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A few people have asked how they can measure energy consumption around the house, so I've written up the details with some examples. Hopefully, it's understandable!

First, get yourself a power meter. The one from Maplins is the cheapest I've found and is accurate with low wattage appliances. It's also possible to plug it in to sockets that are low on a wall.

The more expensive Brennenstuhl PM230 does have the extra facility of being able to put in two electricity tariffs and doing the calculations for you, but it has the display under the plug and can't be used on low sockets with little clearance under them. It's also not very effective when devices draw less than 5 watts where the Maplin one works down to 1 watt. I'm not linking to the PM230 as the prices vary so widely and change frequently - the cheapest I've seen is £19.95 and the most expensive £49.95!


Now, back to the meters. The three most important readings are active power (in watts), energy consumption (kWh) and elapsed time.

While you can use the active power reading, it's not going to be accurate over time for appliances that come on and off (like refrigerators and freezers) nor for those that may be on standby some of the time and powered up the rest of the time. I also find it more convenient for things like kettles to leave them on the meter for a week as I then get a better idea of our typical weekly useage.

But assuming that you want to find out the energy consumption of a kettle as a one-off, I'll start with that.

Plug the meter into the socket and the kettle into the meter. Turn the electricity on at the mains. Check the meter - the display should be on and, if the appliance is off, then the active power should read zero. I say should be because some appliances do draw a very small amount of power even when the switch is off.

With the kettle plugged in (and hopefully with some water in it), turn it on. The active power reading should climb immediately. Make a note of the lowest and highest readings - you may be surprised at the amount of fluctuation. When the kettle has come to the boil and switched off, click the button to display the energy consumption in kWH, note the figure, then display the time and note the figure.

Remember:

Energy consumption (kWh) = Power (kW) x Time (hours)

Things to remember: Convert watts (on the meter) to kW by dividing by 1000 (10 watts = 0.01 kW,180W = 0.18, 2300W = 2.3kW). Convert time to decimal by dividing minutes elapsed by 60 (12 minutes = 0.2 hours; 1 hr, 18 minutes or 78 minutes = 1.3 hours).

To find the cost of boiling the kettle once, multiply the energy consumption by the price per unit of the electricity (in our case £0.0888).

Cost = Consumption (kWh) x price (pounds)

To work out the yearly cost, calculate how many times a day you use the kettle and multiply by 365. Then multiply by the Cost of one boil.


Now, the more complicated version using a TV which is either on stand-by or on.

Plug the meter in as before and then plug the TV into the meter. Operate the TV as you would normally for a week - but if you normally turn off at the mains and don't use standby, unplug the TV from the meter while leaving the meter on. If you switch off at the mains, you'll lose the meter readings!

The power consumption reading is mainly for interest this time - you can see the difference between standby and on.

The key figures this time are energy consumption (kWH) and time.

After a week, you'll have metered your typical useage in kWh and can multiply this by the price per unit (£0.0888) to find out how much it cost to run the appliance for a week.

To find out the yearly cost, you can either multiply by 52 or, more usefully, work backwards to calculate the average power drawn over the the week (remembering that the TV draws different amounts of power in standby and on modes) and then multiply by the number of hours in a year. The advantage of this is that you then know the actual amount of power drawn by the appliance.

Power (kW) = Energy consumption (kWh) ÷ Time (hours)

To find out the annual consumption, multiply power by 8765 (the rounded number of hours in a year). To find the annual cost, multiply the annual consumption by the price per unit (in our case again, £0.0888). This should be close to the number you got by multiplying the weekly cost by 52.


Our first example (using the kettle to boil one cup of water).

On power: Appears to range from 2026 watts to 2075 watts (2.026kW to 2.075kW)
Energy consumption: 0.04 kWh
Elapsed time: 1 minute (0.01666666667 hr)
Electricity cost per kWh (1 unit) = £0.0888

Cost to boil one cup of water: 0.04 x 0.0888 = £0.0.003552 (not even a penny)

Average active power: 0.04 ÷ 0.01666666667 = 2.3999999995 kW (2399.99 watts) Note that this is substantially more than the active power readings appeared to indicate!

