Everything you think you know about EV charging is wrong

OK, maybe not everything. But if you’re one of those folks who make comparisons to the costs of using hair dryers or the costs of charging laptops and smartphones, you REALLY need to read this page. And if your business depends on knowing the costs of providing charging capabilities to your customers, you can’t afford to not educate yourself.

What is electricity and how do you pay for it

A surprising number of people have little or no understanding of what is on their electricity bills every month. Are you one of them? There are some basic recurring charges on your bills and of course everything is taxed. But the amount of electricity you consume (use) and pay for is specifically represented by a unit of measure called kilowatt-hours, or kW/h. A kilowatt-hour means 1,000 watts being used for one hour.

A device that consumes 200 watts takes five hours to consume 1 kW/h.

A device that consumes 2,000 watts takes only half an hour to consume 1 kW/h.

If you don’t know how much you pay for your electricity, by kW/h, and only think about the supposed national average, that may be your first mistake.

If you’re in the US, you can find a table of electric rates by state here. Some cities may be higher or lower but when this article was written, the costs of residential electricity state by state ranged from 10.5c (Washington) to 44.1c (Hawaii) per kW/h. That’s a WIDE range of electricity prices!

In addition to those figures, some utility companies charge more per kW/h at certain times of day, and some charge higher kW/h rates after consumption passes certain amounts. So it can be REALLY important to know how you’re being billed and for what.

Electricity for dummies

Do you need a quick primer on electricity and how it flows? It is good to have a sense for what you’re consuming and possibly providing to others. Don’t feel bad if you don’t know – not everyone can be an engineer. But you don’t have to be!

I like to think of electricity as being a bit like water, which you also pay for by how much you use. And how much you use depends on how long the spigot is turned on and how far it’s opened. Most folks understand water well enough. Unlike electricity you can see it and feel it – usually without risking your life – and many of us measure liquids routinely. You know what more or less water pressure feels like and you know that a drinking glass contains less than a bucket and both contain less than a swimming pool.

The units of measure for water would be gallons or some variation of cubic feet, on top of some delivery and maintenance costs. The more you use, the more you pay for.

Simply put, if you make water available at a higher pressure (in electricity terms, a bit like more voltage and therefore, possibly allowing more wattage), and someone shows up with a greater demand (to fill an empty swimming pool for example), your water may be consumed in more quantity, and possibly more rapidly, and your bill will increase for the amount used.

Electricity is not very different. If you use more, you pay more. Instead of gallons or cubic feet, the unit of measure for electricity consumption is the kilowatt-hour or kW/h.

In water terms, it takes a quart of water to fill the biggest drink cup but it could take a thousand gallons to fill an inflatable above-ground swimming pool. In parts of California for example, that inflatable pool could cost $15 to fill. But you’d have to refill a quart-size sports cup hundreds of times before the cost got near one dollar.

Likewise, a laptop with its relatively small rechargeable battery could take around 0.4 kW/h to fully charge and then use lightly for eight hours, but a popular EV such as a Hyundai Ioniq 5, with its many larger batteries, could consume as much as 80 kW/h to fully charge if it were near “empty”. At NY’s rates that laptop would run up 9c in someone’s bill (less if commercial rates, for example Starbucks) but recharging that car could potentially cost almost $18 in residential electricity.

The analogy becomes more difficult beyond this. Running water at pressures that are too high for the plumbing can result in burst pipes and hoses. And most of you know what happens to your water pressure when someone flushes a toilet while you’re showering. Consuming electricity at a rate that is too great for the wires and breakers or fuses can result in burnt fuses or tripped breakers, possibly wear out breakers and/or increase fire risks. It can also lower the voltage (pressure) to other devices on the circuit, which could cause them to malfunction.

It may be difficult to envision the flow of electricity but every device that carries or consumes it is labeled with a rating for how much it can carry (breakers, fuses, extension cords, power strips, receptacles and plugs) and the most it can consume (appliances and other AC-powered devices). Read the labels, and maybe as an educational tool buy a convenient meter such as a Kill-A-Watt.

What is a kW/h?

A Watt describes an amount of energy. Mathematically, it is a measure of voltage multiplied by amperage.

A kW/h means 1,000 Watts of energy consumed for one hour.

When you see electric devices and appliances rated in Watts, and not kW/h, that is because you’re not necessarily using them for an hour at a time.

So if you have a 110 Volt device and on the device has a label that says 80W, that means it can consume a maximum of 80 Watts, which happens to be about 0.73 Amps (80/110=.73). This is roughly what some laptops are rated for, and the actual requirement is less once charging is complete. If the laptop consumes 80W for two hours and 25W for the next eight hours, it will have consumed 0.36 kW/h.

A hair dryer is often rated directly in wattage, So too are some microwave ovens. A 1,000 Watt dryer or an 800 Watt microwave will consume roughly 1,000 or 800 Watts respectively, but only during the time they are turned on. If you use both for ten minutes one day, they will together have consumed only about 0.3 kW/h.

EV chargers are usually designed to limit themselves to demanding 80% of what is assumed to be a circuit’s rating. If you plug a typical EV charger into a standard US Type-A receptacle, called a NEMA 1-15R or 5-15R, this is assumed to be a 15 Ampere receptacle, 80% of which which works out to around 1,300 Watts. The car charger will continue to consume 1,300 Watts until the car’s battery is charged. But if the car was driven more than a few miles since the last full charge, that could be all day and/or all night.

Poor planning and hazards

For safety, because of the relatively high load an EV charger imposes on a circuit, most EV manufacturers recommend that the vehicle’s charger be the only active device on a circuit. Do you know what a circuit is and are you certain that the outlet being used for a car charger is the only active device?

Your home’s electric panel, whether it has fuses or circuit breakers, takes the electricity from the utility company’s wires outside, divides it up into branches (“circuits”) and routes it to the wall outlets (“receptacles”) and major appliances in your home. Each of those branches or circuits will be on a separate fuse or breaker but each circuit most often feeds more than one receptacle.

The fuses or breakers are a line of defense against electrical fires. They use heat (relative to the mount of current flowing through them) to either melt a small piece of wire or warp a strip of metal and interrupt power to a circuit so that the wires and other connections on that circuit do not over-heat and cause a fire.

Beware extension cords

When weighing the need to provide power for charging vehicles, it is important to consider the pitfalls of running extension cords from indoors. Leaving windows and doors open even a little invites insects, rodents, small animals and birds into your home.

A cord haphazardly strung across areas where people walk constitutes a tripping hazard.

Openings for electric cords allow pollen and dust into your home. They also may allow rain and snow to get in where it can damage window treatments, frames, flooring, etc. This can in some cases also invite mold problems, which can be very dangerous to health as well as expensive to fix.

Extension cords also rob efficiency. Every length of wire, every additional connection, actually consumes electricity. An electrician can show you by using an infrared camera to reveal the losses in the form of heat.

Poorly chosen, badly maintained and/or over-loaded extension cords are a very frequent cause of home fires.


When doing the math of vehicle charging costs, it’s not a bad idea to factor in the losses, however relatively small, in the process of replenishing electricity to rechargeable batteries. (also PFC, etc.)

How can I support EV charging?

J1772 vs NEMA, International, Type-F, etc….

Other stuff – home damage at windows, screens, moving furniture and appliances, leaving amenities unplugged, fuses, breakers…