Power Usage in the Desert

live in Arizona, USA and the temperature can easily reach 103F by 10:00AM on a summer day.  Obviously you have to have air conditioning to live in this climate.  Additionally, I have a house way too big for my needs and cooling it can be a problem.  I have tremendous insulation in the walls and ceiling, but a check of the temperature on an outside wall in the sun exceeds 170F !  Even with the insulation causing a drop of over 70 degrees through the walls, it can get hot.

Our local power company uses "Demand Billing" with daily peak periods and seasonal adjustments.  So, when you need the power the most, they charge you more; basic supply and demand combined with political considerations can lead to a huge power bill.  The current (always subject to change at a moment's notice) rate plan I'm using is:

Winter (November - April billing cycles)       Cents per kWh
On-peak kWh used (Mon. - Fri., noon - 7 p.m.)      $0.05815
Off-peak kWh used (Mon. - Fri., 7 p.m.- noon and all day Sat. & Sun., 6 holidays) $0.04273
Demand charge  $9.203 per KWH
Summer ( May - October billing cycles)  Cents per kWh
On-peak kWh used (Mon. - Fri., noon - 7 p.m.)  $0.08845
Off-peak kWh used (Mon. - Fri., 7 p.m.- noon and all day Sat. & Sun., 6 holidays) $0.04363
Demand charge  $13.404 per KWH

So, obviously, I need to limit my power usage during the peak periods, especially in the summer when I need it most.  To add insult to injury, the "Demand Charge" above represents the highest usage of power during the peak period.  This is calculated based on some obscure algorithm hidden inside the "smart meter" that they can't or won't describe.  This means that one mistake during a month that allows the usage to get out of hand will result in a HUGE charge based on the demand.  So, the first thing I need to do is be able to measure the power.

The power company suggested I use the meter to measure my usage, but that sucks because by the time the meter records it, I owe them the money.  There's nothing that can tell me what I'm consuming right now.  Off to the web I went and discovered the Arduino, a current measuring transformer, software, data collection web sites and blogging.

Necessity is the mother of ......whatever this has become.

Why Bother Doing This?

Earlier, I talked about my power usage and billing.  When I got my August 2009 bill I was appalled.  I hadn’t used much more electricity, but my bill had jumped way up.  My Peak Demand Usage had jumped to 13.9KW and tacked on over a hundred dollars in fees.  My Peak Usage had actually dropped a little bit percentage-wise.

It seems visitors using the pool and opening and closing doors had gotten both air conditioners, pool pump, hot water heater, and to dry their towels, electric dryer going all at the same time during the peak period from noon to 7PM.  Not surprising since this is Arizona and the pool is fun in the afternoon when the temps are over 110F.

So I installed thermostats that had 4 timers per day and all seven days to stop peak period usage, installed a timer on my water heater to stop it completely during peak periods and reset the pool timer to keep the pump off during the peak as well.  As you can see from the chart below, September was the first month with the new changes.

Peak %
Off Peak %

My bill dropped dramatically.  I called the power company and they were amazed.  Before you talk about how Fall had come and the temperatures had dropped, that is somewhat true, but this is Arizona.  The temperatures were still in the hundreds and I had gotten control of the Peak Demand Usage number that costs me close to $20 for each KW. 

The technique is to cool the house waaay down just before the peak period begins and turn off the air conditioner compressors during the peak period.  This allows the recirculation fans to run on a timer and distribute the cool air during the afternoon.  Fortunately, I have tile floors so they cool off in the night and morning then suck up the incoming heat during the afternoon.
As you can see I basically shut everything down at the beginning of the peak period and then turn it on at the end.  The spikes before the peak period are both A/C units running and cooling the house down before the afternoon starts.  The spike after the peak period is the A/C units cooling the house down after it has heated all afternoon.  The small spikes during the peak period are the recirculation fans moving the air to equalize the room temperatures.

My plan is to go into the details of each piece of this as I go along.  I know there are other people out there that would like to do this and they can leverage my work to make it easier.  I also plan on adding an Arduino to do the grab and forward task as well as display the current real power value.  Eventually (months maybe) I want to implement load shedding by sampling the data and turning things off as necessary.


  1. After adding your numbers for monthly power consumption I was shocked. Your total annual electricity consumption is 20x higher than mine. I don't live in house and fortunately don't need air conditioning (yet), so my numbers are not really comparable to yours. I couldn't afford it anyway (paying 35¢ per kWh here). But 20x more... How thick is the insulation on your walls?

  2. My walls run R25 and the attic is R39. I have two heat pumps, one a 3 Ton and the other a 5 Ton. I also have a swimming pool that needs to filter for several hours a day. I have only electrical service so heating is also electric as well as the clothes dryer and water heater. The temperature difference between the surface of the outside wall and the inside wall is often greater than 100F, so I think the insulation is actually doing its job. However, 20X is a large difference, what area do you live in?

