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Explaining Nest Learning Thermostat numbers

Throughout nest.com, on our packaging, and in our ads and videos we tell you how certain Nest Learning Thermostat features can help you save energy, how often they’re used, and how the Nest Learning Thermostat can affect your life. This article summarizes where those numbers come from.

Half your home’s energy

We say that the thermostat controls about half your energy bills. This information comes from the US Department of Energy 2011 Buildings Energy Data Book, Chapter 2.

Programming saves

We say that a properly programmed thermostat can save you 20% on your heating and cooling bills. Auto-Schedule remembers the temperatures you set and programs a personalized schedule for your home, so it allows homeowners to have a “properly programmed” thermostat without actually having to program it.

We reached this conclusion by studying data from our own tests. Here’s how it was done:

  • We had two efficiency trials: one in the winter and one in the summer.
  • Testers across the country were asked to:
    • Use their thermostat manually, like they always had. They set temperatures to keep themselves comfortable, but didn’t program a schedule.
    • Let the Nest Thermostat learn their schedule with Auto-Schedule or program the Nest Thermostat manually.
  • In winter, the average minimum temperature that people set was 61.6°F and the average maximum temp set was 69.5°F. So we determined that a “good” schedule - one that saved homeowners energy - was one where the temperature went down 8°F for at least 4 hours during the day and/or night. In summer people change the temperature less, so to get a “good” schedule, testers only had to increase the temperature by 2.8°F.
  • We normalized weather data and compared the numbers with and without a schedule.
  • Results: in winter, testers with a good schedule saved an average of 19.6%. In summer, testers averaged a 20.1% reduction in AC usage.

More details about how these trials were run and what temperatures people set are available in our whitepaper.

95% of Nest Thermostats have schedules

Of all the Nest Thermostats connected to Wi-Fi in spring of 2013, 95% had schedules.

Nest Leaf

The Nest Leaf encourages people to choose more energy-efficient temperatures. When speaking about the Leaf, we sometimes state that changing the temperature one degree can cut your energy use up to 8%.

This data comes from our own tests and simulations:

  • To determine winter savings numbers, we looked at two kinds of homes:
    • Type 1: These homes are usually occupied in the afternoon.
    • Type 2: There’s usually nobody home in the afternoon.
  • To determine summer savings numbers, we observed the schedules from testers across the country who were asked not to have a schedule, then to use the Nest Thermostat's Auto-Schedule or manually program their thermostat.
  • Results: in our winter tests for two different kinds of homes, simulations showed that a 1°F change to temperatures throughout the day results in a 2-5% change in energy usage (HVAC runtime). In summer, we looked at tester’s schedules and inferred that changing the temperature 1° results in the the HVAC running an average of 8% less while cooling.

More details about how these trials were run and what temperatures people set are available in our whitepaper.

Auto-Away

Auto-Away is a Nest Thermostat feature that automatically turns the Nest Thermostat to an energy-efficient temperature when nobody is home. We say that Auto-Away works in 90% of homes, even when the Nest Thermostat is in a spot you don’t pass on your way out the door.

When a Nest Thermostat is first installed, it will take a few days to learn about its surroundings and figure out if it can sense enough activity to reasonably understand when people are home and when they’re not. When we looked at all Nest Thermostats currently connected to Wi-Fi, 93% of them had high enough sensor confidence to activate Auto-Away. Only 7% of thermostats are in places that don’t get enough sensor activity, meaning they’re in a closet or unused room.

We mention this on the website because many people are concerned that Auto-Away won’t work in their home, simply because their thermostat isn’t in a great spot.

Filter Change Reminder

We say that a clean filter for forced air savings can result in 5% savings. This information is from the Consumer Energy Center.

Seasonal Savings

We say that Seasonal Savings has helped Nest Thermostat users save 5-10% on heating and cooling. This information is from our Seasonal Savings trial, outlined in the Seasonal Savings whitepaper.

