to a lower price time, such as by chilling water at a low cost time for later use in air conditioning, or by charging a battery electric vehicle during off-peak times, while the electricity generated at peak demand time can be sent to the grid rather than used locally (see Vehicle-to-grid). No credit is given for annual surplus production. Excess generation Excess generation is a separate issue from net metering, but it is normally dealt with in the same rules, because it can arise. If local generation offsets a portion of the demand, net metering is not used. If local generation exceeds demand some of the time, for example during the day, net metering is used. If local generation exceeds demand for the billing cycle, best practices calls for a perpetual roll over of the kilowatt-hour credits, although some regions have considered having any kWh credits expire after 36 months. The normal definition of excess generation is annually, although the term is equally applicable monthly. The treatment of annual excess generation (and monthly) ranges from lost, to compensation at avoided cost, to compensation at retail rate. Left over kWh credits upon termination of service would ideally be paid at retail rate, from the consumer standpoint, and lost, from the utility standpoint, with avoided cost a minimum compromise. Some regions allow optional payment for excess annual generation, which allows perpetual roll over or payment, at the customers choice. Both wind and solar are inherently seasonal, and it is highly likely to use up a surplus later, unless more solar panels or a larger wind turbine have been installed than needed. Energy storage Net metering systems can have energy storage integrated, to store some of the power locally (i.e. from the renewable energy source connected to the system) rather than selling everything back to the mains electricity grid. Often, the batteries used are industrial deep cycle batteries as these last for 10 to 20 years. Lead-acid batteries are often also still used, but last much less long (5 years or so). Lithium-ion batteries are sometimes also used, but too have a relatively short lifespan. Finally, nickel-iron batteries last the longest with a lifespan of up to 40 years. A 2017 study of solar panels with battery storage indicated an 8 to 14 percent extra consumption of electricity from charging and discharging batteries. Adoption by country Australia In some Australian states, the "feed-in