Wednesday, April 14, 2010

A Bucket of Electricity

Please Sir, can I have a bucket of electricity? No? That makes about as much sense as asking for a foot of milk. So, then how is electricity measured? Those of us on boats are much closer to this issue than folks living on shore.

Electrical energy is measured in watt-hours (or kilowatt-hours, or megawatt-hours) - it is not measured in amps, volts, or watts. These are units used to measure other things. I know I am patronizing most of those who read this, but since there is so much confusion in the general public (and especially the media - but I don't expect any of them to be reading this), a quick review is in order:

The amount of electricity flowing - a direct measure of the number of electrons passing a point in the wire. It is also called "current", which provides a great analogy to water or hydraulic systems (where the current is typically measured in something like gallons/min). If it helps, think of an amp as "a bazillion electrons per second".

This is the electrical pressure . Continuing the analogy to liquids, in a water system or a hydraulic system, the amount of work the current can do is related to the amount of current and the pressure at which it is available. Think of a pressure washer, for example. The water flow out of the hose bib to the pressure washer is the same as the flow out of the pressure washer nozzle. The difference in the ability to do work is the pressure.

A unit of power, not energy. Power is defined as the rate at which energy is produced or expended. Changing analogies now, the size of your engine determines the power produced; the amount of fuel in your tank determines the total energy available, regardless of the rate at which it is produced. In electrical systems, one way that power can be calculated is by multiplying the current (in Amps) times the voltage (in Volts), directly giving Watts.

Thus, for example, your inverter might be capable of delivering energy at the rate of 1000 watts - at 120 volts, this means the current is going to be 1000/120=8.33 amps on the 120 volt output side of the inverter. The rate at which energy is withdrawn from your 12 volt battery bank is approximately the same (actually it will be a little greater, due to inefficiencies in the inverter). Thus, on the low voltage side of the circuit, the inverter will be drawing a little more than 1000 watts = 83.3 amps at 12 volts (about the same as a starter cranking a big V8 engine).

This, finally, is a unit of energy. Using the units above, Watt-hours = Volts multiplied by Amps multiplied by hours. If you draw energy from your battery bank at the rate of 1000 watts (83.3 Amps at 12 Volts) for one hour, you will have consumed 1000 watt-hours, or 1 kilowatt-hour. (You can purchase a kilowatt-hour from your local power company for 10¢ - 15¢)

This is going to be a little confusing. But diving right in... In systems where the voltage is more or less fixed, it is sometimes more convenient to talk about amp-hours, as a unit of energy, with the voltage implied. This is a bastard unit, since 100 amp-hours (at 100 volts) is a whole lot more energy than 100 amp-hours (at 12 volts). But for boats, you can trade back and forth between watt-hours and amp-hours, as long as you remember that they differ by a factor of the system voltage: 12 volts. That is, when you say that you have a 100 amp-hour battery bank, what you are really saying is that you have a 1200 watt-hour battery bank (one that will store perhaps 15¢ worth of electrical energy).
The media (even, shamefully often, the marine press) get these all mixed up, talking about nonsense such as "amps per hour", frequently to the point that it is not possible to figure out what they are trying to say.

Well, not so quick a review. I apologize - this was originally going to be a post about energy budgets, but I realized after I got writing that defining the term "energy" was a big enough subject for its own post. The energy budget will have to wait for later - but now we are on common ground to talk about it.

1 comment:

Mike said...

Looking forward to reading more on this! Thanks Bob.


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