Now to work out annual energy consumption and cost at six cups of tea a day (between the two of us):
Daily energy consumption: 0.04 x 6 = 0.24 kWh
Annual energy consumption: 0.24 x 365 = 87.6 kWh
Annual cost: 87.6 x 0.0888 = £7.77888



Our second example (with the TV left on stand-by rather than off to set a baseline):

Standby power: 6 watts (0.006 kW)
On power: 52 watts (0.052 kW)
Energy consumption: 3.36 kWh
Elapsed time: 168 hours (7 days)
Electricity cost per kWh (1 unit) = £0.0888

Annual consumption: 3.36 x52 = 174.72kWh

Weekly cost: 3.36 x 0.0888 = £0.0.298368 (30 pence a week)
Annual cost : £0.298368 x 52 = £15.515136

From those figures, I can now work out the average active power drawn by the TV over the week (combining both standby and on modes):

Average active power: 3.36 ÷ 168 = 0.02 kW (or 20 watts)

Double-checking my earlier annual consumption and cost figures:

Annual consumption: 0.02 x 8765 = 175.3kWh (slightly more than the earlier figure as we're using the actual number of hours in a year - 52 weeks gives 364 days or 8736 hours)
Annual cost: 175.3 x 0.0888 = £15.56664

I prefer to work out the average power drawn over a week and then multiply by the number of hours in a year as it's more accurate.



So, if we have six cups of tea a day, don't turn the TV off at the wall and watch TV at our current levels, that's an annual energy consumption of 262.9 kWh at a cost of £23.35 (rounded up).

The big energy hogs are the refrigerator, freezer and washing machine, then the security/work lights although their use is kept to a minimum. The electric fence draws constantly but not a lot (and with our run of fencing a mains energiser is much more effective than 12v). I'm doing the computer and peripherals at the moment.

I hope this helps. (I've triple-checked the figures, but if you spot anything let me know!!)

BTW our CRT TV is quite low in its energy use.

Research for the 40% house found the typical CRT TV uses 75 watts in on-mode while a typical plasma TV uses 350 watts.

Taking our TV as the baseline with 52 watts in on-mode and dividing all three by 52 to give a baseline ratio:

Our TV : Typical CRT TV : Typical plasma TV
1 : 1.44 : 6.73

Yes, buying a typical plasma TV would multiply our energy consumption 6.73 times!!!

Using that ratio to convert our baseline TV energy consumption of 175.3kWH, that would give annual energy consumptions of:

175.3kWh : 252.432kWH : 1179.769kWh

at a cost of:

£15.57 : £22.42 : £104.76

And that's why I want plasma TVs banned. People may well swap their incandescent light bulbs for more energy efficient ones, but the whole gain is wiped out when they swap their CRT TV for a plasma one.


The most energy efficient television are rear projectors, then LCDs. To take account of different screen sizes, CNET did an extensive review of TVs and then calculated a watts per square inch rating:

Microdisplay rear projector: 0.11 to 0.15 watt per square inch
LCD: 0.16 to 0.41 watt per square inch
CRT: 0.25 to 0.40 watt per square inch
Plasma: 0.30 to 0.39 watt per square inch

Of course, modern TVs use more efficient technologies but as TVs have become bigger, the gains in efficiency are often lost as people trade up to a bigger TV.

Phew, now I need a brain break and do some wood chopping!!

I bought one of those meters from maplin yesterday stonehead as they were in the sale and i also bought a 12v trickle solar panel charger as that was £10 in the sale.

The solar panel is already earning its money charging 12 volt lead acid batterys in the garage that ive been collecting for a future project.

The power meter is pluged into the kettle socket to measure how much electricity the kettle uses in a week then after the first week it will be moving to the freezer.

Ive already noticed that the kettle uses a fair bit more electricity to boil a set amount of water straight from the tap as it does to boil the same amount that was left in a jug to come to room temperature first.

So from now on we have a sealed jug on the kitched worktop that gets topped up whenever anyone walks past and it looks low.
This should save a slight amount of electric use and in turn save me a little money as well as bennifit the enviroment.

I'd heartily concur with that! - my pet bete noir at the moment is halogen spotlight bulbs - they are still being installed widely, and soak up enormous amounts of power - I've become a "light counter" - they are probably 50w apiece, so I'm to be found gazing at where they're fitted going - "lumme, that's 30 bulbs in that ceiling, at 50w a pop, that's over one and a half kilowatts" - where the area could be better lit with say 3 20w energy savers!

One thing I just remembered on the computer side:

Turn off all the peripherals you're not actually using. I do this already, but was interested to see what the difference was when metered.

Using two meters (as I use two sockets):

With the computer, CRT monitor, USB hub, network router, scanner, printer and speakers on, the active power read:

194 watts and 35 watts.

When I went back to my usual power-on configuration of computer, monitor, USB hub and router, the active power read:

170 watts and 5 watts.

Obviously the scanner and printer would draw even more when in actual use rather than on standby, but that's still a good drop. I can drop it even further by unplugging the speakers' transformer from the mains, when I get a reading of:

165 watts and 5 watts

So even when they appear to be turned off, the speakers' transformer is still drawing 5 watts!!! Just to heat my toes...

Jolly good posts, Stoney. Shouldn't this all be consolidated into an article for the main site?

