Amps x Volts = Wattage ?

robert_harper_2000

OK I've got a plug in monitor and am testing my TV setup. But on my meter it says
15.2 W
0.16 A
243.2 V

A x V = 38.91

So... I have watts calculating at 38 and registering at 15 - ?

Which one is correct?
To work out how much it costs I need it accurate as
Wattage x Hours used / 1000 x price per KwH = cost £

Thanks for your help

penrhyn

Probably inaccurate at low currents, what readings do you get with the TV etc switched on?

robert_harper_2000

At the moment
83.9 W
0.45 A
244.7 V

So again about 30% off - should be around 110?

penrhyn

Odd, maybe some power factor thing?

Cardew

Volts x amps = VA or KVA and not the same as Watts.


On most plug-in monitors if you toggle the watts button you get VA.


It is a matter of Power Factor as indicated above. If you don't know about Power Factor - you really don't want to know!!


This is the simple explanation;)

In an ac circuit, the power P consumed can be expressed in terms of applied voltage, V, current I, resistance R and the phase angle A between current and voltage. V, I are instantaneous values.

P = V.I. cos A = I*2.R .
From a phasor diagram, we get Cos A = R/Z
At resonance, when Inductive reactance XL = capacuitative reactance Xc; the total impedance is minimum, Zmin = R;
Cos A = 1 and the Power consumed is maximum, Pmax = VI.

Hence, in general, P = V.I. cos A = Pmax. Cos A.
So, the actual power consumed, in general, is less than the maximum power that can be consunmed (at resonance) by a factor, Cos A. Hence, Cos A is called the power factor.

One may define power factor = P/Pmax
One may also defione powerfactor as Cos A =R/Z

Don't worry about VA as you are only charged for the Watts(kWh) you consume.

robert_harper_2000

ah I have heard about Power Factor.
OK so
At the moment
83.9 W
0.45 A
244.7 V

For me to calculate my costings I only need to look at the 83.9W?

Cardew

robert_harper_2000 wrote: »

ah I have heard about Power Factor.
OK so
At the moment
83.9 W
0.45 A
244.7 V

For me to calculate my costings I only need to look at the 83.9W?

Correct - Power Factor mastered!

robert_harper_2000

Thanks for your help.

undaunted

For a simple answer isn't there a plate on the back of your tv / most appliances that tells you what power they consume per hour?

Alternatively try this site for a quick look guide to all manner of appliance running costs http://www.sust-it.net/

penrhyn

A case of pennywise pound foolish, always go for the big energy guzzlers first, i.e the ones that heat stuff. OP May already have done this of course.

Remember if using an electric hob, only use enough water to cover your vegetables, the same applies when taking a bath.

Cardew

undaunted wrote: »

For a simple answer isn't there a plate on the back of your tv / most appliances that tells you what power they consume per hour?

The plate on the back of an appliance tells you the maximum rating, which is useless for calculating electricity consumption.


For instance the plate on the back of my freezer states 500 watts. That is when the compressor is running and door open(light on). Like most I leave my freezer on 24/7 and it doesn't use 4,380kWh a year at a cost of £500.


The same applies to almost every appliance - oven, washing machine, dryer, microwave, Iron computer etc etc.