I need to compare two garch models, I try to do that by Value at Risk. In general, if I have an initial conditional variance, for example, h1, then I can predict the next N days conditional variance by a known garch model, whereafter, I calculate the VaR based on these conditional variance and make the comparison between two garch models, simply saying, to see how many times the daily return exceeds the VaR. My question is that: do I need to make a one day ahead predict based on the standard deviation from real data (get it by high frequency method) each time? or just make N days prediction based on the initial h1. Thanks.