Power dissipated is indeed given by E^2 / R. If E increases by 4%, that means that E is multiplied by 1.04. Squaring 1.04 gives 1 + 2*0.04 + 0.04^2 = 1.0816, that is, a touch more than an 8% increase in power. The approximation (1+e)^2 =~ 1+2e is pretty good for e less than about 0.1. 73, ian K3IMW