Data availability evaluation is the crucial problem to build large-scale, high available peer-to-peer storage systems by governing many unreliable hosts. However, many recent studies equate host availability with data availability and calculate data availability based on the traditional definition of host availability. By analyzing the trace of live P2P system, we argue that there is a significant difference between them and it is far more inaccurate and misleading to use host availability to evaluate data availability because of peers? incentives and rationalities. To achieve a precise data availability evaluation, firstly, we present an evaluation model which combines peers? incentives and host availability together. Furthermore, we illustrate that the traditional definition of host availability has serious drawbacks because it ignores the effect of time-ofday and peers? different online pattern. So we improve the accuracy of our model by considering both factors. Finally, we introduce how to implement this model in practice and propose some its applications to P2P storage systems.