The joule rating is commonly quoted for comparing MOV-based surge protectors. An average surge (spike) is of short duration, lasting for nanoseconds to microseconds, and experimentally modeled surge energy can be less than 100 joules. Well-designed surge protectors consider the resistance of the lines that supply the power, the chance of lightning or other seriously energetic spike, and specify the MOVs accordingly. A little battery charger might include a MOV of only 1 watt, whereas a surge strip will have a 20 watt MOV or several of them in parallel. A house protector will have a large block-type MOV.