Excellent stuff Stonehead. I will come back and have a re read as its a lot to take in, in one go. In the kitchen, I changed 8 50w halogens to 8 5w cold cathodes, great savings, but I do need a back up light (9w) from time to time. First of all the light was a bit of a joke as they take a while to reach any sort of brightness. Like all things you do get used to less powerful lights.

John

I've just seen that Lidl are selling energy monitors from next Monday (13th) for £6.99. Seem to do everything that you need as per Stoney's post. But I think the Lidl offers vary according to region, so check your local shop, and be quick, because some of their offers seem to sell out quickly.

But here's my own personal disclaimer: I'm deeply unimpressed by the quality of some of Lidl's tools and hardware. People tell me the food is OK (never bought any myself, so can't comment), but I've had a number of tools, computer accessories, and other items, and some of it is pure rubbish, with a very brief life span. You might get some decent use out of this item. But then again, you might not. I'm also a little uncomfortable with their pricing policy - making everything so cheap can surely only make us all worse off in the end. Having said that - it's tempting, isn't it?

Paradox say' his 12v trickle charger is charging his batteries. I have one of these. It is rated at 1.5w max current 125ma. I put a multi meter (new purchase from maplins so not to clued up with it yet) on mine in hazy winter sunshine. It read 18.9v and showed 0.03ma. can check these figures. If so you would be unlikely to even counteract the usual normal unused battery loss, perhaps? The idea of adding water to the kettle from room temperature water can be improved upon with say a matt black jug left on a south facing windowsill for example. I have a kettle rated at about 800w that heats about 2 mugs of water, possibly cheaper to run than measuring the cups of water need in the normal kettle. Turning kettles etc. off at the wall is a good idea. Can you not measure costs of using a kettle for example by dividing the kw hour cost of say a 2kw kettle by the time it takes to boil? I have a battery charger from Lidls it is a type recommended as good elsewhere, but lots cheaper from Lidl. The energy monitor Muddypause mentions would seem to be useful at £6.99. In todays Observer (not my usual read) a wireless high tech electricity consumption meter called a Wattson which you can hook up to the internet and join a network of Wattson users. At £350 whats the point? (also diykyoto.com)

John

What I'd like is one of the smart meters under development by More Associates. It shows everything in the house (by name) that is drawing electricity and gives all the details.

I've read that the prototype ones proved especially useful at finding electricity "leaks" - appliances that appear to be off but are still drawing power. Examples included kettles, radios and battery chargers with no batteries in them.

They also identified appliances that had been accidentally left on or drew far more power than you'd expect.

The most sophisticated versions had the expected remote internet access but, even better, allowed you to specify appliances that remain on so that when you left the house you could click a single switch and shut off everything bar those appliances.

I'd love one!

jondy wrote:Paradox say' his 12v trickle charger is charging his batteries. I have one of these. It is rated at 1.5w max current 125ma. I put a multi meter (new purchase from maplins so not to clued up with it yet) on mine in hazy winter sunshine. It read 18.9v and showed 0.03ma. can check these figures. If so you would be unlikely to even counteract the usual normal unused battery loss, perhaps? The idea of adding water to the kettle from room temperature water can be improved upon with say a matt black jug left on a south facing windowsill for example. I have a kettle rated at about 800w that heats about 2 mugs of water, possibly cheaper to run than measuring the cups of water need in the normal kettle. Turning kettles etc. off at the wall is a good idea. Can you not measure costs of using a kettle for example by dividing the kw hour cost of say a 2kw kettle by the time it takes to boil? I have a battery charger from Lidls it is a type recommended as good elsewhere, but lots cheaper from Lidl. The energy monitor Muddypause mentions would seem to be useful at £6.99. In todays Observer (not my usual read) a wireless high tech electricity consumption meter called a Wattson which you can hook up to the internet and join a network of Wattson users. At £350 whats the point? (also diykyoto.com)

John

I havent checked the output of mine to be honest.
When i get a chance i will break out the multimeter and post my findings.
But at ten pounds in the sale for the solar panel im sure it has to be worth it just to keep the batteries topped up?
Im lead to belive led acid batteries discharge themselves quicker in colder winter temperatures so anything thats keeping it from discharging should help.

Maybe 2 or 3 panels connected in parralel would be a better idea as it would give a greater charging current.

Hi Paradox, Yes £10 is a cheap price for your trickle charger, better to put in a little charge than none. Batteries do perform better when warm. With your multimeter you can get an indication of the state of charge by taking a voltage reading. I guess you know that. Can't remember the figures off hand but very roughly a 12v reading would indicate a flat battery and 12.9 say would indicate full charge. Check out the figures, sure someone will know them or Google or I will find them. I am not to clued up with my new multi meter, too many functions. I would be interested in the readings you get from your pv panel. Batteries last longer if kept in the range of half full to full as a rough guide, ie take the power off the top.

John