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

[KD5NRH]

Wow, 14 replies to "what is a watt," and not one instance of the word "joule." After all the physics and electronics classes, I wouldn't have believed there were that many answers other than "one joule per second."

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.

Awww, you mean I shouldn't have wired a bunch of AAs in series to start my car when the battery was dead and the jumper pack was long overdue for a recharge? They were pretty well fried afterward, but it got me going.

[WarHawk-AVG]

A unit of power in the U.S. Customary System, equal to 745.7 watts or 33,000 foot-pounds per minute.

So a 1 hp electric motor uses 745.7 watts well that is if the motor was 100% efficient and didn't waste alot of it as heat
but not in a measurement of a battery

First, to correct the anonymous guest 74.222.99.80, "Whr" is an invalid unit. The correct unit is "Wh" which is the SI unit of power multiplied by time, typically hours, but minutes and seconds are equally valid as in "Wattminutes" and "Wattseconds".

A simple answer is that a 53Wh battery will last half as long as a 106Wh battery. But what does that really mean?

To answer the question more fully, batteries are typically rated in Ah or Amphours which means a battery with a rating of 50 Amphours (like a typical car battery) can deliver 50 Amps for one hour, 10 Amps for 5 hours, etc. Note that this is strictly a current over time rating without regard to the battery voltage. However, Ah is NOT the amount of power a battery can supply.

Power = Amps x Volts = Watts. However, Watts are a RATE of power being supplied, not how much in total. To signify an AMOUNT of power, you need to add time. Thus, Watts x time = total amount of power, or more correctly, energy supplied.

So, taking your question of "What does 53Whr mean in terms of battery life for a new laptop?", we can answer that in terms of how long the battery charge will last because all batteries fundamentally suppy x amount of current for x amount of time regardless of what the battery voltage is. But we need to know the voltage in order to determine how much energy a battery can supply.

If we work backwards, and take 53Wh, we are starting with the total amount of energy the battery can supply. But that does not give any real idea as to HOW LONG the battery will run the computer.

To know how long the battery will last, we need to know just a few of several possible things: the battery's Amphour rating and the current drawn by the computer or, it's Wattage (Watts drawn) and the battery's Wh rating. The latter one you know. Or, it's voltage and Amphour ratings. From these we can convert back and forth to any other unit(s).

Since you only have the battery's Wh rating, to figure out the battery's Amphour rating, one must know the battery voltage because Watts = Amps x Volts.

There are multiple ways to approach this question depending on what units you are given but we can really only make an estimate as the amount of power modern computers use depends on what you are doing. Like is the backlight on high or low? Or, are you typing a note, or playing a high-power game, or watching a DVD etc. etc... .

The only way to mostly answer your question verbatim is to make some assumptions, get an answer, and show you how one calculates the run time. Then, you would have to use actual values to get a "real" answer.

Here are three paths leading to the same answer:

First, let's assume that, Battery rating = 53Wh, and Laptop nameplate power is 35 Watts.

We know the laptop will use anywhere from 35W to maybe 1W so we take the worst case of 35W.

Very simply, Wh = Watts x hours. Thus, hours = Wh ÷ Watts.

So 53Wh ÷ 35W = 1.51 hours or 91 minutes. That's the simplest way.

But, what if instead the laptop's label says 2.92 Amps @ 12 Volts?

In this case, we know the battery voltage, and the laptop's current comsumption, but we need to convert that into energy which is what the battery provides, and is what Watthours are.

Using Ohm's law (sorry if this is getting too technical), Amps x Volts = Watts.

Using our assumed numbers, 12 Volts x 2.92 Amps = 35W.

Appying the first scenario, the run time would then be the battery's 53Wh ÷ 35W = 1.51 hours, the same time we calculated above.

As long as you know the battery voltage, and either Watts consumed, or the current (Amps) drawn, you can convert into Wh and determine how long the battery should run.

