Wednesday, 26 November 2008

Daily Electricity Bill

Way back at the end of July I started looking into how to graph current cost data. I was looking at this from the point of view of putting the information together on a web page somewhere and potentially sending out useful information via e-mail to me and Beth on our power usage.

Recently I've been looking into this again and over the past couple of weeks we've been receiving a daily HTML formatted e-mail with an attached graph image that might look something like this:


This is a really handy reminder each day to try and be as green as possible but also to show us how we did on the previous day. The graph shows two lines, the red one showing the actual power usage in watts and a straight blue line showing our average usage for that day.

Also attached to the e-mail are the charges and a few summary statistics for the day. So on the example above you can see we averaged 240 watts throughout the day with our minimum usage recorded as 61 watts and maximum usage as 2359. The total usage doesn't mean very much without time associated with it, but I've listed it there all the same. The total can be used to work out the kWh reading for the day, total divided by 1440 minutes in the day (and points on the graph) divided by 60 minutes in an hour.

The costs are rather crudely calculated. We're charged for electricity on two levels, our first 225kWh per quarter are at a higher rate than any usage thereafter. For us, this averages out at around 10 pence per kWh cost so that rough average is used to calculate an estimate for the total cost in GBP.

This only a really simple way in which the current cost data can be used, Dale is way ahead of me on other applications for the data. I intend to extend the e-mail we're getting at the moment to integrate into a little web system we can use at home to log in and look through historic records of our usage and also produce other summaries such as weekly or quarterly usage. Also, the ability to add notes to each day will needed so when we look back over historic data we might have recorded what particular spikes in our usage are. All this is an effort to educate ourselves more in our power usage so we can change our behavior to be more efficient with it.

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