Watt-Hour Question

Watts represents power, regardless of voltage, current and power factor. If you already know watts, you don't need to do all the other stuff with current and voltage, just use the watts and the time for energy consumed.

A load of a nominal 200W will consume approximately 200 Watt Hours in one hour.

The electric power company's meter records the integrated watts x time x power factor to bill you for the actual watt hours consumed. Thus it accounts for changes in load as various appliances are turned on or off and the length of time each one is on based on the voltage and amperage from moment to moment.

Hope that clears things up for you.

It depends where your numbers come from. Many devices have a label which gives the maximum power or current used by the device. For example a machine may have heaters, fans, etc. which automatically turn on when required, and turn off when not needed. The actual power used by that machine wil be less than the label value, most of the time.

In that case you are right that you must measure the actual power, which for most people means measuring the voltage and current and calculating the power, since few of us have power meters.

Even for devices that use a constant amount of power, there is variation between different units, even of the same production run; if you need precision measurements of power (rare), then again you would have to measure the actual power (voltage x current) used.

Then there is the issue of devices like refrigerators that only run part of the time. The only convenient way to determine the energy used is to connect the device to a kWh meter for an appropriate amount of time. I rescued the kWh meter from a very old house being torn down, and added a plug and socket so it could be used for such measurements. (very old meant low current rating, so it was good for this use)

Power is actually the rate you are doing work or in other words the rate you are consuming energy. The unit for energy in the SI system is a Joule and by definition if you consume 1 joule over a period of 1 second then the power is 1 watt. Put mathematically

Power = Energy ÷ Time

Watts = Joules ÷ Time in seconds

Now most electricity supply authorities bill you according to watt hours or kilowatt hours therefore

Kilowatt Hour = 1000 Watts x 1 Hour

Kilowatt Hour = 1000 Watts x 3600 Sec = 3600000 Joules

Kilowatt Hour = 3.6 Mj

If you are calculating the power in an electrical device you can only multiply the voltage and current to get the power being consumed for DC devices. You can do this because in a DC circuit voltage and current are scalars.

To calculate power in AC devices you must allow for the fact that the voltage and currents are now vectors and have a direction as well as a magnitude. With an AC supply the voltage is a vector that is continually rotating through 360º 50 or 60 times a second depending on the frequency of the supply.

If you have a purely resistive load the current vector will have the same angle as the power vector. If you have a purely inductive load then the current vector will be lagging or behind the voltage by 90º. For a purely capacitive load the current will lead or be in front the voltage by 90º.

Now every load really consists of resistive inductive and capacitive components so depending on the type of equipment you are powering the AC current will be somewhere between 90º ahead to 90º behind the supply voltage. This is where the power factor comes in, the power factor which is expressed as a fraction less than 1 tells you how much the current is out of phase with the voltage. A power factor of one indicates they are exactly in phase. The subject of power factor and what it means can be complex and has been discussed in depth in other threads at CR4 so I will not repeat it here. If you wish to know more about it do a search of CR4 on Power Factor and read the threads.

Masu-san wrote: "If you have a purely inductive load then the current vector will be lagging or behind the voltage by 90º. For a purely capacitive load the current will lead or be in front the voltage by 90º."

------------

Ooh noo! ELI the ICE man cometh! Wattch out!

Nice write-up Masu.

-e

I know that one by heart: I before E except after C.

Whoops. I before E only when C. Er, no... do I have the right class?

I think the rule is: I before E except in:

C?

E

Chernoff, write U R. I stand collected.

Hope yours was a great Christmas, Mr. Fry!

-e

Thanks! I hope yours was too!

dear

watts simply denotes power demanded by a particular electrical load on ac or dc supply

what ever current , voltage or power factor is there this does'nt mean .

ya, enegy diffinitely will be electrical power multiplied with time span of duration.

MASU took the words out of my mouth. That was a trip down memory lane.

I have always found it helps to imagine the effects of inductance and capacitance (due to the frequency) causing the 'impedance' to constantly change (it's called 'impedance' but it is measured in Ohms). This means the current will constantly change due to the change of impedance as well as the change of voltage.

It becomes a very complex calculation if working with actual amps, volts and ohms. But it is not necessary in your case because the power rating takes all this into account.

You just count the watts over a period of time to get the energy consumed.

Masu did a good job for theory sake. I would only add that AC power on a purely resistive load (current in-phase with voltage - i.e. power factor=1) is called active power , while power on a more or less capacitive or inductive load is called reactive power and you have to draw the triangle of vectors.

In practice, you can use an AC power datalogger like Fluke 1735 (or similar) from www.powermeterstore.com which can store for up to 45 days all power events and aspects vs. time (power, voltage, current, frequency, harmonics, power factor) to monitor power consumption. Finally, you can download the recording on your computer and analyze data as table or graph.

I hope this is answering your question.