What size EV charger do you need?

When it comes to reducing your personal carbon emissions, perhaps the biggest change you can make is switching from an internal combustion engine (ICE) car to an electric vehicle.

This is true even when your local electric grid includes fossil fuel power plants. You can use the EPA’s excellent Beyond Tailpipe Emissions Calculator¹ to find out the difference in emissions between an EV and a gas car. For me, my EV has less than a tenth of the emissions of the average new ICE car.

Lower operating costs and lower emissions are some reasons why a lot of people are making the switch to EVs. But many people are unfamiliar with EV ownership. Perhaps the biggest question people have – and one I’ve often been asked by curious drivers when I’m using a public charging station – is how do charge an EV at home?

If you have a parking spot with an ordinary 120-volt power outlet, you might be surprised to learn that might be all you ever need to charge your car. For others who drive longer distances every day, that won’t be adequate, and an upgrade to a 240-volt line will be needed.

What works for you depends on your vehicle and how much you drive every day. There’s also the special case of extreme cold – we’re talking when it gets down to about 10°F or so – where 120-volt charging stops working well.

The table below will help give a rough idea of what size of EV charger is suited to your driving needs, and then we’ll get into the details of how to make the right decision.

How much range can an EV charger add overnight?

Here’s a chart that describes roughly how many miles of range you can add in a 10 hour charging session for some popular vehicles:

As you can see, there’s quite a large difference in how much range you can add to your EV battery during a charging session, and it depends on the estimated fuel economy, the electrical circuit, and how much charging time your car will have every time. You can look up the estimated fuel economy for any EV or plug-in hybrid at fueleconomy.gov².

Fuel economy for EVs can be expressed in different ways, but the federal government uses “miles per gallon equivalent”. The “equivalent” refers to the energy contained in a gallon of gasoline if that gallon was converted to electricity with 100% efficiency, which happens to be 33.7 kWh. So, MPGe is the number of miles that an EV can drive on 33.7 kWh of electricity.

What is Level 1 and Level 2 charging?

As an aside, you will often hear reference to Level 1 and Level 2 charging. Level 1 refers to charging at 120 volts, and Level 2 is charging at 240 volts. Both L1 and L2 charging use alternating current (AC).

DC fast charging, sometimes called L3 charging, uses direct current (DC) instead of AC current. L1 and L2 charging speeds are limited by the AC-to-DC converter that is onboard the vehicle, while DC charging speeds are limited by the battery architecture.

Volts, amps, and circuit breakers

An ordinary household outlet is 120v and may support either 15 amps or 20 amps of current. (If it’s a 20 amp circuit, one prong of the outlet may be t-shaped.)

240v electrical circuits in a home are reserved for large appliances such as electric clothes dryers and ranges. In rarer cases, you may find a 240v outlet in a garage, where they might be used to power a heavy duty tool or recreational vehicle.

When it comes to 240v, 30 amp and 50 amp outlets are the most common in households, but there is a large number of outlets types that you could potentially encounter.

Looking at the outlet is one way to identify the rating of the circuit, but ultimately you want to go to your electric panel and find the breaker that protects the circuit, where the amperage of the breaker will be clearly labelled.

EV chargers can be plugged into one of these 240v outlets, but it’s often a better choice to hardwire the charger if possible. Hardwiring means that the charger doesn’t plug into an outlet, but is connected directly to the circuit. This is slightly safer because cheap outlets have been known to melt or catch fire when subjected to continuous loads.

The 80% rule for continuous loads

One important detail is that you can operate your EV charger only at 80% of the maximum amperage of the circuit. This is known as the 80% rule, and it applies to any continuous load on an electric circuit, not just EV chargers. According to NEC code, a continuous load is one with a 3 hour or longer duration. This rule prevents a circuit from overheating.

This means that if you are charging your car using an ordinary 15 amp household outlet, the maximum you can set your charger to is 80% of 15, which is 12 amps. For a 30 amp circuit, 80% is 24 amps, and for a 50 amp circuit 80% is 40 amps.

