# How many Kwh to recharge a car battery from AC outlet?

I’m on the Board of a 152-apartment condo. Outside the garage is a common area where owners can wash their cars. Nearby, there’s an electric outlet where owners can plug in a vacuum and clean the inside of their cars at the same time.

Recently, the Building Manager received a complaint that an owner used the electric outlet to recharge the battery on his car. (The complainant implied that this was a crime that should be severely punished.)

I’m trying to figure out approximately how much electricity the owner used. If it really was a large amount of Kwh, then he needs to be billed for the cost. But if the cost was fairly small, perhaps we don’t need to be make a big deal about this as the complainants seem to think.

I Googled to get more info on some of the devices that will perform a jump start from an AC outlet. A couple of the ads described the power output as 600 watts. If so, then running the device for an hour would use less than 1 Kwh (kilowatt hour = 1,000 watts for an hour), which costs less than 10 cents.

Does anybody know whether the above estimate is valid? If not, can you suggest a methodology for getting a better estimate of the cost?

It’s not a lot of power, assuming he just charged one battery, not his Tesla! I think your estimate might be a bit low, but even 10X that is not worth worrying about. ‘severely punished’? Jeeesh!

A normal 10A battery charger would take less than 200 Watts. A bigger 25A charger would probably take less than 500 watts. I wouldn’t worry about it.

I did some googling myself and found this relatively comprehensive answer:

A totally negligible cost. Mind you that the calculation linked above assumes that one recharges the battery for 12+ hours. These things are also called “trickle chargers” for good reason. If the person only plugged it in for 1 or 2 hours while washing the car the charges would be even more ridiculously low.

In that case, if one wanted to play devil’s advocate, once could even argue that a vacuum cleaner uses many times more power than this trickle charger. Probably even the water used for a car wash costs more than what charging the battery cost. Then you get into territory where you have to start asking: Should we bill people for the water (install a water meter) and for the electricity for the vaccum? What about people who go to the car wash? Why are they punished by other people wasting water and electricty. It’s a can of worms but armed with the calculations provided here you should be able to avoid opening it in the first place.

Thanks for the prompt replies with lots of useful info.

The answers.com response is very complete, but unfortunately they state the electricity used as “1275 watts”, which is a rate not a quantity. I think they meant to write “1275 watt-hours”, which is less than 2 Kwh (kilowatt hours), which would cost less than 18 cents at the rate from our condo’s most recent electric bill.

I hope this will help put the matter in perspective and prevent our condo from starting WWIII over this trivial matter.

Get a Kill-a-watt meter. These meters are under 20.00 and will show consumption. We uses them all the time to get an actual consumption figure for modified computers.

When you actually see how much power an overclocked,watercooled computer with several high powered video cards in them running at 100% 24/7 (while folding) you will be set back a bit.

You’re right, the example dropped the ‘hours’, so it would be 1.275 kw-hrs. Nothing compared to a vacuum cleaner, like YT pointed out!

I would guess vacuuming the car would consumer more electricity than charging the battery.

Now there is one possible exception. If someone with an hybrid or full time electric car, then it might be an issue. But for a gasoline car without hybrid capability, it is too small to worry about.

Protein folding Oldschool?

Charging a 12 volt car battery takes an insignificant amount of electricity…Now when they start plugging in their new Nissan Leafs, you might have a problem…

There is a device called a Kill-o-watt (check Amazon). For \$25 you can hook up the charger and determine just how much juice it draws and multiply by the current rate. As a building manager this is a good tool to have on hand to check how much the various appliances around the building. When I found out that my rarely used freezer in the basement was running me almost \$150 a year, I decided it was time to dump and upgrade… but I’m drifting off the subject of cars.

A typical large car starting battery holds about 100 amp-hours.

100AH X 12V = 1200 watt-hours

So if we assume the whole process of charging were 50% efficient we’re talking 2.4 kilowatt hours.

Like most batteries you could (in theory) recharge it several hundred times and still spend less on the electricity than what the battery cost.

Yes, the Stanford program.

If the charger was the typical McParts store model it had an out put of approx 10 amps at 13 volts which equals 130 watts, or .130 kilowatts. Allowing for thermal losses it would draw about .200 kilowatts. If operated for 1 hour it would be .200 kilowatt hours. That’s the equivalent of 2 100 watt light bulbs. I believe that everyone here calculated a similar conclusion but stated in a different manner. If MandyW has gleaned the knowledge she needed already she hopefully won’t read this post. Otherwise this might turn on her light.

Your complaining tenant needs to mind their own business. Or shut down the car-wash facility and post a note as to what the complaint (make sure you also post the facts as to how insignificant the power use) was and who made the complaint, their unit number and possibly phone number also. See how Mr/Mrs Busy body likes that.

Ah, condo commandos. You’ve got to love them. They have so much time on their hands, they have to stick their noses in everyone else’s business.

I agree with the others. A vacuum cleaner probably uses more juice than the average battery charger.

[sarcasm] Oh sure. Escalating in these types of conflicts never backfires. [/sarcasm]

I don’t do any folding, but my SLI’d GTX 480’s aren’t exactly frugal at the outlet. Running them along with my OC’d CPU (QX6700 @ 3.4 Ghz) during some benchmarks will heat up the room nicely. My next build will be water cooled for sure.

100-watt bulbs are really bright.

Is it the standard auto battery being recharged or is it a plug in hybrid. Big difference.