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Charging Your EV at Home, Things to Consider

Posted by Charley Cormany, EFCA Executive Director

Electric vehicles (EVs) are becoming commonplace in California. In some neighborhoods, it seems like every other car is an EV. The rapid adoption is helping reduce vehicle emissions, which represent 40 percent of the state’s emissions, and that’s a good thing. EVs are starting to become a viable and often preferred alternative to their gas counterparts. However, as with many new technologies that represent change, there are some challenges to consider.

Charging at home is the best way to take advantage of your EV and reduce range anxiety. However, there is a fair amount of confusion over how much electricity is required to charge an EV using the electricity supplied to your home.

To make smart choices about home charging, you need to understand some fundamental concepts about EVs.

  • Let’s start with battery size. EV batteries are rated in kWh (Kilowatt hours). The bigger the battery, the longer it takes to charge. Most electric vehicle batteries are between 60 kWh and 100 kWh, but some fall outside this range. Big, heavy vehicles, like the Ford F-150 Lightning, have two battery options: the standard is 98 kWh, and the extended-range battery is 131 kWh. A plug-in hybrid, like the Prius Prime, uses a much smaller battery with a capacity of 13.6 kWh. Knowing the capacity of your battery will influence your home charging solutions.
  • Your driving habits will also impact home charging. Do you most often take short trips around town, or do you have a long commute every day? Depending on your answer, you might need to charge your EV less than you expect.
  • Most people think charging their vehicle is like filling up with gas. They drain the tank to near empty and then fill it full again. In practice, charging an EV is more like charging your cell phone or laptop. I suspect you seldom run your phone or computer to one percent or less before you charge it.
  • Do you know there is really no drawback to plugging in your EV every day, even if it does not need to be charged? Most EVs include software that protects the batteries from overcharging. Plug it in when you get home and you are always ready to go.
  • Battery technology impacts charging as well. Most EVs today use Lithium-Ion (Li-ion) technology, just like your phone, laptop, or power tools. Li-ion batteries work best if they are charged between 20 and 80 percent of their capacity. If you want to extend battery life, don’t deplete them to 1 percent or charge them to 100 percent unless you need to. This is why some manufacturers suggest you only charge them to 100 percent if you are planning a long drive the next day. We are starting to see a new type of battery enter the market: Lithium Iron Phosphate (FLP). Regularly charging LFP batteries to 100 percent is no issue. The technology has some downsides, but that’s another discussion.

Next, let’s explore the types of chargers available by order of capacity.

  1. Level One: Level one chargers use the existing wall outlets in your home or garage. The circuits provide 110 volts of electricity, which is what we are used to. If you are charging a small battery, like a plug-in hybrid, you will likely be fine using your existing 110v infrastructure.
  2. Level Two: Level two chargers use 240v circuits. Most homes have a few 240v circuits as they are typically required for high-draw appliances such as cooktops, ovens, electric dryers, electric water heaters, air conditioners, and other appliances that need more electricity than 110v circuits can provide. Most home EV charging solutions rely on 240v circuits and are level-two chargers. Most can operate on a variety of power levels, which is determined by the circuit breaker and wire size used in the electrical run.
  3. Level Three: Level three chargers are often referred to as fast chargers. Most use direct current (DC) instead of alternating current (AC). Level three fast charging allows you to rapidly charge the vehicle, sometimes in thirty minutes or less. Level Three chargers are typically found along highways or in other public places. Tesla Superchargers are level three chargers. I don’t suspect many homes, if any, would support level-three charging.

The more powerful the charger, the faster the battery will charge. However, there are some considerations. Rapid charging creates heat in the battery pack and may cause degradation over time. Cold weather extends the charging times, which is something you should consider if you’re taking a long trip. Many of today’s EVs have battery cooling and heating systems that ensure the pack stays at an optimum temperature during charging.

Let’s talk about Level Two charging, as this is what most EV owners will use at home, work, or overnight at a hotel. One of the most common questions is, how many amps (a measure of the electrical capacity) of electricity do I need for level two charging at home? If you call an electrician and ask for a home EV charger installation, the default solution will be a 50-amp circuit breaker and a new electrical run to your EV charging equipment.

Some vehicle manufacturers suggest a 60-amp circuit, or even 90 for big EVs. Adding 60 amps or 90 amps to an existing electrical panel may require a panel upgrade, and that gets expensive. If you drive an F-150 long distances every day and use the truck as a generator for your power tools on-site, you may need a 90-amp circuit. For most people, 50 amps is plenty, and I will make the case that it is more than you really need.

We manage a rebate program for the Sacramento Municipal Utility District (SMUD). SMUD supports rebates for EVs through the Charge@Home rebate program. Incentives are available to cover some of the costs of home charging, such as installing an electrical circuit for the charger and the chargers themselves.

To qualify for the incentives, SMUD limits the circuit breaker size to 40 amps. We get a fair amount of pushback on this limit, but in practice, I think it’s a smart solution. The reason SMUD limits the breaker capacity is to reduce the impact of EVs on grid infrastructure.

Let’s consider how most people charge their EVs. You come home at the end of the day and plug your car into the charger. A best practice is to configure your charging to occur after midnight when electric rates are cheaper. If this is your standard practice, do you really care if it takes 8 hours to charge versus 6? So long as the battery is ready in the morning, you are good to go.

