Understanding Kwh

[ey143]

I'm trying to better understand Kw hours and how monetary charging is done.

If I buy an appliance like an electrical heater or air con unit and it states that it is a 1kwh system, does that mean it is consuming 1000 watts of energy all the time (assuming its on max setting) and if electricity is priced at 13p per kilowatt, it would therefore cost me 13p per hour to run that equipment?

Also when a washing machine is being marketed and we're told it consumes 2kw per wash, it means that 2000 watts of energy have been used (rather than 2kw of constant usage) and therefore costing 26p of electricity?

 

[JohnW2]

If I buy an appliance like an electrical heater or air con unit and it states that it is a 1kwh system, does that mean it is consuming 1000 watts of energy all the time (assuming its on max setting) and if electricity is priced at 13p per kilowatt, it would therefore cost me 13p per hour to run that equipment?

Not necessarily - that's probably the maximum it might cost. Most domestic appliances - whether for heating, cooking, washing or whatever will use significant power only intermittently (i.e. will turn themselves on and off, often by means of a thermostat) and the power quotes (1 KW, 2kW or whatever) is usual the peak of the power when 'on'. The average over an hour (which is whjat matters in terms of determining kWh, hence cost) may therefore be much less than that 'peak figure'.

Also when a washing machine is being marketed and we're told it consumes 2kw per wash, it means that 2000 watts of energy have been used (rather than 2kw of constant usage) and therefore costing 26p of electricity?

"2 kW per wash" is meaningless. Are you sure that it's not "2 kWh per wash"? If it were, then it would, as you suggest, be saying that an entire wash (however long it took) would cost 26p - but if it really says "2 kW per wash", then it's anyone's guess what they mean!

Kind Regards, John

 

[JohnD]

Also when a washing machine is being marketed and we're told it consumes 2kw per wash, it means that 2000 watts of energy have been used (rather than 2kw of constant usage) and therefore costing 26p of electricity?

A washing machine uses a high current for a short time if it is heating the water for a hot wash. It will use more for a hot cotton wash than for a cool synthetics wash or a cold wool wash.

And a low current for a longer time when it is soaking, churning, spinning or pumping.

that claim does not make sense.

look it up again.

Does it say "2kW per wash?" (nonsense) or does it say 2kWh per wash (quite possibly true but looks rather high to me)?

 

[big-all]

kwh is a unit off energy for a given time [one hour ]
look at a power tool battery it has volts x watts times time as a total energy stored as in18v times 5a times 60 mins [18x5x60=5400w]
virtually no item will consume a constant amount off energy it will tend to fluctuate small amounts to wildly different and unpredictable this is why annual consumption in units are often quoted but can be miles out even then as there are at least 100 variables effecting the outcome lol

 

[ericmark]

There are 3600 kilojoules in a kWh, the thing is a watt is a joule per second, so the watt/hour cancels out the time, so since there are 3600 seconds in an hour, it is 3600 times larger than a joule. So in spite there being hour in the name the kWh has nothing to do with time.

So some thing like a freezer has the average energy it will use per year, for example a 70 watt freezer at 70 litres, will likely use 199 kWh/annum. So it will use around 0.55 kWh per day, so it will run 550/70 = 7.7 hours per day.

Washing machine is hard, it says £23.4 per year to run, but I tried using an energy meter on a washing machine, and it depends not only what program is used, but also the weight of the load, so really impossible to judge. But with the freezer if the energy label says 199 kWh/annum and one measures 0.65 kWh per day then likely it is OK, but if it measures 0.75 kWh it is likely the insulation is damaged and it's using to much.

There may be a good reason to measure what a freezer used per day, but with a washing machine, rather pointless, and with a TV it is clearly marked on back, so really no point measuring what it uses, and to measure all electrical equipment together will tell you very little if anything, which makes the Smart meter so daft, as it measures everything, you need to measure each item, there are some surprises, in stand by equipment should use less than 1 watt, however there are exceptions, for example a Sky box auto changes times if the programs are not at the time published, so the LNB has to be powered to get that info, so it uses well over one watt on stand by.

However at 15 watt, and 13p per kWh it will cost 4.68p per day to be sure not to miss a program, I can measure how much power my charger uses to charge my caravan battery but why bother, as it needs charging, so does not matter how much it uses, I have two options, throw it away or charge it, so since I need a battery in the caravan I will charge it, does not matter how much it uses.

 

[AndyPRK]

As you say washing machines are very variable. And modern ones take a long time to be able to wash at lower temperatures.

therfore stating a wash is 2kw would be a good way to tell a buyer how much a wash will cost and allow comparison between different brands

 

[FrodoOne]

There are 360 kilojoules in a kWh, the thing is a watt is a joule per second, so the watt/hour cancels out the time, so since there are 360 seconds in an hour, it is 360 times larger than a joule. So in spite there being hour in the name the kWh has nothing to do with time.

Since 1 Watt = 1 Joule per second
and
there are 3600 seconds in 1 hour (60 * 60)
1 Wh = 3300 Joules
and
1 kWh = 3600 kJ (kilojoules)

 

[charliegolf]

It's sometimes easier to think of the kWh as 'one click' of your electricity meter. It's also easier to group appliances as being 'constant' or 'intermittent' users of the electricity. If you put a 100W incandescent light on, it will go 10 hours for one click. A 10kW shower will do 6 mins for a click, and so on. Fridges and ovens use energy to maintain a temperature, so are not so easy to gauge, as has been said.

