Measuring energy consumption
One of the first steps to take in reducing your personal energy consumption is to find out how much energy you and your family actually use.
I’ve mentioned this on Selfsufficientish and to friends and neighbours, with the inevitable result that they ask me how to do it.
Well, here’s the Stonehead Guide to Measuring Energy Consumption…
First, get yourself a power meter. The one from Maplins is the cheapest I’ve found in the UK 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! Do a bit of Googling to find the best offer.
Now, back to the actual meter. The three most important readings for our purposes are active power (in watts), energy consumption (kWh) and elapsed time.
While you can use the active power reading for instant readings, 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 socket, 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 elapsed 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).
Active 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 = £0.003552 (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’ve triple-checked the figures, but if anyone spots anything let me know!
This is very interesting mate. I think I would like to try such a device out down here in Aust.
As we also have pumps (plural) for our water supply the calculations would be of considerable interest for us “bore and tank” water people. For example, our washing machine operates from one rainwater tank via an electric pump.
I will do some Googling to see what I can find that fits our power system.
The Australian equivalents are a total rip-off with prices of several hundred dollars! The Maplins one I mentioned in my post would cost Aus$33.46 including VAT.
Genesis Automation do a rather crude and old-fashioned looking one for “only $685 plus 10% GST”.
There’s a more consumer friendly one (similar to the UK ones) called the Sparometer digital power meter, which has “a new low price of $180″ but it seems to be out of stock everywhere.
And I’ve found mention on a couple of Government websites of one called a Power Mate.
I’ve just found your website and find it most interesting.
So far, I’ve been checking the energy consumption of various items in the house by counting the revolutions on our electricity meter and using a stop watch. This gives pretty useful figures, especially for items which take a very low current – but it does mean that I have to switch off or unplug all the unused items in the house!
I divide the product of 320 x 60 by the number of seconds it takes for the disc to make one revolution. This gives the wattage of the appliance.
And so, I’m thinking of buying a plug-in wattmeter….
I’ll certainly visit your site from time to time in the future.
Best wishes,
Spurred on by your posting last winter, we now have a meter set up to measure consumption. It is fascinating!
Funnily enough, though, even without looking at it, it reminds us to turn things off, particularly stand-by lights!
I am also pleased to find that my computer is not too expensive to run, but worth turning off when I leave it overnight, or for any length of time during the day.
Firstly, I must congratulate you on an amazing blog, and pursuing an awesome lifestyle.
With so many great articles here to read I wasn’t sure at all where to add my praise, though having found this one I thought it a linkback via my sig to a blog post I wrote about Green Computing. Though I imagine your PC use is quite low, it’s an area that few consider savings can be made.
Another thing that doesn’t seem to be covered or considered is using heat pumps to supply you with heating. These work via a heat exchanger and running piping below the ground to make use of ground temperature, allowing you to then heat your home and water. Running costs are quite low, though the initial installation/purchase is the expensive part. I imagine that you could keep down the costs of installation by digging the trenches yourself though whether you’re able to stretch to the cost is another matter.
Anyway, am bookmarking this blog of yours and will be back to read more in the future!
Roo, thanks for the kind words.
We have a Mac laptop, which is fairly energy efficient, and an old Mac desktop, which isn’t. We’ve wanted to replace the desktop’s energy hungry CRT display with an LCD one for a while (we could cut monitor energy consumption by 70-80 per cent) but it would mean diverting money from elsewhere. However, the priority may change as the image on the CRT monitor, which is about eight years old, has started blinking and distorting.
What we do to cut energy consumption is leave all peripherals off unless they actually need to be used (including the speakers), shut down completely and turn the power off at the wall when the computers are not in use, and try to keep our overall usage down.
Anyway, must dash as lunch is finished and there’s still a lot to do.
Hold your horses on that, I may be able to help you out and clear out my cupboard at the same time…you can’t keep everything for a rainy day!