Bidirectional Charging 101: Vehicle-to-Home

What is Vehicle-to-Home (V2H)?

Previous generations of electric vehicles (EVs) simply drew electricity from the grid or perhaps a behind-the-meter generation source like solar to charge their batteries. Now, bidirectional charging unlocks your EV's potential as a mobile energy hub - sending power back to your home when you need it most. Today’s EVs have large batteries and hundreds of miles of range. Tapping into this reservoir of energy with vehicle-to-home (V2H) technology turns your car into a powerhouse that can back up the average household over a multiple day outage.

When paired with solar panels or a dedicated home battery, you’ll improve your control, backup duration, and self-sufficiency.

Equipment Requirements for Vehicle-to-Home

All vehicle-to-home systems require the following equipment:

• Bidirectional-capable EV: EV batteries are charged through unidirectional charging – pulling electricity from an external source to store as chemical energy in their batteries. Bidirectional-capable EVs can also discharge the energy stored in their batteries back out to external systems.
• DC to AC inverter: When your car’s battery discharges, it converts stored chemical energy to electrical energy resulting in a Direct Current (DC) flow of electricity. To power the equipment in your home, this must be converted to Alternating Current (AC). V2H systems require a DC to AC inverter; in some systems this is located in the car, in some systems it’s external.
• Bidirectional EV Supply Equipment (EVSE or charger): A specialized charger that allows power to flow both to and from your EV’s battery.
• Islanding Controller: A device that detects when the grid is down and switches your home's power source from the grid to your EV while preventing back-feeding into the grid. This enables the vehicle to start powering your home.
• Service Panel Capacity & Dedicated Circuit: Homes with 100A or smaller service panels may need physical upgrades to handle V2H power flow. Also, a 40 – 60A circuit breaker for hardwired connection of your charger to a 240/120V electrical panel will be needed.

Estimating Backup Runtime from Vehicle-to-Home

To estimate your backup runtime, start by finding how much energy you use in a typical day. This is measured in kilowatt hours (kWh) and can be found on your electricity bill. Learn how to interpret your electricity bill here. According to the US Energy Information Administration, the average American home uses around 30 kWh per day¹. Next, identify your electric vehicle’s battery capacity (kWh) by checking the manufacturer’s specifications, and divide battery capacity by daily usage to arrive at backup runtime.

Approx. backup runtime = Battery capacity (kWh) / daily usage (kWh)

For example, if your vehicle’s battery capacity is 90 kWh, and your home typically uses 30 kWh/day, your car’s battery can power your home for roughly three days.

3 days = 90 kWh battery / 30 kWh daily usage

Note that this calculation assumes you are starting with a fully charged battery. Be sure to consider use your actual state of charge to estimate actual runtime in an outage.

Estimating Backup Runtime from Vehicle-to-Home with Solar

Installing rooftop solar where you charge your bidirectional-capable EV increases your backup runtime by powering your home directly during daylight hours and recharging your EV battery with any excess generation.

For a simplified estimate of how solar increases runtime, you’ll need your average daily solar generation in addition to the numbers above.

Since solar panel production only occurs during daylight, start by dividing your daily usage (kWh) in half to reflect a typical 50% day / 50% night pattern (adjust based on your actual habits for better accuracy). Subtract daily solar generation from your daytime usage. Depending on whether you’re a net producer or user of energy during the day, you’ll charge or draw the difference from your EV battery in an outage.

Approx. backup runtime = Battery capacity (kWh) / [Nighttime usage (kWh) + (daytime usage (kWh) – solar generation (kWh))]

Continuing with the above example, assume your home uses 30 kWh per day (15 kWh during the day and 15 kWh at night), your car’s battery capacity is 90 kWh, and your solar system generates 10 kWh daily. In this scenario, you draw just 5 kWh from your EV battery during the day, adding an extra day and a half of outage protection.

4.5 days = 90 kWh / (15 kWh + (15 kWh – 10 kWh))

If your solar generates more energy than you use during the day, your runtime will be even longer. Assume instead of 10 kWh your solar system generates 20 kWh per day.

9 days = 90 kWh / (15 kWh + (15 kWh – 20 kWh))

In this example, you draw 15 kWh from your EV battery at night and recharge it 5 kWh every day, doubling your runtime over the previous example’s.

These are simple examples for demonstration purposes. A more precise estimate should also consider overall system efficiency, more precise daily usage rates, and external factors such as weather.

Benefits over Fossil Fuel-Powered Generators

Historically, fossil fuel (natural gas, gasoline, or diesel) generators served as homeowners’ only option against grid outages. Today, battery-powered home backup provides many benefits over traditional backup generators. Benefits include:

• Quiet: Fossil fuel-powered generators are loud, producing noise ranging from 60 to 90 decibels at 20 ft., depending on the model, size, and design. During extended grid outages, this can create a persistent hum and rumble from multiple units in a neighborhood, while the noise from battery-powered backup is negligible.
• Clean air: Fossil fuel-powered generators emit pollution and degrade local air quality. Pollutants can include carbon monoxide and increase the risk of carbon monoxide poisoning in the home if the generator is not positioned and vented safely. Bidirectional charging produces no exhaust, offering a cleaner, safer alternative.
• Less maintenance: Generators require regular service to ensure reliability, efficiency, and safety - weekly, monthly, or annually, depending on usage rates, and after a certain amount of runtime². Just as EVs require less maintenance than gas-powered vehicles³, battery storage requires little to no maintenance in comparison to conventional generators.
• Potential for energy independence: Generator fuel depends on a fragile supply chain that is vulnerable to disruptions and cost spikes, and reliant on electricity at every step. Batteries from your EV paired with rooftop solar provide reliable, grid-independent power from your own property.
   

Given its substantial capacity, minimal maintenance, quiet operation, and zero emissions, V2H offers many homeowners a substantial upgrade over fossil fuel generators.

Is Vehicle-to-Home Right for You?

Consider the following factors to determine if V2H is the right backup solution for you.

• EV ownership: If you have bidirectional-capable EV, it’s tough to beat the value of V2H for outage protection. For example, the hardware for Powershare Home Backup, Tesla’s V2H solution, is $1990 and standard installation starts at $1800-$2200. The bundle includes a Universal Wall Connector, so, for roughly $4000, you can charge at home when the grid is up and power your home using your EV in an outage.
• Outage frequency and duration: Places like California, Nevada, Texas, and Florida often face prolonged outages from wildfires or extreme weather. Increasingly, utilities cut power before a wildfire starts to reduce risk in what are known as a Public Safety Power Shutoffs⁴. In areas with unreliable grids, backup power delivers essential peace of mind. Consider the frequency, duration, and impact of outages in your area.
• Intended daily power usage in an outage: Depending on the size of your home and power needs of the loads you’re backing up, you may want to supplement your EV’s battery capacity with home backup batteries. One of the primary benefits of a dedicated home battery is its ability to handle large loads requiring large amounts of power when they initially turn on. This is commonly referred to as LRA or Locked Rotor Amps, and appliances such as air conditioners typically exceed the rating your vehicle can support. As a result, you’d have to either exclude these loads from your backup circuit if opting for a V2H only system or upgrade to a dedicated home battery to be used in conjunction with V2H.
   

To learn more about Tesla’s approach to vehicle-to-home technology: