We hear so much these days about fake news, scams, and marketing. Anyone recently go out and invest in an energy efficient, energy saving, automatic setback, smart thermostat recently as the marketing says they will save you money? Yes, you did! I know you did! Well, guess what, you are not alone. I did as well. Well, I really should say that I upgraded a three year old version to the most recent Nest ™ thermostat because I wanted the tracking and reporting that the Nest has as a part of its interface. We recently installed Solar Photo Voltaic panels and have a little over 5Kw per hour of generation capability and are about to add another 2Kw per hour to bring our totals up to 7Kw. Why upgrade the thermostat? As air conditioning in Central Florida is our largest expense, and a appliance you cannot live without, I wanted to make sure that we were running our air conditioning at peak efficiency with all of the energy savings in mind.
So, we installed the new thermostat at the start of a three week period with temperatures in the upper 80’s and low 90’s. During the first week of operation we set the temperatures how we have always set them. We both work from home so we keep the house at 75 degrees from 6:30am to 10:00pm. At 10:00pm we drop the temperature 2 degrees to allow us to have a cooler room for the night. Since the bedroom does not have it’s own thermostat and we have six greyhounds in the room overnight we need to start off with it cool. The history of the Nest during this period showed that our system ran between 6-3/4 and 7 hours a day. Our A/C takes 5Kw of power per hour to run. The average cycle time for the system is about 15 minutes. To keep it simple we will use the higher number of seven hours of runtime times 5Kw for a total of 35 Kw hours of power. Our power is billed at 0.1279 per Kw, so our A/C costs us $4.46 per day to operate on a 90 degree day. A further look at the thermostat history showed that between 6:30am and 3:30pm the system really never ran. Peak use was between 3:30pm and increasing in activity until 9:00pm. Sun goes down around 7:00pm during this time.
Well, I went nuts over the 7 hours of runtime that I saw. Hey, I was installing solar panels in order to cut our power bill and perhaps sell a little back to the power company. 35 Kw of power per day was going to cause problems. So, after some discussion we made a change in the programming. We started running the system from 6:00am until 7:00pm at 78 degrees, an increase of temperature of 3 degrees. Then we dropped to 75 from 7:00pm to 10pm and then to 73 degrees overnight. In essence the entire time the sun was up we had the temperature set at 78 degrees. In terms of weather conditions they were the same as the prior week. We ran this method for a full week and looked at the history. With the new setbacks we ran the system between 6-1/4 hours and 6-1/2 hours per day. Let’s take the maximum savings of 3/4 hour (7 down to 6-1/4). So, 6.25 hours multiplied by 5Kw comes to 31.25 Kw hours of power used for our air conditioning. The history also showed that peak run times started at the 5:00pm area and ran all the way until 9pm cycling about every 15 minutes. Multiplying this slightly lower use out by 0.1275 (our power rate) we come up to a $3.98 (and some decimals) of cost per day, for a savings of 0.48 (48 cents) per day. For a total of $14.40 per month of savings. Sounds awesome? Right? Well, not really.
For this savings we had wild temperature fluctuations and humidity fluctuations around the house. One of our offices we up to 82 degrees during the daytime hours as the sun hits that area pretty hard early in the day. The other office is on the other side of the house and became slightly unusable during the afternoon hours as it is South facing and gets the midday and afternoon sun. The really interesting part was that the late evening run-time actually went up. Why? OK, a lesson in building materials comes into play here.
When sun hits the side of a building the heat has to transfer through all of the building materials, insulation, and sheet rock to get into the space. The rate of this transfer depends on the materials the wall is built from, and the temperature difference between the inside temperature and outdoor temperature. Nothing else really matters. Oh, and the amount of sun hitting it depending on the time of day and the direction the wall is facing. So, the sun hits the East corner of the house first, moves towards the South, travels across the South facing surfaces, and then “attacks” the West facing surfaces during the late afternoon and early evening.
The next thing you need to know is that heat transfer is not “immediate.” It takes time for the heat to come through the material. For example, glass is almost immediate. Wood takes slightly longer. Wood with insulation and sheet rock takes even longer, and then we have concrete block and brick. The walls in our house are build with Stucco, air gap, concrete block (with air gaps), insulation, and sheet rock. The air gaps are important as they are an insulator, they do not allow heat to pass without first heating the air in the gap. According to Manual J 7 (the Bible of Heating and Air Conditioning design) it can take up to 4 hours for the heat to work it’s way through this combination of building materials.
Back to our house. The sun starts hitting the East wall at 7:30am in the morning (no windows on this side), the sun then begins to warm the South facing wall (lots of windows and brick) around 11am and gets stronger on that surface until 3:30pm, and then begins to work on the West side of the house until the sun sets around 8:00pm. Make sense? If you put this into perspective:
The office on the East side of the house starts to get warm around 10:30am (3 hours after the sun starts to hit the East wall) and continues radiating heat into the space until 3:00pm in the afternoon. Then, the South facing rooms begin to get heat immediately at 11am as the sun hits the windows and then begins to get the heat from the walls around 3pm in the afternoon. This heat transfer continues until about 7:30pm in the evening. The West side of the house starts getting heat into the conditioned (inside spaces) at 6:00pm and continues transferring heat until 10:30pm-11:00pm at night.
Let’s put it all together and see if the setback actually makes sense? We setback the thermostat at 6:30am in the morning. Heat does not start radiating into the house until about 10:30am the temperature of the house starts to rise and eventually reaches 78 degrees (3 degrees above our desired 75 degrees) at about 1:00pm. The system starts cycling as the South facing rooms are already getting heath through the windows and then the walls. Then, the heat continues to build on the South and then the West side starts adding in. So, all of the walls are heated above outside temperature by 11:00pm. Then, in the middle of this heat transfer we drop the temperature again to 75 degrees and then again to 73 degrees at 10pm.
Here is the critical part: Not only is the air conditioning trying to maintain the temperatures based on the heat working its way through the walls, the air conditioning is attempting to drop and maintain a lower space temperature at the same time, this causes longer run times in the late evening hours (7:00pm and 10:00pm). It takes about an hour of runtime to do a temperature drop of 3 degrees. Concrete block walls have a lot of mass to cool.
Please note, this conversation did not take the roof into account, nor the material the floor is made out of. We also did not take into account the damage that the higher humidity does, or the wear and tear on the compressors in the refrigerator and freezer that runs harder, nor the additional stress on the Air Conditioning compressor caused by the higher refrigerant pressures and the higher temperatures.
So, back to the original question. Is it worth it? For $14.48 per month I think I would rather be comfortable. One additional 500w solar panel will take care of the .48 cents per day in 7 hours of sunlight. I think that makes sense.
Payback period on a 500W solar panel that is grid-tied? Let’s say $600.00 for the total package (reality) divide by $14.48 per month, about 41 months. That’s less than 4 years of a solar panels 25 year lifespan. Once the solar panel is paid for you are not paying for the comfortable factor and you are starting to reduce your electrical use without being “uncomfortable” in the process.