The "normal" charge time, from zero to full, on a 120 volt system is "about 12 hours ± 1 hour" (except minus does not happen often, so more like 12 to 13 hours). At 240 volts it drops to half that, for the simple reason that energy is the integral of power.

Remember, batteries store *energy*. Think of energy as if it were water. A battery is a tank or a bucket: it is not water (energy) itself, it is just a storage container. There's a low water mark labeled "empty" (in this case, there's some water/energy left at "empty" because the battery lasts longer that way) and a high water mark labeled "full". The *rate* at which you drain or refill the bucket is the electrical *power*.

So: "energy" is much like stored water, and "power" is rate at which you drain or refill the water in the tank.

In electrical systems, power (rate of drain or fill) is measured in watts. Watts are like gallons-per-minute. Energy is, or at least should be, measured in joules (or some multiple, like megajoules, because "1 joule" is really quite tiny—it's like measuring a water-tank capacity in drops of water).

Because joules are tiny, and because of our history—people's home use of electricity mainly started with electric lighting—the measure most people are familiar with, that you see on your electric bill every month for instance, is "kilowatt hour". A kilowatt hour is the amount of energy you could dump into an energy-tank if you filled it at a rate of one kilowatt, for one hour.

(Side note: I'm going to add another analogy here, because in my experience, a lot of people get lost at this point. They're not used to thinking of filling the bathtub "at 3 gallons-per-minute for 1/6th of an hour" and the unfamiliarity throws them. So another useful trick in wrapping your head around this stuff is to think of "power" in terms of "miles per hour on the road", and "energy" in terms of "distance traveled". If you can average 30 mph, and you need to travel 10 miles, how long will it take? It turns out it will take 20 minutes: 30 mph is one mile every 2 minutes. If you can go 70 mph on the freeway, and you drive for an hour and a half, how far will you go? That's 70 miles/hr times 1.5 hr = 105 miles. So if you see "watts" or "kilowatts" in any claim, try mentally substituting in "mph", and see if the result seems to make sense. When someone says "I have a fancy new battery that stores 2000 watts" you can mentally translate it to "I have a storage tank that holds 2000 mph" and you will instantly realize that there's something fishy here. Whoever it is, either misspoke, or has no idea what he is talking about.)

Anyway, OK, so once you have energy = "stuff" (like water) and power - "flow rate of stuff" (like gallons per minute) down, now we can take a look at "volts" and "amps" in electricity. Take a moment to go outside and play with a hose and your garden/lawn, or try remembering when you were a kid, playing with the hose (I did as a kid, anyway). Turn the water on as a sort of dribbles-out-of-the-end-of-the-hose rate: high enough that it flows well, but not so high that it makes a long arc before splashing on the lawn or driveway.

Hold the end of the hose and watch the water dribble out at your feet. Then, take your thumb and put it over the end of the hose, blocking it off as much as you can without completely stopping the flow. What happens? The water squirts out through any gap you leave, and now it actually *sprays* rather than dribbling. It's a thin spray, only as big as whatever opening you leave with your thumb. Open the end wide (take your thumb off again) and it's a wide-mouth dribble.

Without changing the faucet setting, get yourself a bucket and try (1) dribbling water into the bucket and then (2) spraying the water into the bucket. Observe how fast the bucket fills with water each time. The interesting thing here is that whether you spray the water in a thin but spray-full stream, or let it dribble in, *the bucket fills at the same rate*. If it takes 60 seconds for the water level to climb one inch, it takes 60 seconds regardless of "big dribble" or "thin spray".

Here's the really cool bit: with electricity, voltage (measured in "volts") is like "force of spray", and current (measured in "amperes") is like the width of the opening out of which the liquid (the "electric energy") either dribbles or sprays. The product of the two—volts times amps, or "volt-amps"—is (in simple DC electrical systems anyway) the rate of the energy flow. And it turns out that "volt-amp" is another name for "watt" (well, until you get to alternating current and "phase angle" but never mind that!).

We have:

**volts: pressure**

**amperes: width** (cross-section)

**watts: flow rate**

**energy** (in joules, or watt-hours, or whatever): **integral of flow-rate over time**

It's easy to see, when playing with the garden hose, that you can increase pressure by decreasing width, or vice versa, without changing the total energy flow rate (the same amount of water comes out of the hose, regardless of whether it's a fat dribble or a thin spray). This is how alternating current electrical energy is sent long distances over skinny (read: cheap) wires: convert it to high voltage (high pressure) and it fits in a thin wire! Of course, at high pressure, it's also very ready to escape, so we down-convert it to a more convenient voltage at the house.

Now, if you want to get energy (water) into your battery (storage tank) faster, there are two ways to do it: build a bigger, fatter pipe and use the same voltage/pressure, or, keep the same skinnier pipe but raise the voltage/pressure.

If you double the pressure (120 volts to 240 volts) while keeping the pipe the same width (same amperage), you'll double the flow rate (twice the watts) and hence move twice as much energy in the same time. This is why, ignoring secondary effects having to do with losses in wiring and converters and so on, charging your car at 240 volts makes it charge up twice as fast as doing it at 120 volts.

Note that if we could go to 480 volts while keeping the amps the same, the charge rate would double yet again, and the charge time would drop to about 3 hours. 480 volts is common in industrial applications (although 415 is also common, with the difference having to do with "delta" vs "wye" configurations of the three phases in standard industrial 3-phase AC power, but never mind that). However, almost every building in North America has 240 volt power run to it (240 volt "center tapped", with 120 volts from center-neutral to either side; that's where the 120 volt power comes from).

(The actual voltage at any outlet in your house can vary from a low-normal of about 110 to a high-normal of about 125 and the nominal "correct" voltage is actually "117 volts AC RMS" but there's so much slop in the system that people just say "110" or "120" and we call it good....)