Finally, a third way to get the same answer: Again, assuming the battery is 12 Volts, and is rated at 53Wh, we can say 53Wh ÷ 12V = 4.42 Amphours (Ah). Assuming the same current draw of the laptop of 2.92 Amps, we can calculate 4.42Ah ÷ 2.92Amps = 1.51 hours, again, the same time.

Interestingly, you asked about a "new" laptop. As the battery ages, it's Amphour and thus Watthour rating will slowly decline until it's energy capacity becomes low enough to become a pain in the butt. Then, it's time for a new battery!

http://wiki.answers.com/Q/What_does_53W ... new_laptop

So in effect that 60Wh battery could keep the laptop alive for approx 15hrs before the battery was completely drained.

[jimlongley]

Awww, you mean I shouldn't have wired a bunch of AAs in series to start my car when the battery was dead and the jumper pack was long overdue for a recharge? They were pretty well fried afterward, but it got me going.

All I was trying to convey is that, as you said, you will destroy your battery pack by exceeding its rating.

I thought explaining joules in order to explain Watts was just going too far.

[KD5NRH]

I thought explaining joules in order to explain Watts was just going too far.

Why would that be so hard? A joule is just a coulomb volt, and a coulomb is just an amp second.

Thus, a watt is a joule per second, or a coulomb volt per second, or an amp second volt per second. Since a volt is a joule per coulomb, it is really a trivial task to give up on trying to reduce the definition by hand and Google the simplified definition of a watt in terms of base units as 1 newton meter per second.

[jimlongley]

I thought explaining joules in order to explain Watts was just going too far.

Why would that be so hard? A joule is just a coulomb volt, and a coulomb is just an amp second.

Thus, a watt is a joule per second, or a coulomb volt per second, or an amp second volt per second. Since a volt is a joule per coulomb, it is really a trivial task to give up on trying to reduce the definition by hand and Google the simplified definition of a watt in terms of base units as 1 newton meter per second.

You're right I was being lazy, but I really didn't want to have to try to explain to the uninitiated, that a newton is not a cookie and no they cannot have seconds.

[KBCraig]

A unit of power in the U.S. Customary System, equal to 745.7 watts or 33,000 foot-pounds per minute.

So a 1 hp electric motor uses 745.7 watts well that is if the motor was 100% efficient and didn't waste alot of it as heat...

Yeah, I love Sears power tools that claim to be 6 or even 6.5 hp on 115V household current.

I've gotten several blank stares when I asked salesmen where I was supposed to get a 115V 40 amp circuit.

[jimlongley]

A unit of power in the U.S. Customary System, equal to 745.7 watts or 33,000 foot-pounds per minute.

So a 1 hp electric motor uses 745.7 watts well that is if the motor was 100% efficient and didn't waste alot of it as heat...

Yeah, I love Sears power tools that claim to be 6 or even 6.5 hp on 115V household current.

I've gotten several blank stares when I asked salesmen where I was supposed to get a 115V 40 amp circuit.

Someone was taking on a fellow home depot associate the other day, in a similar vein, wanted to know how many BTUs per hour a particular electric heater would produce, and I knew I had a party lined up. I jumped in to the conversation grinning like Davy Crockett (it's my department, too) and left the customer standing bewildered with no substantive comeback beyond "But, I thought . . ."

I explained that when speaking of BTU, the hour is usually included, and is understood by those who know what they are speaking of, and that saying "BTU per hour" is kind of like saying "knots per hour" when describing wind or nautical velocities, where the second "per hour" either implies acceleration or acknowledges a less than perfect understanding of the subject matter, and then illustrated with an example in "miles per hour, per hour."

Then I went on to explain that a BTU is approximately equal to .293 Watts, or 1 Watt equals about 3.41 BTU, without the hour and thought of going into Joules and horsepower, but didn't have all the figures in my ever present PDA, so just left it at that. Then I explained that while there was no real direct conversion for the 5000 Watt electric heater he was referring to, he could assume that its heat output would be roughly equivalent to a 17000 BTU gas heater.