The table above of charging times takes this 80% rule into account.

How to calculate charging times

To calculate charging times yourself, multiply voltage by amps to get watts. For example, if your EV charger is connected to a 240 volt circuit and is set to 24 amps, multiply the two numbers:

240 volts × 24 amps = 5,760 watts

Usually this would be expressed as kilowatts, which is 5.76 kW in this case. If you charge for one hour, that’s 5.76 kilowatt-hours of charge per hour.

10 hours of charging at that rate would be 57.6 kWh. Many EV batteries today have a capacity of about 60-80 kWh, so those 10 hours of charging would be enough to nearly fully charge a depleted battery for most cars.

To convert that to added range, you need to know your average fuel economy, which will be displayed on your EV’s dashboard. Multiply your fuel economy figure by the amount of charge to calculate added range. For example, I get about 3.6 miles per kWh in typical highway driving. If I gain 5.76 kWh of charge in one hour, I multiply the two numbers:

3.6 miles per kWh × 5.76 kWh = 20.7 miles

This means I would gain about 21 miles of range for every 1 hour of 24 amp L2 charging.

Efficiency losses: L1 vs L2

One additional detail is that charging isn’t 100% efficient. Some energy will always be lost as heat in the charging equipment and in the battery. Those losses are smaller with higher voltages, so L2 charging is more efficient than L1 charging.

The actual losses will depend on a number of factors, such as the length of the wire from the EV charger to the electrical panel. (Longer distances result in higher losses.)

As a rough estimate, in the table of charging times above I’ve assumed 95% efficiency for L2 charging and 90% efficiency for L1. You’ll probably find that your efficiency differs from this guesstimate, especially during temperature extremes.

Cold and hot weather EV charging issues

Lithium batteries can be damaged by charging at very cold or high temperatures. To avoid damaging the battery, an EV will either use a battery heater to warm up the battery in cold temperatures or slow down charging in high temperatures.

With L1 charging in extremely cold weather, so much electricity can be diverted to the battery heater that very little is left over to actually charge the battery.

How cold is too cold? One Reddit user in Northern Canada tested charging a Tesla with an L1 charger at -25°C (-13°F) that normally delivers 1.4 kW of electricity³. They found that all of the energy was consumed by the battery heater, and none was left over to charge the battery. When temperatures were a little warmer at -20°C, they found that charging was able to work, but only at 40% efficiency.

This is less of a problem with L2 charging, which can deliver much higher wattage. There will still be losses for battery heating, but the higher wattage will allow the battery to charge.

Very high temperatures are also a problem, and can result in the car using active cooling or throttling the charging to prevent the battery temperature from rising too high. However, this tends to be less of a problem than cold temperature charging because battery cooling using fans is less energy intensive than operating a battery heater.

Understanding circuit breaker sizes

To find out exactly what amperage your charging circuit can accept, you’ll need to look at your electric panel. The panel should have the breakers labelled. If not, you might need to experiment by switching off breakers to find out exactly which breaker is for your EV charging circuit.

The amperage of the breaker will be clearly labelled as a number on the switch, like this 50 amp breaker:

Do you need to upgrade your panel?

A common question that people have is what size electric panel you need to support an EV charger.

This is usually the wrong question to ask. Instead, you need to know how much electricity your home will be using when you are charging your vehicle. For example, do you plan to run your heat pump, pool heater, and electric range at the same time that you’ll using a 50 amp L2 charger? If so, you’ll want to double check your panel has the maximum capacity to support it.

However, EV owners commonly charge their vehicles overnight, especially because of utility time-of-use plans that offer cheaper electricity at night.

Because your home will usually be using less electricity in the middle of the night – maybe the only major draw in addition to the EV charger would be air conditioning or space heating – most homeowners won’t need to worry about the capacity of their electric panels.

You can read more about this issue in my article on electric panel upgrades.

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