Electric vehicle charging has been a hot topic for years, but it's mostly been a one-way discussion: how the EV charges from a given home charging system. Toyota turns the conversation around, experimenting with how the home can draw power from the EV. Toyota's vehicle-to-home (V2H) charging system offers two-way charging between the Prius Plug-in and a home power supply.

Obviously, the Prius Plug-in's small lithium-ion battery isn't enough to pull a home off the greater electric grid, but Toyota envisions the two-way system working as an emergency generator during blackouts. It also envisions the Prius Plug-in drawing power during off-peak hours and then feeding that stored power back to the home during peak times, helping to lower energy costs. Toyota says that the Prius Plug-in can power an average Japanese home for up to four days.

The V2H system equips the Prius Plug-in with an AC100 V inverter that converts the car battery's direct current into the alternating current used in homes. Communication between the car, the charger and the Home Energy Management System (HEMS) facilitates power flow between the car battery and home.

The V2H charging tech is part of the Toyota City Project, an ongoing research project on smart grid technology supported by Japan's Ministry of Economy, Trade and Industry. Toyota City is one of several regions selected to participate in METI's Next-Generation Energy and Social System Demonstration project. A series of model homes are serving as the test bed for the project. The homes use a combination of smart appliances, solar panels, household power storage, home energy management and two-way vehicle charging to stabilize power demands and optimize energy usage. The project also aims to optimize energy usage at the community level. According to Toyota, 40 households are currently participating.

The Toyota City Project began in 2010, with Toyota completing construction of the homes last summer. In one of the next phases of the project, Toyota will begin testing the V2H charging system in about 10 of the model homes.

Toyota isn't the only automaker working on a system that allows for car-to-home energy flow. Nissan has previously floated the concept and last week announced the "Leaf to Home" system . Like Toyota's system, the Leaf to Home uses special charging equipment to enable the Leaf to send electricity from its 24 kWh battery back into the home. Nissan says that the Leaf can power an average Japanese home for two days. It plans to bring the system to Japanese showroom this month.

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