General kitchen applicance power consumption calculation KWh, KW etc

[Kay]

I'm playing around with max demand and diversity figures and thought I'd try and get better/more factual figures. Most appliance manufacturers tend to quote in KWh which doesn't make much sense for instantaneous power calcs and for A rated appliances the actual power seems to be very low with my calculations.

If Watts are the unit expression of Joules per second would it be fair to suggest that:

KWh * [(1/60)/60] = Power as Joules per second or Watts

 

[Silly Sausage]

I'm playing around with max demand and diversity figures and thought I'd try and get better/more factual figures. Most appliance manufacturers tend to quote in KWh which doesn't make much sense for instantaneous power calcs and for A rated appliances the actual power seems to be very low with my calculations.

If Watts are the unit expression of Joules per second would it be fair to suggest that:

KWh * [(1/60)/60] = Power as Joules per second or Watts

When they are quoting kWh figures you need a time period in order to calculate the power consumption, eg an appliance uses 240kWh in 24 hours then its power rating is 240/24=10kW -> 10000/230 = 43.5A

kWh is a unit of energy, in Joules.
So kWh = 1000*60*60 = 3.6MJ

 

[lauriehurman]

As I understand it a kWHr is an amount of energy.
240kWhrs over a 24Hr period would be an average of 10kWHrs each hour.
That's an average of 10/60 kWHrs each minute.
That's an average of 10/(60 * 60) kWHrs each second.

Think of pouring sand into a pile. If 240kWHrs is the size of the pile of sand after 24 hours then the pile of sand will be increasing at an average rate of 10/(60 * 60) kW each second.
So the rate of flow of sand must be 10000/(60 * 60) Watts, which equals 2.7 Watts.

For a 230 supply thats 0.012 Amps.

Your original question was about rating the cooker in kWHrs instead of Watts. I would guess (I don't know) that this is to take account of the way that the thermostat switches the current on and off in order to maintain a particular temperature. The kWHr is a way (I'm guessing) to present an average power use.
So if you have your cooker on for an hour that's how much energy you will use. That's how big the pile of sand will be but the flow of sand is being switched on and off so you have no information as to the rate of flow when it is on because you don't know what proportion of the time it is on and what proportion it is off.

You can only say that to accumulate 1kWHr in the period of an hour would require an average power flow of 1000/(60 * 60) Watts.

Hope that helps.
Laurie

 

[JUD]

You can only say that to accumulate 1kWHr in the period of an hour would require an average power flow of 1000/(60 * 60) Watts.

Your electric bill must be high!!!

To accumulate 1KWh in 1 hour would require a power flow of 1KW, not 0.27W.

 

[Kay]

Thanks for the replies on this one. The pile of sand analogy made it easier to visulize. What wasn't making sense to me was how the quoted power rating on an appliance related to it's energy rating in KWh. After some further investigation I was reminded that motor driven appliances would be inductive and also found out that nominal power ratings quoted on these appliances may refer to the drive shaft output power rather than input power plus with higher starting currents and power factor also needed to be considered nominal power ratings imo shouldn't be used for diversity calculations.

Anyway the reason I started looking into this was because I saw some crude math showing how a ring final supplying a modern kitchen would be inferior to a number of dedicated radial circuits. e.g.

washing machine 1800W
dishwasher 1500W
kettle 1900W
fridge freezer 600W

works out to just over 25 Amps Add to this the other common appliances such as microwaves, televisions, radios etc and it would seem that the 32A limit is close to being breached.

However individual nominal power ratings for motor appliances (inductive loads) taken at full power do not show a true picture of actual demand.

An interesting link for typical values (can't comment on accuracy of data)

Electricity Consumption of Domestic Appliances

 

[malcolmsanford]

In real life I have yet to see a ring final at 32 amps ever come close to it's load condition in a domestic situation. They could be windows of perhaps a few minutes when you could have the kettle, dishwasher, washing machine and F/F all active, but it is very rare.

Though I always have advised my clients not to have appliances on rings if possible, especially if they all integrated into a kitchen rather that say a utility room, not for loading but just for division of installation. If your dishwasher tripped the ring and everything was on it, then everything goes off.

 

[i=p/u]

1kwh = 4A so if a customerr says i got new cooker @ 3kw to me that means 12A and if electricty provider charges at 14 pence per kwh , to me that means the appliance costs 14p x 3kwh = 42p per hour on full load. am i correct ?????

 

[Engineer54]

1kwh = 4A so if a customerr says i got new cooker @ 3kw to me that means 12A and if electricty provider charges at 14 pence per kwh , to me that means the appliance costs 14p x 3kwh = 42p per hour on full load. am i correct ?????

Actually NO!! It would if it was a constant load, but as you know, these elements will be switching on and off during that hour, so the actual cost will be lower!!

 

[i=p/u]

aye so would you treat as diversity of 42p per hour multiplyed by 0.4 as a average if it wasn't a constant load

 

[Engineer54]

If you wanted to go that far, i suppose you could!! ...lol!!