The kilowatt-hour is an odd unit.

A kilowatt-hour is nothing but 3.6 megajoules. A watt is 1 joule/second, so if you multiply that by an amount of time, you get right back to joules. Why isn't electrical grid usage simply measured in megajoules?

In the future, they measure power in megajoules, not kWh.

Non-SI units are used all the time e.g.

mAh to measure battery charge capacity instead of coulomb = amp second

MPH or KPH to measure speed intead of metre per second

Low calorie cola intead of low joule cola (I've actually seen this before - it sounds really stupid!)

etc. etc. etc.

TigerhawkT3 said:

A kilowatt-hour is nothing but 3.6 megajoules. A watt is 1 joule/second, so if you multiply that by an amount of time, you get right back to joules. Why isn't electrical grid usage simply measured in megajoules?

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Because people are used to appliances and lights that are rated in watts; they can grasp the idea that if they run a 100 watt light for 10 hours, they've burned 1 kilowatt-hour of electricity. Most people have no idea what a joule is, or how it relates to watts.

whats a Jewell?

nuggett said:

whats a Jewell?

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A hot blonde singer with a great rack and less than perfect teeth.

Who cares about her teeth?

The_LED_Museum said:

In the future, they measure power in megajoules, not kWh.

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No, we'll measure _energy_ in J, not power. Power will continue to be measured in W. Or was that exactly what you meant by your . I hope so.

GO SI!

Good topic i was going to start something like this, so here it goes.. How do you figure out how much it costs to run an appliance per hour. half hour, 1/4 hour..?

..I usually just post a question in Cafe..

but seriously, I once knew a bartender named Joules...
besides it's convenient because amps times volts equals watts, so you can figure.

You can use the Kill-A-Watt meter to measure how much energy each appliance (up to 15 amps at 120 volts AC) uses.
I have one of these, and I would recommend it. :thumbsup:

xlt2000:

Look at your power bill... Say it is $0.10 per kWhr... So:

$$=KWhrs * $0.10

or

$$=100 watt (light bulb) * 10 hours (running time) *1/1,000 (W/kW)* $0.10/kWhr = $0.10


$$=1,500W (toaster) * 10 minutes/60minperhour * 1/1,000 (W/kW) * $0.10/kWhr = $0.025 (2.5 cents) for 10 minutes

$$=1,500W (room heater) * 1 hour * 1/1,000 (W/KW) * $0.10 = $0.15 per hour
$$=1.5 kWatt (room heater in kW) * 1 hour * $0.10 = $0.15 per hour


Does this help?

-Billl

AAARRRHHH
Now to go to the "pet peeve" category

Watts = volts * amps ONLY for DC and NON REACTIVE AC

Otherwise it's
V*A * cos(Phase angle)

Don't get me started on even the use of Ohms, Amperes, and Volts.......

I'm with monkeyboy on this one:

It should all just be coulombs/second or some variation of that set [#of electrons/time unit]


Not to mention using gauges for wire instead of just measuring the diameter

KC2IXE said:

AAARRRHHH
Now to go to the "pet peeve" category

Watts = volts * amps ONLY for DC and NON REACTIVE AC

Otherwise it's
V*A * cos(Phase angle)

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There are so many reactive and inductive loads (including grid line capacitors) throwing phases one way and the other, that things get balanced out and for practical purposes volts times amps is a good measure of power. I know this because I have an off-grid solar cabin and can watch the actual DC load coming from the batteries as I use different AC appliances with different power factors.

Josey said:

There are so many reactive and inductive loads (including grid line capacitors) throwing phases one way and the other, that things get balanced out and for practical purposes volts times amps is a good measure of power. ...snip...

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Until your dealing with one item

I had a real strange problem one day that really drove home the difference between VA and Watts

I think I've said that I used to be an electronics tech, back in the old days. One of the units I worked on was a programmable AC power supply (designed in the REAL old days - mostly analog, except for the digital programming section whic was DTL logic (Now THAT dates the system a tad - was moved to TTL eventually)

Anyway, I had this strange problem - I was getting output voltage - no problem. I was getting current. But I wasn't getting POWER!! Huh? Headscratching time. Take the Tektronix 461A O'scope (That ALSO gives you an idea of when it was), put it across the output - nice sine wave, take channel 2, put it across the current sensor (it had programmable current limits), also a nice sine wave - EXACTLY 90 degs out of phase!!! Given that Cos(90) = 0, NO WATTS - I had what ever voltamps I programmed

I can't remember what exactly was wrong - probably a wiring error in the chassis (yeah - that dates it too - all wiring harnesses, and wirewrap backplanes for the digital logic boards)

I can remember that it used to take about 10 work days to troubleshoot the average pair of boxes (took 2 boxes to make 1 unit) IF you had a known good set of cards to put in the boxes. You's start with a good set of cards, put them in an unknown chassis. Debug the chassis, and then return the cards to the box you borrowed them from. Debug the next set of card, then send the chassis off for 2nd inspection/heatshrinking of harness leads etc, while using the cards to debug the next set - and just keep going. 22-30 sets a year, depending on what the Navy ordered that year

The worst was when we got boxes back from the fleet for "SLEP" (service life extention program - aka overhaul) I don't know WHERE the Navy stored these units on the Carrier, but by the FILTH, and GRUNGE, and OIL, and other GUNK that was in the units, I don't doubt it was in the bilge somewhere. The FIRST thing we did after removing the covers and opening the doors in the chassis was take them out back in the parking lot, and take a garden hose to them and wash the stuff out of them - then the cards went right to the vapor degreaser to be cleaned, and the boxes went to inspection, then the production/wiring line to be fixed as best as we could BEFORE I would gt it back

I think the reason we(Americans) use the kiloWatt/hour measurement is because megajoules sounds too metric. Our government made a big push for us to convert to the metric system and it pretty much failed. That's why our cars are measured with miles per gallon instead of kilometers per liter, and our wrenches are sized like 1/4" and 5/8" instead of 4mm and 12mm.(just an example, probably not the right conversion, so don't beat me up)

It would be helpful if we finally did convert to the metric system fully, but I don't expect to see it in my lifetime.

The problem with America and our continued use of the Imperial system is not so much the different units and confusion surrounding them as it is all the convenient industrial standards we have based on imperial units.

You deal with voltage and amperage all the time in electrcity, so the kilowatt-hour is a natural extension of power over time. I would think that dealing with megajoules would be more inconvenient since it's another conversion if you need to get back to the base units for whatever reason.

Hey it's better this way. Now our kids will learn both the Imperial and Metric system.

or neither system.