Rush Hour Rewards

We tell you about how much we think you could earn from Rush Hour Rewards for each of our energy partners, for example “You could earn around $60 from SCE this summer.”

How much you get paid depends on your energy provider, the weather, the number of energy rush hours that summer, the size of your air conditioner, and the amount of electricity you save across your whole home during a rush hour compared to your normal electricity use on non-rush hour days. Our estimates can’t account for all of this and even though we attempt to include relevant factors, a precise amount can’t be guaranteed. Here are examples of how we calculated how much you could earn:

How we calculated “around $60” for Southern California Edison:

4 hours

Length of each rush hour

3 kW

Average air conditioner size, per the California Energy Commission

33%

Average AC runtime reduction during a rush hour using Rush Hour Rewards. More details can be found in the Rush Hour Rewards whitepaper.

$1.25 per kWh

Account credit from SCE for each kWh of energy saved during a rush hour compared to your average kWh use during the same time on non-rush hour days.

12

Hypothetical assumption of 1 event per week during the summer (12 weeks).


$5 per rush hour = [(4h * 3 kW) * 33%] * $1.25 / kWh
$60 per summer = $5 per rush hour * 12 rush hour days per summer


How we calculated “around $20” for Reliant:

2 hours

Length of each rush hour

3 kW

Average air conditioner size, per the California Energy Commission

33%

Average AC runtime reduction during a rush hour using Rush Hour Rewards. More details can be found in the Rush Hour Rewards whitepaper.

$0.80 per kWh

Account credit from Reliant for each kWh of energy saved during a rush hour compared to your average kWh use during the same time on non rush hour days.

12

Hypothetical assumption of 1 event per week during the summer (12 weeks).


$1.60 per rush hour = [(2h * 3 kW) * 33%] * $0.80 / kWh
$19.20 per summer = $1.60 per rush hour * 12 rush hour days per summer


Reliant customers that use less electricity during the requested high demand hours compared to the same time of day during the five previous weekdays will receive the Degrees of Difference with Nest account credits within one to two billing cycles. The program is not available to customers enrolled in the Reliant SmartStart Plan® and participants must be in good standing to earn Degrees of Difference with Nest account credits.


Saving energy example homes

On the Saving Energy page we created three simulated homes to show how different Nest Thermostat features interact to save energy. We used real data to simulate three realistic scenarios:

Our basic premise was that these homes - with their different needs, locations and residents - can all save energy with a Nest Thermostat. Prior to having a Nest Thermostat, we assumed that, like 89% of people, none of these families programmed their thermostats. Here’s a precise explanation of how we imagined their lives and the data we used to establish their schedules and savings numbers:

A. Johnny and Kate, Chicago, IL
Family with two kids. Kate stays home but is constantly on the go, dropping off/picking up kids, running errands, and going on an occasional outing with friends.

  • Lives in a 3,000 sqft home. Forced air. Gas furnace. AC.
  • Heating schedule: 7am 70°F, 9pm 66°F. Away temp 62°F.
  • Cooling schedule: only turn on AC when needed. Away temp: 82°F.

Kate wants to turn down temperature when she is away to keep her heating bills low, but her schedule isn’t predictable enough to do so during the day. However, they do turn down the temperature at nighttime by 4°F, which Auto-Schedule remembers and results in 9% savings.

With their irregular schedule, Johnny and Kate take full advantage of Auto-Away. Auto-Away turns on three times a week for more than 4 hours at a time on average. Because Kate set their Away temperature to be 8°F lower than her usual temperature during the day, she is saving 16% on days the thermostat goes to Auto Away. 16% for 3 days/week amounts to 7% savings every week. All together, winter savings from a Nest Thermostat total 15%.

During the summer, Kate and Johnny don’t use the AC too often. They turn it on during really hot afternoons, but they turn it off by early morning. AC typically runs fairly consistently during this time so Airwave doesn’t run too often. Total Airwave savings are 2%.

