KW-hr Question

kW-hr (kWh) is a useful unit in practice.

For example, you have a 2kW heater (heaters are usually sold with the power rating on the box). You use it for 5 hours a day. Energy consumed = 10kW-hr. The power company will probably charge £££ (or $$$ or whatever) per kW-hr, so you can easily work out how much they're going to sting you per day.

Similar calculations apply from LEDs to to power stations - you can specify consumption or output in kW-hr (while they're on). Ok, maybe μW through to GW is a more useful range, depending on your dicipline. Tho' I used to be a physicist, I'm more of an engineer now, and this (kWh) is a more practically useful unit than any other I know.

You are absolutely correct, another engineering feat, that has no reasonable answer in this time and space medium.

From Wikipedia, the free encyclopedia

The watt (symbol: W) is the SI derived unit of power, equal to one joule per second.

As others have stated energy is measured in JOULES and power is measured in WATTS such that if you expend 1 WATT for 1 SECOND you have 1 JOULE. In other words

Joules = Watts x Seconds not Watts/Seconds.

As for why we use the metric system to measure energy do you remember the equation;

KE = ½ MV² and PE=MgH

That are used to calculate the amount of potential and kinetic energy and quess what the unit is JOULES again. Maybe its time we had a blog to go through all the SI units and explain how simple the SI system is.

Pardon me, but "your" (as in the line at the bottom of YOUR entry) indicates something that belongs to you. You're going to be more credible if you use the correct words! I believe this is true whether in British, US, or Australian English. Am I mistaken?

Congratulations, you get a gold star and can go to the head of the class. I was wondering how long it would take for somebody to pick that up, its taken nearly three weeks. Stay tuned for the next subtle deliberate mistake.

Oh, by the way dkwarner YOU'RE disqualified from the next deliberate mistake.

Battery manufacturers agree with you and refuse or neglect to reveal the amp hour rating of batteries. CCW, VDC, AMP, deep cell, whatever reveal little to nothing about the product. True amp-hour ratings of batteries are difficult to impossible to find. Guess why? Amp-hour rating of an auto battery reveals the limited capacity of the battery for comparison purposes. Batteries worth purchasing power will always state the amp-hour rating.

Utilities would rather keep the public barefoot and expecting an unknown monthly bill rather than educate how to read and compare kw-hr per month consumed. Which month should also be included to be precise and accurate so that the Kw-hr per winter month as compared to the Kw-hr per summer month can be used to determine heating vs air conditioning cost.

You are confusing what is being measured. Watts is a measure of RATE of work or power. To get to quantity of energy consumed, WATT-HOUR, you must mustiply the rate of doing work by the amount of time the work was done at that rate. So yes, if you want to rearrange things joules per second is a measure of rate of work and joules per second * seconds is a measure of quantity of work done. It is correct and neccessary for time to appear twice as it represents two different measurements.

The actual power being consumed changes rapidly as loads are switched on and off. The meter responds to the power being consumed at any instant and advances the reading at a rate proportional to the instantaneous power. The process is actually mathematical integration of power over time, or addition of the tiny increments of energy consumed during tiny intervals of time. The units kWhr are indicative of the actual process, We could call them joule meters, but that is less informative to the user.

1 KW = 1 Kilo Joule/second x 3600 not 1 hr so it is 3600 Kilo Joules

I think all what you say is correct. The only diference is convinienece. Since the power of electric motor, generator and others appliances are measured in KW and production time is measured in hour then kW-H is the convenient (practical) ways to measure energy but if you want you can measure as you say by converting to J.

Have you converted some times Britsh unit to SI and Vice versa? is it all ways so easy? so fast?

Allways the technology tend to follow practical approach.

If you talk to electrcians Joules of energy then you will never get a communication thru. Tell him 10 Units and he understands it as 10 kW-H which is either 1kW for 10 hours or 10kW for 1 hous. Some time batteries have Amp-hour for their fixed volts. No one write Joules on batteries. Most confusing one is kilocalories.

Doctor says you can't eat 1000 calories in a day in your food and you drink 1L water or coke near zero degree C and you end up removing solid 37000 calories out of the body. Where do you get so much calories to lose and still remain warm and alive. This fuss is really bad in technical communication with non-physics background people. Energy physics does not go well with doctors.

Shyam that is exactly why you should stick to one measurement system. The SI unit of energy is a Joule and if you measure everything using the SI system it minimizes the chances for errors. Just ask NASA what happens when you mix measuring systems. In Australia any nutritional information lists the energy content in KJ (Kilo Joules) as well as food calories which are actually KCal (Kilo Calories).

The only exception to this is in the aviation sector where nautical miles and feet are still used. The nautical miles because they are equivalent to 1 minute of arc measured from the center of the earth, making navigation easier and are an accepted SI unit. The feet are used to measure altitude because it was not possible to convert every altimeter in every aircraft simultaneously so they stuck with it but 1Km = 3,000 feet is normally close enough in this situation.

Fellow CR4 people you may find this helpfull. It is a conversion calculator and is easy to use.

I have a shortcut to it on the Start menu and use it all the time. Click on the following link to install it.

http://www.mcp.com.au/convcalc/convcalc.zip

It doesn't have every conversion but covers just about everything that I have ever needed. You do however need to know what you are talking about first though.

Good.