What element must be present in order for the excuse of necessity to be valid?

The harm that would be inflicted by the action must higher than the harm inflicted by the offense that the defendant is being charged with.
Necessity can be used in many countries as justification for the criminal behavior but the defendant must prove that the deed was done in order to prevent some other greater harm. For example, the a person could drive over the speed limit so they won't be kidnapped. Also it is applicable when it comes to self-defense.  In the English common law there is no defense by necessity for the charge of murder. 

The damage to be be careful about was more noteworthy than the offense charged.    
An activity that is unsafe yet laudable is supported, though an activity that is hurtful yet should be pardoned might be pardoned.Litigants looking to depend on this contend that they ought not be held at risk for their activities as a wrongdoing in light of the fact that their acton was important to keep some more prominent mischief at bay and when that lead isn't pardoned under some other more particular arrangement of law, for example, self protection