Understanding EV Charging Basics: Amps, Volts, and Watts!

Volts, Amps, and Power

Electricity can be a daunting topic for some people. Rest assured, the basics are easy to understand with a little bit of practice, and we want you to master them. Importantly, we want to show you relevance and provide real world examples of how you can use this knowledge to be a more informed customer. We will go over:

  • Volts (V)
  • Amps (A)
  • Power (W)

A good way to remember the electrical basics is through comparison to a fluid system.

The energy of a river depends on how much water is flowing down the river, and the driving force in which the water flows. A river like the Bow, or the North Saskatchewan, have massive flows of water. We can increase the energy output of the river system by channeling them into smaller channels (increasing the resistance) to create a high-pressure flow of water. 

The same thought is used for electricity, but the terms are slightly different. The energy provided by an electrical circuit depends on how many electrons are flowing through the circuit, and the potential energy difference of a system that drives the electric current. We can increase the potential energy difference by increasing the resistance of a circuit. 

Current (Amps)

An electrical current is the measure of the number of electrons flowing through a location at a moment of time. Think ‘volume of water’ in the river analogy. Amps (A) measure the current of an electrical circuit. Amps are relevant to EV charging for two main applications.

Charger Capacity:

 This is typically measured in Amps, and this translates to how quickly the charger will charge your electric vehicle. The charger most commonly used in residential homes is a level 2 charger, and these range in size from 16A-48A (Amps). Assuming your onboard charger can handle up to 48A (some vehicles can only receive a maximum of 24A for example), then a 48A current will charge your vehicle 4 times faster than a 16A current (48/16=4). Another way to think about amperage is comparing it to different size straws. A smaller straw will require more effort and time to drink with, whereas a larger straw can move more fluid in a shorter amount of time. A small 16 Amp charger is comparable to a small straw, and a 48 Amp charger is more like a large straw.

    Home Service Capacity:

    Home service size is also measured in Amps. It’s the total amount of current flow your home can handle based on the gauge of wire coming into your property and the size of your panel to receive it. Electricians will complete a total load calculation on the panel which measures the total potential draw of all electrical devices in your home. This includes appliances, lighting, hot-tubs, electric ranges, and of course EV chargers. The average service size for homes in Alberta ranges from 60-125A, however larger homes may have 200+ Amperage service sizes. Service size is determined during home construction, but can be upgraded later (costly). 

    When you install an electric vehicle charger a load calculation must be completed to determine if the home has enough unused capacity to support it. Many homes are close to their maximum capacity due to add-ons like hot tubs and basement suites, so this step is very important (also required for permitting). If the home is at capacity you have a few options:

      1. Reduce the Charger Size: Customers will sometimes use a charger with less Amperage  to avoid completing panel upgrades or installing an energy management system. This is the cheapest option, but sacrifices charging speed for the electric vehicle. If your driving habits can accommodate less charging speed this is a good option. 
      2. Energy Management System Installation: This is our recommended option for customers wanting to charge at high speeds without completing an expensive panel upgrade. This system monitors the entire electricity usage of the panel, and will shut off the charger if there is a danger of going above the home service size. This is a good option because EMS systems are significantly cheaper than completing a panel upgrade, and once installed require very little maintenance.
      3. Panel Upgrade: This is the most expensive option and not recommended unless your home is already above its maximum capacity. As most homes now have underground service, this may require trenching a new line into the home as well as wiring in a higher capacity panel. This work needs to be completed in collaboration with your utility provider.

    Voltage (Volts)

    Is the electric potential of two points along a closed circuit. It is the driving force that drives the current around the circuit. Volts (V) measure the voltage of a circuit. A typical household outlet will supply 120 Volts. This will power small appliances like blenders or hairdryers, but is not enough for larger appliances like washers and dryers. Level 2 chargers (The type you would have in your home)  require 240 Volts.

    Power (Watts)

    Power is measured in Watts (W). It is the rate of electrical energy consumption or transfer. People will typically find power measured in kilowatts, kW (1000 watts). For example, your car charger may be able to charge at 11kW.

    A kilowatt-hour, kWh, is the total amount of energy used over a period of time. For example, you may use 10kW of power over 10 days in your home. You then have used a total of 100kWh of power over this time. Battery capacities are measured in kWh.

    To find the Power supplied by your charger it’s actually quite simple! Multiply the Amperage by the Voltage and you will have Watts.. For example, for a typical 40A charger operating on a 240V circuit the amount of Power will be calculated using this formula:

    Power = Amperage x Voltage

    Power = 40 Amps x 240 Volts = 9,600W = 9.6kW

    How do I know the power required to charge my electric vehicle?

    If you read the specification sheet for your specific car model it will indicate the onboard charge rate. By knowing this number, you can identify the maximum power your vehicle is capable of receiving. If the charger power is greater than the onboard charger receiving capacity, your vehicle will charge at it’s maximum rate. You can use this number to determine the size of charger you might want to install. Let’s look at two examples:

    2022 Audi E-tron with 11.6kW onboard charge rate:

    Working backwards, 11.6kW divided by the Level-2 charger voltage (240V) = 11.6kW/240V = 48 Amps. That’s a decent size on board charger! Knowing this information you may want a 48 Amp charger installed because your car is capable of handling the power, and will charge quickly! a 40 Amp charger would still be a decent choice due to its ease of installation, however a 16 Amp charger would likely be too small if your driving habits demanded a quicker charge and your vehicles battery was large.

    2021 Rav 4 Prime SE with 3.9kW onboard charge rate:

    Often plug-in hybrid electric vehicles will have smaller on board charge rates due to having smaller batteries and ranges. Working backwards, 3.9kW divided by the level-2 charger voltage (240V) = 3.9kW/240V = 16 Amps. You may be more inclined to do a smaller 16 Amp charger for this vehicle. a 48 Amp charger would still work, however the vehicle would never charge at that speed so it might be redundant. We would always encourage you to look at your residential situation holistically and consider the total installation cost, electrical capacity, and future EV purchases. For example, if the installation cost is the same you may want to purchase a 40 Amp charger because the cost will be the same, however you will be ready for potential future needs without having to upgrade again.