All EV charging circuits have some reserve capacity built in (typically about 20 percent), which is why the maximum output is less than the breaker size. The following chart provides details on how much electricity is provided in a variety of scenarios. Notice that the max amp draw is less than the circuit breaker capacity. As you can see, the larger the breaker, the more power output in kilowatt-hours (kWh).

In this example, the last column shows how many miles you add for each hour of charging for a Tesla Model 3. Battey size impacts charging miles per hour. The bigger the battery, the longer it takes. If your vehicle has a large battery, you may need a larger circuit. (Source – Tesla’s website)

Circuit (amps)Max Output (amps)Power outputTesla Model 3 (MPH)
240 volts (Level 2)   
604811.5 kWh44
50409.6 kWh37
40327.7 kWh30*
30245.7 kWh22
110 volts (Level 1)   
20163.8 kWh15
15122.8 kWh11

If, like most people, you drive less than 50 miles per day on average, and charge overnight, the rate of miles replenished per hour is not a huge factor. In fact, a Level 1 charger might meet the bulk of your charging needs. Knowing your vehicle’s charging capacity could save you money on your EV charger. For example, some Tesla configurations (*Model Y rear-wheel drive and Model 3 rear-wheel drive) are limited to 7.7 kWh per hour. Installing a 50-amp or 60-amp service would be a waste of money for those vehicles. Make sure to take this into consideration as you are considering what vehicle meets your needs.

As I mentioned earlier, the standard EV charger uses a 50-amp breaker. There are several good reasons to use a smaller electrical circuit for charging your EV.

From the utility perspective, supporting 40-amp chargers versus 50, 60, or 90 amps is a big deal.

With 40-amp chargers, an entire neighborhood could adopt EVs without transformer and feeder line upgrades. On the other hand, if every home installed a 60-amp circuit, the neighborhood might require transformer and wire upgrades, a costly endeavor that is often passed on to the end user, the homeowner. Smaller amperage chargers also put less strain on your battery.

Ideally, you should charge your EV to meet your needs and driving habits, and not more.  It’s a bit like Goldilocks. If the circuit is too big, your vehicle might not be able to take advantage of the extra capacity, which means you paid more for your electric run and charger than you need to. If the circuit is too small, your vehicle might not reach a fully charged state overnight, and that could be a problem.

I think most EV drivers will be able to meet their needs with smaller chargers than they imagine. Smaller circuits are cheaper to install, place less strain on the electrical distribution grid, and are easier on your EV battery. If your charger has multiple amp settings, I suggest using the smallest one that meets your needs. This will have less impact on grid loads, which is becoming more of a concern as we electrify our buildings.

If your electrical service panel’s capacity limits you, you have a couple of options. Load sharing and load management devices are gaining traction and might be the perfect way to avoid a costly and time-consuming electrical service panel upgrade.

Load-sharing devices take advantage of limited-use appliances, by allowing you to use two appliances with one circuit. How much do you really use your electric clothes dryer, for example? A couple of hours a week? Most load-sharing devices allow you to prioritize one device over the other. For example, you can make the clothes dryer the priority device and the EV charger as the secondary device. If you are doing laundry and the EV is charging, the EV will be put on hold until the laundry is done. When you consider that most folks are charging overnight, there is seldom a conflict unless you like doing laundry at 2:00 am.

Load sharing is not limited to laundry equipment; you can share a circuit with an electric oven, an electric cooktop, potentially an electric water heater, and the list goes on. If you have an older home with a smaller electric panel, a load-sharing device might be the perfect way to avoid a costly panel upgrade.

Another strategy is a load management device. These are similar to load-sharing devices but work a little differently. Typically, you install a dedicated electrical service to the EV charger and the load management device is installed in line with the electrical supply. Load management devices measure the load on the electric service panel in real time and will shut off the EV charger if the panel reaches a pre-determined capacity. A common threshold is 80. Again, this is a great way to support an EV load with a smaller service panel.

If budget is not a concern, there are “smart” electrical service panels that include sophisticated load management software tools. Smart panels can be a great way to manage your site-produced electricity; they can send it to your batteries when there is surplus generation and manage loads when there are lots of devices drawing electricity at the same time. If you have solar PV with storage and an EV, a smart panel might be a great solution.

The final consideration is your electric rates. Are you aware that most utilities have special rates for EV users? Many utilities offer discounted rates after hours, such as between midnight and 6:00 a.m. when grid loads are at their lowest levels. Make sure to check with your electricity provider and look at their EV rate plan options; they might significantly impact your electric bill and EV charging costs.

Home charging is the cheapest way to charge an EV. If you have the ability, it can be a game changer.  Having the ability to charge your vehicle at home while you sleep is something you can’t do with your fossil fuel-powered vehicle.

If you are considering an EV, I suggest you try renting one for a few days and find out how easy or difficult charging can be. I have done this and experienced a carefree experience with DC fast chargers. I have also experienced range anxiety when my charge was low, and all I could find was level two chargers, which were too slow for my needs.

Do your homework and consider the vehicle battery size and your current electrical service capacity. Far too many people buy an EV first and then figure out home charging. In practice, I think most people can get by with a low-power charger, even on a car with a fairly large battery, but it’s worth thinking about before you purchase your vehicle.  

EVs are here to stay, and understanding your charging options should be part of your purchasing consideration.  Hopefully, this information will help you figure out what works best for your situation.

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