fair is fair. If you are supplying some material to the shop, the clock can't start ticking until he has everything in his hands.
Incentives for faster performance, as mentioned above, are a good way to keep your supplier from feeling abused. Usually the incentive is some percentage of the penalty.