He gave up and said he was going to Lowe's, and my opinion is that he should, he's too stupid to shop at Home depot, even if I didn't say so at the time.

Then we had the guy who bought a 18000 BTU air conditioner last week, and when he went to plug it in he found that the socket in his (110V) outlet didn't match the (15A 220V) plug on the airconditioner, so he came in to the electrical department to get the "right" outlet - luckily the guy in electrical kind of gathered that the customer didn't know what he was doing and called me over to confirm that the A/C was indeed 220. We have been explaining that one for weeks now, to lots of customers.

[Pinkycatcher]

Then we had the guy who bought a 18000 BTU air conditioner last week, and when he went to plug it in he found that the socket in his (110V) outlet didn't match the (15A 220V) plug on the airconditioner, so he came in to the electrical department to get the "right" outlet - luckily the guy in electrical kind of gathered that the customer didn't know what he was doing and called me over to confirm that the A/C was indeed 220. We have been explaining that one for weeks now, to lots of customers.

O man, that's a bad combination, I do stage lighting and we regularly run into 220v sockets, luckily some of the dimmer packs split into 2, 100v extensions, but the problem is you have to find 2 different circuits for that. Good thing they have breakers in houses, it's to prevent people doing something stupid like that.

[Keith B]

..called me over to confirm that the A/C was indeed 220.

Ahh yes, "220, 221, Whatever it takes."

[jimlongley]

..called me over to confirm that the A/C was indeed 220.

Ahh yes, "220, 221, Whatever it takes."

Well, I was going to ask if he meant V(RMS) V(peak) or V(peak to peak) but also didn't have that presentation in my PDA, so I just told him "Yup, that's pretty much what it are."

The thing I get a kick out of is the A/C boxes are VERY clearly marked on the outside, but some of our braniac customers still buy the wrong ones and then blame us. These are A/Cs on display near the entrance to the store (you've been there and know what I mean, I'm sure) so people just take them on carts and take them home, which is what they are intended to do, it's just that they don't pay any attention to what they are buying, or the pictures on the boxes, or whatever. I even had a guy arguing with me about one yesterday, that the 110V and 220V plugs looked identical to him.

[KD5NRH]

Then we had the guy who bought a 18000 BTU air conditioner last week, and when he went to plug it in he found that the socket in his (110V) outlet didn't match the (15A 220V) plug on the airconditioner, so he came in to the electrical department to get the "right" outlet - luckily the guy in electrical kind of gathered that the customer didn't know what he was doing and called me over to confirm that the A/C was indeed 220. We have been explaining that one for weeks now, to lots of customers.

Doh! Glad you reminded me; in all the excitement of the wife's pregnancy and looking at houses, I've been completely forgetting to look for 220 outlets in the houses. I remembered to put suggested locations for a few on the plans we were using to get estimates from builders, but in buying something prebuilt I'll probably have to settle for one in the W/D hookup area or possibly the A/C closet, if any at all. My goal is to have at least one in each of those areas, plus one outside and one in the garage and/or workshop, since 220 extension cords aren't exactly typical WalMart items.

[dukalmighty]

I got a little battery bar on my cell phone when It beeps it's time to recharge or no talky

[mr.72]

a Watt is a measurement of heat. It is conveniently used to express power consumption of an electrical circuit given that it is calculated by multiplying the current in the circuit (in Amperes) by the Voltage. It can be converted to horsepower or joules or calories or other units of heat measurement by simple arithmetic.

Regarding battery life in a laptop, well that's a far more complicated question and the battery chemistry as well as the minimum operating voltage and DC-DC converter topology come into play. Common Li-Ion and NiMH batteries have very flat discharge curves and their voltage stays high until they are very nearly dead and then drop rapidly, unlike lead acid batteries like in your car. However the power management system in laptops have a great effect on battery life with all other things being equal. So a laptop with better power management and a better design (lower consumption overall) may well run longer on a smaller battery than a less well-designed laptop with a larger battery.