  3. It's 100 degree's outside and you use a clothes dryer?

  4. Heh, good point. I tried drying outside twice. The first time I wound up with bird droppings on the sheets. The second time a wind came up and blew the stuff across the yard. Since I live alone and only have to do laundry every couple of weeks, I just take the lazy way out. Besides, have you ever noticed that clothes dried outside are stiff? They do smell good though.

    1. I thought about the clothes line a little bit more, and while it's definitely a good idea, it's not a solution. For example, I hung a rung over the fence to dry once and when I brought it back inside, half of it was faded by the sun. A nice red rug was half red, half pink. Another time I put a large rug on the patio to dry (out of the direct sun) and a cactus wren (bird) pulled a number of tufts out of it to build a nest with. So my 8' x 10' wool blend rug has bald spots in several places. I forgot a towel once and found it a year later in a pack rat's nest. So, if I use a clothes line, I'd have to be careful of what time I put the clothes up, make sure they come in the house before the sun gets them, and stand guard to keep the critters from hauling parts of them away.

      I guess I'll just have to live in the 21st century and use a clothes dryer. Except for the big rugs; I'll have to stand guard over them.

  5. Hello,

    I can understand the clothes dryer because if you forget the sheets outside for a full day you'll end up with a plank.

    What I don't understand is the water heater. With all that sun, heating water should be the least of your worries. :| Have you considered these solar panels made to heat up water? Or wouldn't that be a good enough investment?

  6. Look around a little more; I have a solar water heater. It's actually really nice. The only time it consumes power is on those overcast days when the sun doesn't come out, and here in AZ, there's about 8 or so of those a year.

    Come to think of it, I have two of them. There's also a solar water heater for the swimming pool up on my roof. It gives me two extra months of swimming pool time a year.

  7. What do you think of evaporative cooling? http://en.wikipedia.org/wiki/Evaporative_cooler

  8. I grew up with evaporative cooling. When I was a kid, large refrigeration units were way too expensive and most homes had swamp coolers. They work great in dry weather, but don't do much when the humidity goes up. They move the air around and make it more comfortable. I still have one on my garage. I can turn it on and make the garage more comfortable. They also keep smells out of the house by simply blowing them away.

    The problem is that they inject a lot of water into the air. Your wood starts to swell a bit, the floors are a little bit damp, and sheets feel clammy. They use a lot less power than refrigeration, but really can't do the job needed during humid periods.

    So, when the temperature is under 100, I open the door between the garage and the house and feed the swamp cooler into the house for a few days. This lowers my overall usage and blows out the house. Currently, you just can't beat refrigeration for keeping it cool during those 110-126 degree days though.

  9. haha, think about if you'd be paying 0,12 € / kW-h (about 0,23 $, +taxes) from noon to 22h, and 0,06€+taxes from 22h to noon, like here, in Spain. Maybe that is one of the reasons why nobody lives in a big house here. Congratulations for your great work and for sharing it with everybody.

  10. Sorry, it was 0,17 $/kW-h +taxes (24,8 % in total)

  11. I have cheap power compared to other places. In Los Angeles, they have a 'baseline' tiered rate that amounts to a huge bill. I don't know the details anymore, but someone may chime in and tell us, or I guess it can be looked up online. I remember it being about .25 per kWh after you passed the baseline. You ALWAYS go over the baseline.

  12. "The temperatures were still in the hundreds and I had gotten control of the Peak Demand Usage number that costs me close to $20 for each KW. "

    I'm thinking you meant $0.20 for each KW. By comparison, I live in So Cal (high desert area) and my average price was $0.13 per KWH. It's a tier system as you know, that's $87 / 682KW. It's a 5300 sq. ft. house I live in alone, pool (cut the pool pump from 4 hours/day down to 4 hours every couple days or so), refrigerator, 2 AC units (those weren't really used at all in March). My job keeps me away from home nearly all weekdays, 1 or 2 weekends, and 10 evenings per month.

    The other day, I went through the entire house shutting down nearly every device individually to see power draw (I have a Rainforest EMU-2 networked with the house's Zigbee smart meter and noted the KW drop with each device).

  13. I should have added that you're absolutely correct about everyone going over the baseline, even with my relatively "low" power consumption, I still make it way into the tier 4 range.

    1. With a 'demand' billing system, when you use just enough to bounce the 'peak demand' up a kW, you get slammed with a huge bill. So ... If I bump the demand number up a kW, it costs me and average, by season, 19+ dollars for it.

      That's only the 'demand' number. The kWH usage is still in the cents for each one. Where one gets into trouble here is that one day, they turn on the dryer and stove at the same time on a week day. That sets the 'demand' number for the entire month and thy're screwed.

    2. Easy enough... have an "ask permission" button alongside the heavy hitter appliances. For example, before someone uses the dryer, they hit the "mother may I" button attached to the dryer. Your real time kWH monitor compares current kWH usage with anticipated usage by the requesting device and if there's enough headroom/margin, enable the circuit breaker; otherwise, raspberry sound and alarm and keep the circuit off. Lol, of course I'm kidding.

    3. Don't kid too much I seriously thought about doing something like that. The problem with the dryer and the water heater is that they use so much power that just turning them on will mess me up.

      They're not allowed on at all during peak periods.