You're going to be paying a standing charge no matter which provider you go with - so you can ignore that for your calculations. I've made the simplified assumption that I'm saving 35p for every kWh which passes through the battery. Perhaps sometime is costs more or less, but it will do as a simplified calculation. After a year, add up the total use. For solar and export, it is slightly trickier. I wrote a post about how to calculate solar savings.