What is a watt, and how do I measure its usage?

[Kalrog]

A watt is an instantaneous measure of power. It is an output in the same way that 25 horsepower is a measure of instantaneous work being performed (I know, not quite the physics definition). If you want to get at how long something will last, you need to look at a different unit of measure - like watt/hour. Or the number of watts you can output for so many hours. If you look at your electric bill, I bet you are charged a certain amount per kilowatt hour used. That is you use 1000 watts for 250 hours (or whatever the number is). Measuring it is a bit trickier... takes some electronic stuff usually. Although many cyclist know what their wattage output is at various effort levels...

[lunchbox]

a watt is a function of amps and voltage

[Kalrog]

I am looking for the simple definition just so that I can understand and purchase a laptop in the future that is good on battery life.

Oh boy... there is so much more involved in that calculation than just the capacity of the battery. Screen size, processor speed, HD stats, RAM, even OS all make a difference. The best thing to do on this is check review sites. Smaller usually = better battery life. Until you get into the ultra portable ones which have a smaller battery as well.

It seems to me that a lot of websites are lazy and put "15W power consumption" when they mean 15Whr

No, they usually do mean 15W. As in the instantaneous draw.

[lunchbox]

[WildBill]

The watt is a measure of power consumption. There are many variables that affect power consumption of a computer. These include the type and size of the monitor, speed of the microprocessor, disc drives, and DVD readers/burners. Calculating real power consumption isn't an easy task.

Batteries are usually rated in Amp-Hours. Since batteries are a [relatively] constant voltage they are rated to supply so many amps of current for so many hours. There are many ways to test battery life, i.e. constant load, but under real-world conditions most batteries don't last as long as their ratings would have you believe. How they are charged and discharged also affects battery life.

This is why I agree with Kalrog that the best way to choose the computer/battery combination is by reading reviews that are based on actual testing. Actual testing of a computer in normal operating conditions will provide more realistic expectations than just reading the specifications published by computer and battery manufacturers.

[Kalrog]

Now what do you mean by instantaneous draw? If a device draws 15 watts all of the time, and a battery is rated 45 watt hours, would the device theoretically last 3 hours? Is that how it works?

That is a pretty good way to look at it - you will never get exactly that, but you would be okay. Although I think most batteries are rated in mAH right? Mili Amp Hour. Then you can do your math.

[jimlongley]

I am looking for the simple definition just so that I can understand and purchase a laptop in the future that is good on battery life.

Oh boy... there is so much more involved in that calculation than just the capacity of the battery. Screen size, processor speed, HD stats, RAM, even OS all make a difference. The best thing to do on this is check review sites. Smaller usually = better battery life. Until you get into the ultra portable ones which have a smaller battery as well.

It seems to me that a lot of websites are lazy and put "15W power consumption" when they mean 15Whr

No, they usually do mean 15W. As in the instantaneous draw.

The AHr rating on a battery is the current draw it will support (although other limitations do sneak in) so, mathematically speaking, a 4AHr battery will give you 4 Amps for 1 Hour, 1 Amp for 4 hours, and so on. Some of the other limitations will be the ability of the battery pack to sink the amount of heat involved in the current drain, so unless the battery pack is rated for it, you sure wouldn't want to try to draw 128 Amps for 7 minutes, you would at minimum destroy the pack and possibly start a fire.

That said, the likelyhood of overloading the rating of a battery in the device it was designed for it somewhere between slim and none, but poeple do short them out inadvertantly with disasterous results.

Dell's problems with battery packs a couple of years ago was a result of the pack being unable to support the drain of the laptop, apparantly a design flaw in either the battery pack or the charging system (Dell blames the battery, Sony blames the charger) which resulted in a couple of immolated laptops.

Another example is the nine volt battery I made out of Blue Bell Cells a very long time ago, to run my little transistor radio in my phone truck. For the uninitiated Blue Bell Cells were manufactured by the Bell System for use by the Bell System in various telephones and applications - yes, phones used to have batteries in them - Blue Bell Cells were about the size of a popular energy drink can - half again as tall as a beer can and the same diameter. I wired up six of the cells, to get nine volts, and taped them together, wired them to the radio, which was one of those little ones about the size of a pack of cigarettes that would run for days on a little nine volt battery, turned the radio on, and it ran for years without ever changing the battery. I don't recall how many Amp Hours the cells were, nor what the current draw of the radio was, but it hardly put any stress on the battery at all.

Now I have to resist going into a lecture about the differences between cells and batteries, suffice it to say that those AA, AAA, C, and D "batteries" you buy at wherever - which are labelled "battery" are NOT batteries, they are cells, batteries are cells wired in series or parallel to get a different voltage or current rating - OOOPS, I did it anyway didn't I?

[anygunanywhere]

Ohms law deals with electrical properties.

Volt (E) = Potential.
Ampere (I) = electric current flow
Ohm (R) = resistance to current flow
Watt (P) = power consumed

One amp of current flow through one ohm of resistance with one volt of potential will produce one watt of power consumption.



The internet is full of information.

Anygunanywhere

[jimlongley]

And then we can get into further esoterics, such as transformer ratings in Volt-Amps (power company pole pigs usually are rated in KVA (kilo-volt-amps)) Volt-Amps are the theoretical power ratings, while watts are the "real" power flow.

In a strictly resistive circuit, which is purely theoretical and can not exist in reality, Watts and VA will be equal, but as soon as you add complex reactances due to inductance and capacitance, they depart from each other.

The difference (ratio) between the two will sometimes be seen listed as Power Factor, not to be confused with the minimum power factors used in determining suitability for compettition ammunition.

VA can be thought of as the instantaneous and continuous power usage in a circuit, which varies over time, while Watts refers to power usage averaged over time.

Don't get me started on capacitive and inductive reactance.

[The Annoyed Man]

I am looking for the simple definition just so that I can understand and purchase a laptop in the future that is good on battery life.

I recently purchased a MacBook Pro 17" laptop. There is no doubt that Macs are more expensive than most PCs, however, the new ones do have improved battery life. I am typing this on mine. It is fully charged, and I just unplugged it. The battery life meter tells me that, under normal energy usage, I have 4 hours and 18 minutes of active use charge. That seems about right according to my experience with this unit. I'm not sure what that would translate to on standby mode with the unit closed up.

[anygunanywhere]

Mrs. Anygun's new Dell runs over 4 hours on the battery. Not bad for as light as it is.

Anygunanywhere

[Owens]

I am looking at the back of this laptop battery, and it says "10.8V - 4.0AHr".

This is similar to an automotive battery. It says basically that the the battery will provide 4.0 amps for one hour without the voltage dropping below a certain level. What the level is, though, is determined by the battery mfg. and or the BCI (Battery Council International). BCI has more to do with automotive batteries though.

As an example: (thrown in for the heck of it)
An auto battery with a 75 Ah rating would be able to provide 75 amps of current for one hour without the voltage dropping below 10.5 volts or 1.75volts/cell.
Reserve capacity (stated in minutes) is essentially how long a vehicle can be driven or operated without a working charge system without the voltage dropping below 10.5. I believe this is based on an estimated 25 amp draw.