Bryan Palmintier is a fellow on the Energy and Resources Team at Rocky Mountain Institute

Imagine heading to the office one day in the not-so-distant future. You park, press a button on your dash, plug in your car, and head inside. 

After work you unplug, jump inside your car, and head home, where you do the same thing: park, press a button, plug in, and go about your business. 

Sounds easy, right? 

But behind the scenes, things are very high-tech. When you plug in, rather than simply charging your car's battery, the power grid and your battery are in constant communication. The battery charges when there is excess power available on the grid and discharges to provide energy back to the grid when it is needed. 

To the car owner, the system is transparent -- no meters to monitor, no hardware to manage. The only difference is that at the end of each month, you'd see two big differences on your electric bill: 

1. There would be a list, similar to your cell phone bill, itemizing the details of each period of grid services provided, and
2. An on-bill credit for supplying services to the grid

Welcome to world of vehicle-to-grid -- or V2G for short.

Mobile power plants
The average car in the United States produces about 150 kilowatts (200 horsepower) at max power, which is about 400 times less than the average electric power plant. But there are well over 200 million cars in the United States alone, compared to only about 17,000 power plants. That means there is over 30 times the power generation capacity in cars and light trucks than in the entire power grid. 

Though it would be possible for vehicles, if electrified, to temporarily provide more than enough power to run the entire grid, it is unlikely that we can use electrified vehicles to replace electric generators. 

Why not? There are two big reasons. 

First, the maximum amount of energy (power supplied for a period of time) that will be stored in electrified vehicles would only be enough to run the grid for a few hours or even less if energy is saved for driving. And, second, vehicle batteries are only a storage system; the energy they contain has to be generated somewhere else, in this case from other sources on the power grid. 

It is also important to note that as vehicles become lighter-weight and electrified, the maximum power per vehicle will come down significantly. But even if it were 10 times lower, there would still be substantial power capacity available in vehicles, even if only a fraction of the cars participated in V2G.

Giving back power
The simplest V2G strategy is to charge vehicles at night, when electricity use (and price) is lowest, and then use the batteries to provide power during the peak demand (typically in the afternoon). This way, V2G could replace the so called "peaker" power plants, which run for only a few hours to meet the highest loads, and are typically the least efficient and most polluting. 

To be effective however, this peak shifting would require millions of V2G-capable vehicles on the road (or, more precisely, parked in the afternoon), which will take some time to happen. In the short run, the most promising use for V2G is to provide "ancillary services" to help stabilize the grid.

Ancillary what?
Ancillary services are utility services that provide increased power quality and allow the grid to respond to compensate for unpredictable variations and events. 

Today, there is essentially no storage on the grid. Every time someone turns on a light, an air conditioner kicks in, or more importantly a large industrial factory starts up, the grid responds by calling on generators to produce more power. 

When averaged over thousands of customers, the resulting pattern of electricity usage is surprisingly predictable, and utilities plan in advance to turn on and off generators or buy electricity from elsewhere on schedule to meet the changing load. But some power fluctuations are unpredictable, and many of these are compensated for by ancillary services. 

How vehicle-based power storage can help
Historically these services have been provided by utilities directly by varying the output of traditional thermal plants. But in areas with deregulated power markets, roughly half of the needs for these services are filled by bids on the open market, allowing generators to focus on, well, generating. 

In such systems, payments are made for being connected and available to provide ancillary services, plus payments for any energy exchanged. As a result, there are ways for V2G to provide services when charging, while maintaining charge, and for providing energy only in times of critical power needs. 

The ability to provide V2G services with only occasional discharges is important, since today's battery technology loses a noticeable bit of life with each charge/discharge cycle. Advanced battery technologies just emerging on the market can be cycled a total of 10-20 times more, making V2G with frequent, significant discharges a more attractive option in the not-so-distant future. 

How big is this opportunity? In northern California alone, it would take 20-30 thousand V2G cars to provide the ancillary service market needs, representing a compelling opportunity for large scale pilot projects and initial production vehicles.

V2G and renewables
Another exciting possibility for V2G is to help compensate for (or "firm") the variable output of intermittent renewables such as wind and solar. 

When the wind blows or the sun is out, vehicles could charge their batteries, while still leaving plenty of power to run other loads on the grid. Then when the wind slows, or a cloud covers the sun, power from the vehicles would be used to continue to provide the same level of power to the grid.

A win-win strategy
All in all, V2G has the potential to be a big win for everyone involved. 

Car owners will get some money to help offset the cost of their battery. Utilities will get access to valuable storage, which can be used to provide services to the grid. 

And everyone benefits from cleaner air, a more reliable power grid, and reduced greenhouse gas emissions.