# Electric Usage Comparison... PC vs Kettle

Found 11th Apr 2009
Hi

Can somone please settle an aruguement between me and mum.

I need to know the difference (in cost), between boiling a half kettle of water and 3hrs on a PC (which is then shut down, but not switched off at the mains).

iirc, we were told at school, its more or less the same, and that it does no harm leaving the PC on as such? Can someone please clarify.
Depends On The Power OF The Pc, My Setup Dimmes The Street Lighting
Are you using a CRT or LCD?
This guy has done a few measurements of his system, you need to see how long your kettle takes to boil the water so you can work out the consumption. Hope you payed attention to maths at school.
A 3Kw kettle will use .15 kWh in 3 minutes
For a pc to use this amount over 3 hours it would have to be drawing 50 Watts, most pcs use more than this.
Mine draws about 20 Watts when shut down, this depends on the quality of the power supply.
But whatever the figures, mums are always right.
vengod;4893636

Are you using a CRT or LCD?

tft 19"
Going off a basic example, this is PC specific by the way:

positech.co.uk/cli…=21

Your monitor might be around 30 W, your PC around 190 W if gaming, router around 8 W, and I'll assume no other equipment.

Total usage therefore when your PC is running like this is about 228 W.

Hence over 3 hours, you use 0.684 kWhr.

Your kettle being half full is a little more difficult, a kettle might state a power usage but during actual usage, it is likely to start low then get high. For a worse case scenario, we can assume it uses its constant maximum, though.

After a test, my 2.2 kW kettle takes 2 minutes and 18 seconds to boil approximately 850 ml (it has a 1.7 litre capacity). Hence at worst case, the kettle uses 0.084 kWhr.

However, as a more theoretical check, the heat transfer in a kettle is governed by Q = MCpdT and Q = UAdT, the former being most important in this case. We know M (0.85 kg of water), Cp (approximately 4.18 kJ kg^−1 K^−1) and dT (approximately 85 degC, from 15 to 100, units not important as it is all relative), hence this gives an actual heat requirement of Q = 0.85 *4.18*85 = 302 kJ. 1 kJ = 0.000278 kWhr, and therefore the theoretical kettle needs to supply 0.0840 kWhr.

Absolutely amazingly, the theoretical kettle and actual kettle have "used" the same amount of energy, I didn't expect that, that suggests that the kettle is either higher rated than it suggests, uses an AVERAGE of 2.2 kW during its process, did not quite have 850 ml in and/or heat loss is negligible. You would expect the theoretical value to be lower.

Hence the answer is no, you can boil approximately 8.14 half filled kettles for every three hours your PC is on. That's 28 cups of tea.
If your heating is on then chances are that the pc is making the heating work less so it's effectively free to leave the pc on.
Benjimoron;4894459

If your heating is on then chances are that the pc is making the heating … If your heating is on then chances are that the pc is making the heating work less so it's effectively free to leave the pc on.

Heating themostats are usually not where the PC is running, therefore this is not exactly true, instead the temperature of the room will just go up and you will end up opening a window instead.
pghstochaj;4894511

Heating themostats are usually not where the PC is running, therefore … Heating themostats are usually not where the PC is running, therefore this is not exactly true, instead the temperature of the room will just go up and you will end up opening a window instead.

The temp of the room would go up, and if you opened a window then that heat would go out the window. But if you didn't then that heat would spread around the house.

I have no heating in my kitchen, but it's not cold.
I shudder to think how much electricity my pc uses
Benjimoron;4894538

The temp of the room would go up, and if you opened a window then that … The temp of the room would go up, and if you opened a window then that heat would go out the window. But if you didn't then that heat would spread around the house. I have no heating in my kitchen, but it's not cold.

It doesn't really work like that, heat and hot air are different but do similar things. If you didn't open your window, the temperature of the room would increase and the insulation provided by the walls will keep most of the heat in. The heat that does leave will be caused by a high to low temperature driving force, and (for example) if a corner room has two external walls and two internal walls, heat will be lost in both directions. The external walls will be better insulated but the internal walls will have a lower temperature driving force, thus a calculation would have to be done to see how much heat goes which way (Q = UAdT) for a given degree day (or range of). The heat that leaves the room internally will take a long time to reach the thermostat (for example, if in an upstair's bedroom) and thus you create a hotspot area in that part of the house.

What actually happens is that the computer causes a local temperature increase and a window is opened or you put up with the heat, nothing particularly interesting. If what you were saying was true and all the heat would go to heat up the house, you would only ever have to have one room heated and this would heat the rest of the house.
Ok it depends on where it is I guess. My pc is near the middle of the house downstairs. About 3m from the thermostat.
Benjimoron;4894622

Ok it depends on where it is I guess. My pc is near the middle of the … Ok it depends on where it is I guess. My pc is near the middle of the house downstairs. About 3m from the thermostat.

In which case, you may find that your PC is turning off your thermostat earlier than is comfortable for elsewhere (or you actually end up having it on higher due to compensate for the local high temperature area).

Your argument works to an extent with low energy bulbs, but again has similar issues (and more) to those that I have brought up.
pghstochaj;4894262

Hence the answer is no, you can boil approximately 8.14 half filled … Hence the answer is no, you can boil approximately 8.14 half filled kettles for every three hours your PC is on. That's 28 cups of tea.

