Despite scalable computing resource facilities and industry's ability to accurately model individual components with simulations, fine-grained physical level power simulation for today's typical digital design is found to be too expensive, especially, when parametric variations need to be accounted for modern processes. This paper presents a new linear solver based on-line modeling method that exploit existing slow but accurate simulators to obtain highly accurate sub-circuit power estimates while using significantly less processing and memory resources. Comparative simulation runs for a number of benchmark circuits using industry standard SPICE simulators and our simulator suggest that our technique can accelerate existing simulation speed up to 100x with less memory without sacrificing power estimate accuracy.