Amps to Wattage formula

[stean_5085193]

Hi all

Can anyone tell what the mathamatical formula is to work out how many watts of electric you can use on a given amount of amps

Many thanks

Stean

 

[RogerL]

volts x amps = watts

eg 230V x 4A = 920w

 

[cooky1]

V = I x R.

Same thing - Voltage = current x Resistance.

 

[CheapnCheerful]

No it is not Graeme! You have quoted ohms law, which does not even have watts in it, let alone being the same thing!

Roger you are correct.

 

[cooky1]

Oops. Sorry, my physics is obviously a bit rusty.

 

[JeremyClarksson]

Watts = Volts X Amps

Or if you need to work it from the other direction for supply

Amps = Watts / Volts

i.e. 2.4 kW (/240V) requires a 10A supply

 

[Guest]

Use 230 volts as the 240 has been superceded by European directive.

 

[JeremyClarksson]

A new european standard has come in and the the old british 240V standard falls within it, nothing has been done to change the infrastructure and in most uk houses you will still measure closer to 240V than 230V in the same way that in most european countries you will still measure closer to 220V.

It is only the standard and tolerances that are different.

 

[JeremyClarksson]

A bit more from Wikipedia

"Following voltage harmonization all electricity supply within the European Union is now nominally 230 V

 

[RogerL]

Just to echo what Jeremy has posted - the NOMINAL voltage has changed to 230V - the actual voltage still varies exactly like it always did - in practice appliances were designed for 200-250v long before we joined the Common Market.

 

[BigAl1]

So, given that an appliance needs the same operating power (Watts) regardless of what the available voltage is, That means that the current (Amps) will be higher if the voltage is lower.

So we a caravanner gets less capability from a 16 Amp hook up.

 

[RogerL]

The current will drop as the voltage drops due to Ohm's Law V=IxR, the resistance remaining constant - so the power (W=VxI) drops with the square of the voltage.

240V from a UK site will give 19% more power from a particular mains appliance than a continental site at 220V.

 

[JeremyClarksson]

You beat me to it Roger - a nice clear explanation.

Big Al

You could expect a kettle to take longer to boil, although you probably would not notice it in a warmer climate.

An appliance does not need a certain level of power, it is what it is rated at and would normally be available at the rated voltage (ie what it says on the box)- back to standards again really.

 

[Rubix]

RogerL : "240V from a UK site will give 19% more power from a particular mains appliance than a continental site at 220V."

Are you sure ? If the available current is the same, yes, but I thought that the continent had more current available or do most sites have 16A.

 

[rob_jax]

Hi,

Roger and Jeremy are correct, and maybe I can explain why.

People has seen that an appliance is rated at 1000 Watts, and if the electrical supply in the UK is 240 volts - then from the Watts = Amps x Volts equation that means a current of 1000 / 240 or 4.17 Amps. But the same 1000 Watts on a 220 volts supply would need 4.5 Amps - and it's this difference that BigAl and Rubix are asking about.

But this difference doesn't actually exist - the constant in this is not the rated wattage of the appliance, but the internal "resistance" of the appliance. And the rated power consumption in Watts differs with differing supply voltage.

Our appliance with a rating of 1000 Watts was measured at the nominal 230 Volts - a current of 4.35 Amps. The internal "resistance" of the device doesn't change, so if you operate it on 240 Volts - the current flow will be approximately 4.5 Amps and the device will actually use 1080 Watts. Likewise running the appliance on 220 Volts will use 915 Watts.

So a kettle will take slightly longer to boiler on a lower voltage supply.

But in any case - this is all a bit simplistic - AC devices don't have "resistance" they have "impedance" and the mathematics is seriously more complex that the DC equations this thread has used - but they are good enough for home calculations.

And the blue IP44 caravan hook up lead and continental Schuko-style mains plugs - are all rated for safe continuous operation at 16 Amps. Caravan sites may fit 16 Amp trips on each power pole (or a different rating) but that doesn't mean that the appliance "expands" to draw more power - just because the site has fitted 16 Amp rated sockets.

Robert

 

[Bigfoot]

All I know is...it stings if you stick your finger in it !!!!

 

[Rubix]

And it hurts more if you lick your finger first !

 

[ProfJohnL]

Spot on Rob_jax

As for Rubix's last post, I doubt you would actually remember whether you had liked your finger or not, though the undertaker might have to put gloves on you.