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July 14, 2008

Green Gadgets: Calculating Electricity Cost

This is another article in our "Green Gadgets" column written by Shane Conder, a guest blogger.

In our last article on the Kill a Watt, we talked about using it to measure how much power your devices are using. Now we are going to talk about how to use that information to calculate the costs of operating your devices. It doesn't matter now if you use a Kill a Watt or something else that will tell you how many kilowatt-hours or current watts an appliance uses.

There are multiple ways to calculate the cost of using a device. One method is to use your current bill to find the total cost for a month and then divide that by the total kilowatt-hours (kWh) listed on the bill. This will give you an average cost per kilowatt-hour. For instance, if you had a $91.20 bill and used 480 kWh, your current average price per kilowatt-hour is 19 cents ($91.20/480). Now, taking that value, you can then multiply it by the number of kilowatt-hours that a particular device uses.  The example we used last time was a 100 watt light bulb, which will add 0.1 kWh every hour to your bill, so we now know that every hour that this device was on 1.9 cents was added to your bill.

This method is useful if your electricity is billed at a flat rate. It is also useful to see an average of how much each device you currently use is costing you.  In New Hampshire, we have an electric bill that is always charged at just about 16 cents per kilowatt-hour (cheap, from our point of view).

In California, though, we had a tiered rate (you're charged different rates depending on how much you've used - California charges higher rates the more power you use in order to reduce power consumption). Simply, your rate starts low, but if you went over a certain limit, you get charged at higher rates for the additional use. This had interesting ramifications, because your power bill might normally be charged at the low rate until you bought a new appliance, which added enough power consumption to your bill to bump you into a higher rate. Kind of like taxes. 

In California, our peak tier was 33 cents per kilowatt-hour. If our bill was over about $85 per month, additional use would fall into the peak tier at the highest rates. This means that any new appliance we plugged in or additional power we used was billed at 33 cents per kilowatt hour, not at the average rate from the previous bill. This is a very important distinction. Again, adding a device in a tier rate billing environment will add a cost at the most expensive tier you pay at and maybe even the next tier up. In the above example, this means if we had added the 100 watt light bulb, we would be adding 3.3 cents per hour of cost. When compared to the 1.9 cents average from the previous bill, our calculations would have been wrong by nearly 75%!

Now for a couple of examples: If you pay 16 cents per kilowatt-hour flat rate and you add an appliance that will be on all the time and uses 250 watts of power, how much will it cost?  An average month contains about 730.5 hours (see the tip at the bottom for an easier way to do these calculations). Multiplying 16 cents by 0.25 kWh by 730.5 hours/month, we see it will cost more than $29 per month or over $350 per year! What if you have a 100 watt porch light that is on for 8 hours each night and we pay 33 cents per kilowatt-hour? That would cost about $8 a month. For a light bulb that costs less than a buck, that's a lot.

If you let your measuring device, such as the Kill a Watt, add up the total for a month you have less calculating to do. Just multiply the kilowatt-hours by the rate. For instance, we left it measuring our TV (with regular use) for a month and saw 73.5 kWh listed. We then just multiply that by 16 cents per kilowatt hour to get a cost of $11.76 for the month.

Another important note is that an appliance plugged in may use power even when it is switched off. Many devices have idle modes or power saving modes. These usually still use some amount of power and it adds up fast. For example, we once measured one of our printers at 21 watts when just sitting idle. At our rate of 16 cents per kilowatt hour, that adds $2.45 a month to our bill. This power use is often referred to as "Standby power" or "Vampire power".

TIP: Did you know you can use Google to quickly calculate the cost of an appliance for a particular time period? You can. I calculated the examples by typing these search terms in to Google: "(16 cents/kWh) * (1 month) * 250 watts", "(16 cents/kWh) * (1 year) * 250 watts", and "(33 cents/kWh) * (1 month * 8 hours/day) * 100 watts" (the capital case of the "W" in "kWh" is critical) (click on the links to see the results). That's basically just the rate per kilowatt-hour, the time period (you can try 1 week or 1 day or 1/2 year, and so on) the appliance will be on for, and the amount of power used. It's fun and easy.

Now that we've learned how to calculate the cost a new appliance will add to our electric bill, we're going to go around our house and measure a bunch of stuff.  We'll then post our findings and see how they compare and we will tell you about any surprises we found (there have already been two!).

Posted by shane at July 14, 2008 2:46 PM