Hot! Hot! Hot!

Over the course of the next two weeks I’ll be doing a series of posts related to the mechanical systems and/or utility services found at the Byrne residence.

Today’s post is all about electricity including daily usage patterns, how much is used and for what purpose, plus a relative comparison to similarly-sized homes in the area.  Graphics were snipped from our account page on the Arizona Public Service (APS) website. I should also mention APS installed so-called “smart meters” in our community earlier this year, (I believe it was back in February). The level of detail regarding energy usage available on the APS website is very, very impressive – I only wish the smart meter had been installed years ago!

This first graphic, (below), shows daily usage trends over the course of a recent billing cycle, (3rd week of June through 3rd week of July).  There isn’t much day-to-day variance, but that shouldn’t be surprising, as there aren’t any spikes in demand – the thermostats are set to a constant temperature, (roughly 80 degrees in the summer and 65 degrees in the winter), the exterior low voltage lighting is on timers, (as is the irrigation system), and nothing else consuming electricity is used very much during the summer, (e.g. dishwasher, refrigerator, washing machine, clothes dryer, etc.).

What is striking, particularly when you take a look at how the energy is being used, is the proportion of energy consumed by the A/C system. Now take a look at the second graphic, (below). Over 85% of the electricity consumed during a typical summer month goes toward cooling the interior! We’ve taken steps to minimize energy usage during the warmer part of the year, (generally outdoor temperatures start to rise above 90 degrees regularly in May and don’t really drop back below 90 degrees until late September/early October), but even with the addition of 3M film and the extensive use of window coverings, the thermal load generated by the large expanses of green “solex” glass on the east, south and west-facing elevations is significant.

It will be interesting to see what the December to January usage breakdown looks like – I should be able to answer the question “Does heating consume roughly 85% of the electricity during a typical winter month?” and will undoubtedly post the answer here!

At first glance, you might think the Byrne residence is not energy efficient, but the next graphic, (below), shows how much more efficient – relatively speaking – the Byrne residence is compared to similarly-sized homes in the same area. In fact, this trend has been very consistent since well before I started tracking the data downloaded from the smart meter several months ago.  The energy usage breakdown, (pie chart), is derived from answers to an online survey on the APS website, and the comparison between your home and the average for the surrounding area has been a feature on the APS website dating back to when we first activated the account in 2010 – rarely, if ever, does our actual usage approach the average noted in the graphic, summer or winter.

For the June to July billing period, (based on data supplied by APS), we used 17% less energy than comparable homes, (i.e. our electric bill, when compared to the average bill for similarly-sized homes, was 17% less).

Update #1: Our August electricity bill arrived the day after I posted this entry. I’m happy to report the same trend described for the June to July period held true for the July to August period as well, (see graphic below). In fact, it got a bit better – we used 23% less energy than comparable homes, and the usage breakdown was nearly identical – 86% of the electricity consumed went toward cooling the interior. …consistent with the title of the post, it’s been “Hot! Hot! Hot! outside, no doubt about it!

Update #2: Our latest electricity bill arrived around the 21st of September. I’m happy to report the same trend described for the July to August period held true for the August to September period as well. In fact, it got quite a bit better – we used 36% less energy than comparable homes, and the usage breakdown was identical – 86% of the electricity consumed went toward cooling the interior.

What’s most intriguing – to me – is we’ve cut our usage by nearly 40% when compared to last year, (see graphic below). I think a combination of factors are contributing to the reduction in usage we’ve observed over the course of the past several months:

  1. Replacing the heat pumps – the 5 ton unit in July 2010, the three smaller units, (3, 2 and 1.5 tons respectively), in April 2012.
  2. Replacing two thermal expansion valves, (TXV), in May 2012.
  3. Installing an additional A/C return for the largest of the units, (the one serving the Living Room, Dining Room and Kitchen), in July 2012.

Update #3: Our latest electricity bill arrived around the 20th of October. Energy usage for the September to October period continued to decline, (this is expected as temperatures start to drop from their summertime highs), and we’re seeing the beginnings of a trend – an 18% decline when compared to last year, so we’ve seen pretty significant year-over-year declines for 3 months in a row, (and during the hottest time of year).

I was also struck by the relative efficiency of the Byrne residence compared to equivalent homes in the same area – we used ~54% of the electricity comparably-sized homes did! I’m beginning to think these new heat pumps, additional A/C return for the largest part of the house and our UV reduction efforts are really paying off!

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