EV Charging With a Home Battery

An EV is the biggest single load most homes ever add, and a home battery is the biggest single store of energy. The Alternative Fuels Data Center reports that most EV drivers charge overnight at home using Level 1 or Level 2 equipment. This guide explains the charging levels, how they compare with a battery's output, and what pairing the two means for your power planning.

Home charging levels

Home charging comes in two speeds. The Alternative Fuels Data Center puts Level 1 charging on a standard 120 volt outlet at approximately 5 miles of range per hour, which suits a plug-in hybrid or a low-mileage driver charging overnight.

Level 2 charging is much faster. The Alternative Fuels Data Center reports Level 2 on a 240 volt circuit at approximately 25 miles of range per hour, with most residential chargers delivering 7.2 kW at up to 30 amps. That speed makes Level 2 the practical choice for a full-battery EV charged at home each night.

The overnight charging window

Timing is what makes EV charging pair well with storage. The Alternative Fuels Data Center notes that most EV drivers charge their vehicles overnight at home using Level 1 or Level 2 charging equipment.

Overnight is off-peak on most time-of-use rates, so charging then is already cheaper. A home battery can extend that logic by storing off-peak or midday solar energy and feeding part of the charge, which shifts even more of the EV's demand away from expensive grid hours.

Battery output versus charging load

Whether a battery can charge a car depends on how its power output compares with the charger's draw. EnergySage lists the Tesla Powerwall 3 at 11.5 kW of continuous power, which comfortably exceeds a typical 7.2 kW residential Level 2 charger.

Capacity is the tighter constraint. A 7.2 kW charger running for several hours can consume more energy than a single battery holds, so using storage to fully charge an EV can drain a home battery quickly. Many households instead let the battery cover part of the charge or reserve it for home backup while the grid handles the car.

Planning the combination

Pairing an EV and a home battery works best when you decide their roles up front. If the priority is backup, reserve the battery for the house and charge the car from the grid during off-peak hours. If the priority is maximizing self-consumption of solar, route surplus daytime production into the battery and draw on it when the car plugs in at night.

Either way, the numbers guide the design. A 7.2 kW Level 2 charger adding about 25 miles per hour tells you how long the car needs power, and your battery's capacity tells you how much of that the storage can supply before the grid takes over.