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How to Choose a Home Charging Solution - Part 1 - Acronyms

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Most first-time EV buyers ponder the Home Charging question and there are a lot of foreign terms and concepts that take some effort to learn before making a choice. The purpose of this series of posts is to act as a bit of a primer for those in search of a home charging solution.

Part 1 explores acronyms new EV owner may wish to understand to begin their exploration of Home Charging alternatives.
Part 2 covers some math, or how to calculate how powerful of an EVSE you may need (or not need).
Part 3 covers some characteristics of a variety of EVSE choices in the market (Smart vs Dumb, Portable vs Wall Mounted, Hardwired vs Plug in).

Unlike internal combustion engine vehicles (ICE), EVs live in the world of electricity which is something we often take for granted. Understanding some of the concepts can go a long way towards understanding how EVs work.
  • AC/DC - Alternating or Direct Current. Home electrical systems and the grid supply AC. Batteries store energy as DC. EV Motors are typically AC. So, inverters are used to change AC to DC, and vise versa.
  • kW - Kilowatt. A measure of power at a given moment. A kW is 1000 watts.
  • kWh - Kilowatt hour. A measure of stored or consumed energy over a 1 hour period. This is typically the unit of measure utilities use for billing, and often used to represent the storage capacity of an EV battery pack. Thus, if you use or supply 5 kW for an hour, you will have consumed or supplied 5 kWh of energy.
  • A - Amperes or Amps. A measurement that describes the amount of electrical current flowing through a system. An important additional consideration is heat, higher Amps generate more heat. We will touch on this in a subsequent part of this series.
  • V - Volt. A measure of electrical force. Homes are typically wired for both 120V, and 240V. A bit more below.
  • NEMA - National Electrical Manufacturers Association, a body which approves standards for electrical equipment. Electrical outlets and plugs are often described by the NEMA desigation, for instance the common 3 prong 120V plug and outlet in our homes is a NEMA 5-15p (plug) or NEMA 5-15r (receptacle\outlet). A chart is supplied below to help identify the plug and outlet types and electrical ratings for each.
  • EVSE - Electric Vehicle Supply Equipment. EVs are commonly equipped with an on-board "charger", or more accurately an inverter which converts AC from the grid to DC for the battery. The "charging" cord, unit mounted on the wall, or public charger is therefore a means of supplying power to an EV, thus EVSE.
A few common calculations can come in handy.
  • kW = (V * A)/1000. Example: 240V * 32A = 7,680 watts, or 7.68 kW. Understanding this can be extremely helpful when choosing an EVSE or public charger.
  • Miles/kWh. This is the EV equivalent of MPG in an ICE car. The calculation is simply miles/kWh where kWh is the measure of a quantity of energy like a gallon of gas. Most EVs will operate at around 3-4 mi/kWh in normal conditions, perhaps a bit lower at highway speeds and in colder temperatures.
  • Wh/mi. Tesla and some other EV manufacturers prefer using this measure of efficiency. The logic is sort of reversed here, the lower the consumption (Wh) per mile, the higher the efficiency. For example, 250 Wh/mi = 4 mi/kWh and 330 Wh/mi = 3 mi/kWh. It takes a little more calculating to make the comparison meaningful to an ICE owner.
  • MPGe is the EPA equivalent efficiency rating used for alternative energy vehicles. Since the EPA is firmly established in the MPG concept for ICE vehicles, they sought a formula to state the equivalent for alternative fuel vehicles to simplify comparisons. According to EPA, a gallon of gasoline contains as much energy as 33.7 kWh of electricity. So, an EV owner with a 68 kWh capacity battery could say his\her EV has a 2 gallon fuel capacity (33.7 * 2 = 67.4 kWh), yet can drive 250 miles on those 2 gallons or 125 MPGe. Yes, EV are up to 95% efficient in converting energy to motion, most ICE are 20-40% efficient in converting fuel to motion with 60-80% of the fuel wasted as heat.
The Handy NEMA Plug Chart:
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A lot of great information here. Very helpful.
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