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But an amp-hour is not the final and full measure of energy use. It doesn’t include one factor—voltage. When we talk about how many amp-hours a device uses in a given time, or the size of our battery bank, we also have to specify voltage to give the whole picture. Forty amp-hours at 12 volts is not the same as forty amp-hours at 120 volts.

To bring voltage into the equation, we have to talk about watts, which will be my topic for next time.


Author: Ian Woofenden, PO Box 1001, Anacortes, WA 98221 • Fax: 360-293-7034

Amp-hour— Quantity of Charge

Ian Woofenden

©1999 Ian Woofenden

In my last column I defined an amp as a rate of electron flow. It’s not a quantity—it’s a certain number of electrons passing a point in a given time.

There are lots of rates used in scientific descriptions, and some that are fairly common in our everyday speech. Miles per hour (MPH), revolutions per minute (RPM), gallons per minute (GPM), and cycles per second (Hz) come to mind.

What if I told you I had seen a car going 100 miles per hour per hour? You’d think I was a bit confused, and you’d be right, because it doesn’t make much sense. In the same way, saying “amps per hour” does not make sense. An amp is a rate; miles per hour is a rate. There is no such thing as amps per hour!

When we want to describe an amount of “charge” that has gone through a circuit, we use another term—amp- hour. If I turn on a light that draws 2 amps and leave it on for an hour, the battery will pump 2 amp-hours of charge through the circuit. If I leave it on for four hours, the total charge will be 8 amp-hours.

Deep cycle batteries are rated in amp-hours. Theoretically, we can say that a 220 amp-hour battery will deliver charge at a rate of 220 amps for one hour, or at a rate of 1 amp for 220 hours. This is not actually accurate because we are ignoring efficiency and the rate of discharge, among other things. But I hope it gives you a feel for what an amp-hour is.

When we say amps, we’re talking about charge flow rate. When we say amp-hours, we’re talking about the amount of charge that has passed through a circuit, or the amount of charge that’s pumped by a battery before its energy is depleted.

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98 Home Power #70 • April / May 1999

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