 

[big-all]

as an aside
some years ago i used a plugin flow meter
washing machine full load 60 degree wash cold fill 1600 spin speed 2kwh max rated machine
tumble dryer 2kw rated as well full cycle the washing machine used around 750wh[3/4 off a unit] the dryer used about 1.75kwh [1 3/4 units]over say 90 mins clothes 100% dry
so in total 2.5 units at 12p so 30p to do a full load wash and dry for around 30p quite a surprize

 

[davelx]

As you say washing machines are very variable. And modern ones take a long time to be able to wash at lower temperatures.

therfore stating a wash is 2kw would be a good way to tell a buyer how much a wash will cost and allow comparison between different brands

Stating a wash is 2kW tells you nothing at all, other than that the people writing the spec/instruction book haven't got a scooby.
For clarity, since the terms kW and kWh have been used erroneously by more than previous poster:

2kW is a RATE of use of energy (2000 Joules/second), not AN AMOUNT of energy, in the same way that miles per gallon is a rate of fuel use, not an amount of fuel.
You would not express the amount of fuel needed for a given car journey as 45mpg - you have to multiply by the number of miles to find out how much fuel is used. In the same way, you cannot express the amount of electricity needed for a wash in kW (which is analogous to mpg) - you have to multiply by the time that the energy is used for (assuming it's constant - it get more complicated with a washing machine which has various stages that consume energy at different rates)

So, a 1kW electric heater users 1000 Joules/second. If you run it continuously for 1 hour, it uses 1000x3600 Joules of energy 360,0000 Joules (or 3600 kJ(oules)) or 1kWh (i.e. 1kW for 1 hour) which is an easier unit to work with and is why our electricity meters still measure in kWh.

 

[ericmark]

Since 1 Watt = 1 Joule per second
and
there are 3600 seconds in 1 hour (60 * 60)
1 Wh = 3300 Joules
and
1 kWh = 3600 kJ (kilojoules)

Yes error corrected, thank you.

As you say washing machines are very variable. And modern ones take a long time to be able to wash at lower temperatures.

therfore stating a wash is 2kw would be a good way to tell a buyer how much a wash will cost and allow comparison between different brands

This is the problem with using the kWh unit, people get kWh and kW mixed up. The idea was to make it easier and also have a smaller unit, so a washing machine has likely a 100 watt motor, and a 2 kilowatt heating element, the latter is used for a very short time, so using the £23.40 per year and running the machine once a day it would mean 427 watt/hours per wash.

I have never worked out why washing machines stopped having hot and cold fill? My father-in-law had hot water solar panels, which had they worked mean he wanted to use hot water from the store, and with gas or oil hot water heating it is cheaper than electric, but finding a hot/cold fill washer is near impossible today. Idea is better if cloths washed first with cold so anything that heat will set washed off first, egg for example, but when hot water is there to heat up cold water still not good.

To get a zigbee socket adaptor so I can set outside light to switch on just in the morning for milkman costs £7.99 from Lidi, the bulb uses around 12 watt, so saving 20 hours per day by switching it off, so approx 1.68 kWh per week, so 25.2p per week, so 32 weeks to pay off cost of adaptor, so question is if worth the effort?

 

[JohnW2]

...therfore stating a wash is 2kw would be a good way to tell a buyer how much a wash will cost and allow comparison between different brands

Eh? As I and others have said, not only is a not "a good way" of determining the cost of a wash, it's not a way at all. Unless, as I said at the start, the OP misread it and it actually said "2 kWh", it's not clear what it means (presumably the peak instantaneous power utilised?) but, whatever it were, it would not, alone, enable one to determine what "a wash" would cost.

As has been said, power (Watts) is a measure of energy usage per time - so quoting just a power would be like saying that a car used "5 litres of fuel per hour" - which, in the absence of other information would not enable one to compare its fuel; consumption with any other vehicle.

Kind Regards, John

 

[winston1]

Hot and cold fill washing machines rarely used the hot (only for the very hottest wash). No machines mixed hot and cold for cooler washes so no point in hot and cold fill.

 

[JohnW2]

Hot and cold fill washing machines rarely used the hot (only for the very hottest wash). No machines mixed hot and cold for cooler washes so no point in hot and cold fill.

Even less point in recent years (which is presumably why they stopped having 'hot fill') since they now use such small puddles of water that (unless the hot water cylinder/combi is very close to the machine) little, if any getting to the machine would have actually been hot, so the only effect of 'hot fill' was to move hot water into the pipewort, from where it would be 'lost'

 

[FrodoOne]

Hot and cold fill washing machines rarely used the hot (only for the very hottest wash). No machines mixed hot and cold for cooler washes so no point in hot and cold fill.

I do not know if these "Fisher And Paykel" New Zealand made machines are/were available in the UK, so winston1 may not know about them!
However, they do connect to both the Hot and Cold water supplies, have no heater for the water and do vary the temperature by mixing hot and cold water as required.

(https://usermanual.wiki/Fisher-And-...tiveEcoIwl12UsersManual119880.1472772770/html See Pages 14 and 18)

Even less point in recent years (which is presumably why they stopped having 'hot fill') since they now use such small puddles of water that (unless the hot water cylinder/combi is very close to the machine) little, if any getting to the machine would have actually been hot, so the only effect of 'hot fill' was to move hot water into the pipewort, from where it would be 'lost'

Fortunately, the machine which I have is located within about 4 metres of insulated pipe work from the Solar Hot Water tank.