This article focuses on simulation accuracy of photovoltaic (PV) systems designed for local energy usage in households. The results of simulations can be misleading when the input data is estimated not correctly or the input data has a low time resolution. Load profile is difficult to estimate due to the changes in user behaviour and possible high power fluctuations. In contrast to many other studies, 15s data are used as PV power and household load data input. Using this data for simulations we show the influence of averaging interval on the results and thus we indicate the variance in the simulated system performance (rates of self-consumption, energy independence and grid interaction).