In order to decide what is cost effective, you will need to know the following three factors. The expected cycles of your batteries, the price of your batteries and the cost per unit of electricity from your council. You can calculate the cost per kWh from your battery based on the expected life cycle of your batteries and their cost, and compare this to the cost you are being charged by the council. When used at night, Li-Ion batteries, which have far higher life cycles, are more likely to save you money.