Electricity - How Much Does It Cost? III

Just a quick post about the power energy monitor I mentioned in this post. I know a couple of you have bought one so I thought I'd give a couple of tips on how best to use the device.

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The monitor will display all manner of useful information such as RMS Voltage, RMS Current, Active Power, Apparent Power, Line Frequency, Power Factor, Energy Quantity and Time. Now, unless you're an Electronic Engineer most of that is of little interest. What we want to concentrate on is Active Power (measured in watts) and Energy Quantity (measured in Kwh).

Active Power: We would use this reading when connected to a device which has a constant power consumption (i.e it's power consumption does not vary over time). To use the table lamp example I've mentioned before we know that a 40w bulb will consume 40w for the duration the lamp is turned on. It's power consumption will not change over time. To convert watts into Kwh (remember, that is what your electricity supplier will bill you in 1 unit = 1Kwh) you just divide by 1000. Therefore the 40w lamp will use 0.04Kwh (0.04 Kw per hour). This reading also applies to devices on standby etc.

Energy Quantity: We would use this reading when connected to a device whose power consumption will vary over time or will last for a finite time. Take for example a sandwich toaster. When you first switch it on it will consume power in an effort to heat the plates up to the desired temperature. Once it has reached that temperature it will switch the heating element off (usually indicated by the wee light on the front of the toaster switching off). If during the toasting of the sandwich the plates cool down enough the toaster will switch the heating elements back on to top up the temperature to the desired level. This can happen numerous times during the cooking process. As the toaster is already hot it will use less power to top up the temperature thus making it unsuitable to read the active power value on the meter as it will be constantly changing over time. For this we use the "Energy Quantity" value displayed on the meter. Helpfully this is already converted into Kwh for us so no further calculatuion is required.

So to get a reading on how much power was consumed to make that cheese and ham toastie you plug the toaster into the meter, switch it on and activate the button on the meter to display Kwh. Toast your sandwich and when it's all done read the value displayed on the meter. This is the total amount of Kwh the toaster consumed while making the sandwich. To find out how much it actually cost you to cook simply multiply this value by the value of 1Kwh of electricity you calculated using your electricity bill (as described in this post). Remember these aren't wild approximations or industry overhyped values this is the real, personal cost to you.

You'd use the same method for calculating the cost of a load of washing, or a dishwasher or to boil a kettle etc. Not very difficult when you actually get down to it.

I've now got a spreadsheet listing every electrical device I have plugged in on a room by room basis and am slowly taking readings of everything. Saving my cash (and the planet) one plug at a time.

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