That is of course dependant on the size/volume of the cups.;-)
Inactive;4894671

That is of course dependant on the size/volume of the cups.;-)

I had to use numerous assumptions throughout, that one being 250 ml cup sizes
so to clarify, which uses more.. a kettle or a pc?
Disco;4898789

so to clarify, which uses more.. a kettle or a pc?

It depends entirely on the PC, a modern PC will use from a couple of watts (a portable laptop) up to about a hundred watts (a PC with high power components and monitor) when idling or doing basic tasks like web browsing. Under maximum load you can multiply those figures by four or five to about 15-20W for a good laptop or 300-400W for a powerful PC and monitor.

So a normal PC with LCD would use maybe 60W when web browsing, resulting in 648,000J. A 3KW kettle boiling for three minutes would use 540,000J, so the PC uses more but not greatly so.

EDIT: J is joules, one watt is one joule per second. You can convert it into watt hours by dividing by 3600.
Disco;4898789

so to clarify, which uses more.. a kettle or a pc?

Are you joking?
EndlessWaves;4899264

It depends entirely on the PC, a modern PC will use from a couple of … It depends entirely on the PC, a modern PC will use from a couple of watts (a portable laptop) up to about a hundred watts (a PC with high power components and monitor) when idling or doing basic tasks like web browsing. Under maximum load you can multiply those figures by four or five to about 15-20W for a good laptop or 300-400W for a powerful PC and monitor.So a normal PC with LCD would use maybe 60W when web browsing, resulting in 648,000J. A 3KW kettle boiling for three minutes would use 540,000J, so the PC uses more but not greatly so.

You won't find many normal desktop PCs that use 60W in total!
pghstochaj;4899291

You won't find many normal desktop PCs that use 60W in total!

I thought most did these days while idling? A 22" TN monitor uses 20W on reasonable brightness so I assume a 19" is proportionally lower, and 45-50W is perfectly reasonable. Looking at the ]latest motherboard test from TechReport shows a system using 57W when fitted with a high powered Xenon CPU and WD Raptor.
sounds like someones getting told off by mum for the high leccy billshe must of just received

lol trying to justify leaving ur lappy on with her having cups of tea.....haha

better get a paperound n start contributing
EndlessWaves;4899356

I thought most did these days while idling? A 22" TN monitor uses 20W on … I thought most did these days while idling? A 22" TN monitor uses 20W on reasonable brightness so I assume a 19" is proportionally lower, and 45-50W is perfectly reasonable. Looking at the ]latest motherboard test from TechReport shows a system using 57W when fitted with a high powered Xenon CPU and WD Raptor.

Honestly, use a PC and you will see it uses far more than 60 W in total.
so does this make sense .......... i've included my Sky box too.....

HP Media Centre PC in use = 110W
Shutdown but still connected = 2W

Boil half a kettle of water = 100W

Sky HD box on standby = 20W

So your PC use approx. 330W + 42W = 372W per day
Boiling 4 half full kettles of water = 400W
Sky HD = 480W

So they are all quite close unless you only brew up once a day

The power rating on a PC's PSU indicates the maximum power output, not the consumption. The more cards/drives etc. you have in the more power it uses.

No, not a single bit of it makes sense, sorry, a watt is a Joule per Second, you need to include a time element to have it as an energy - your a dimension out at the moment, sorry.

Look at my post on the previous page for a methodology and also the answer to your specific question.

Disco;4899556

so does this make sense .......... i've included my Sky box too.....HP … so does this make sense .......... i've included my Sky box too.....HP Media Centre PC in use = 110WShutdown but still connected = 2WBoil half a kettle of water = 100WSky HD box on standby = 20WSo your PC use approx. 330W + 42W = 372W per dayBoiling 4 half full kettles of water = 400WSky HD = 480WSo they are all quite close unless you only brew up once a dayThe power rating on a PC's PSU indicates the maximum power output, not the consumption. The more cards/drives etc. you have in the more power it uses.

pghstochaj;4899539

Honestly, use a PC and you will see it uses far more than 60 W in total.

Well I can't speak from experience as I'm a keen PC gamer so I do have to put up with fairly high power consumption from my PC (running BOINC doesn't help with that either :-) ).

Disco: Watts are similar to miles per hour. You can drive at 60 miles per hour, but if you drove for three hours that doesn't mean you did 180mph.

Likewise, watts are joules per second. Something that consumed 3W for three seconds used 9 joules of energy.

Watt Hours are just equivelent to the energy of something using 1 watt for 1 hour, or 3600 seconds. So 1 Watt Hour is 3600J.

20W for 24 hours (you don't turn it off at night?) is 20W*60s*60m*24h = 1,728,000J, or 1,728,000J/3600 or 20W*24h = 480Wh. So you've basically worked it out in watt hours for the sky box and PC but labled it with the wrong unit.

The kettle isn't correct though. 3kW means 3kJ/0.8333Wh per second, so it'll be using that for however long it's on (I think the volume of water just affects the time it takes to shut off rather than the power draw). If it's on for 9 minutes and 36 seconds total throughout the day then it'll be using the same amount of power as your sky box.