Because their heating bills are 10 times higher than their cooling bills, they are saving 14% overall from heating and cooling bills with a Nest Thermostat.

B. Angelina and Dillon, Seattle, WA
Newlyweds. Both work many hours. No kids. One dog.

  • Lives in a 1,500 sqft condo downtown. Heat pump with AUX. Forced Air. No AC.
  • Heating schedule: 6am 71°F, 8am 65°F, 7pm 70°F, 11pm 65°F. Away temp: 65°F.

Angelina and Dillon bought their Nest Thermostat because they heard it can reduce energy bills for heat pumps with AUX, which are common in Seattle. Their Nest Thermostat learned a schedule that reduces the temperature while nobody is home as well as when they’re asleep, but they have an older dog and want to make sure he’s comfortable. So they set both their afternoon temperature (while they’re at work) and Away temp to 65°F. Because their daytime temp and Away temp are the same, they don’t save much from Auto-Away. This schedule, however, is saving them 14%.

The Nest Thermostat activated Heat Pump Balance for the couple, and they set it to Max Savings. That saves them 9%. Finally, they pay attention to their Nest Thermostat's Filter Reminder and change their filter regularly, saving an additional 6%. Put it all together and their heating bills are 26% lower with a Nest Learning Thermostat.

C. Tina and Joe, Phoenix AZ
Retired couple. Home a lot. Grandchildren stay at their house in the afternoon.

  • Live in a 2,000 sqft home. Forced air. Electric heat. AC.
  • Heating schedule: 7am 72°F, 9pm 68°F. Away temp 62°F.
  • Cooling schedule: 7am 74°F, 9am 76°F, 3pm 74°F, 9pm 73°F. Away temp 79°F.

Phoenix summer is hot. While Tina and Joe’s schedule doesn’t have a large increase in temperature during the day, they made a conscious decision to increase the temp by 2°F when the grandkids aren’t over. This schedule is saving them 10% on their cooling bills.

They like to go out to play golf about twice a week, so Auto-Away saves them 20% on those days. This is 6% savings total. Finally, Airwave is working quite well in their dry, hot home and cutting their AC runtime by 30%. That saves them 9% on their cooling bills. All together, they are saving 23% with their Nest Thermostat in the summer.

The winter is mild so heating doesn’t turn on much during the day. However, it turns on a lot at night because it gets cold. They’ve turned down the night temperature, so they are saving 18% (mild weather places have larger percentage savings). Auto-Away in the winter is saving them an additional 2%. In the winter, they are saving 20%.

With cooling costing 5 times more than heating, they’re saving 22% on their heating and cooling costs with a Nest Thermostat.

There are two different ways that these savings number interact.

A. First, it is good to understand how numbers “add” up. If Angelina and Dillon are saving 14% from Auto-Schedule and 9% from Heat Pump Balance, the savings aren’t exactly 14+9=23%. The savings are actually: 1 – (1-0.14)*(1-0.09) = 0.2174 (21.74%). So if the monthly bill without the Nest Thermostat was $100, with the Nest Thermostat it’s $78.26.

B. It’s also good to explain how Auto-Away savings work. Kate set Away temperature to be 8°F lower than her usual heating temperature. This choice is saving her 16% on days she goes to Auto-Away. She goes into Auto-Away on average three days a week, so to get the overall Auto-Away savings, this must be calculated into a seven day a week number. The calculation is: 16% x 3 days / 7 days = 6.86% savings overall from Auto-Away.

C. Heating savings and cooling savings add up, but based on where a family lives and whether they have an air-conditioner, these numbers add up differently. For example, assume that cooling bill savings are 20% and heating bills savings are 10%. If cooling bills are twice as expensive, then the overall savings are: (20%*2 + 10%*1)/(2+1) = 16.7%. So if the total cooling bill is $200 without a Nest Thermostat , it is $160 with a Nest Thermostat. If the total heating bill is $100 without a Nest Thermostat, then it’ll be $90 with a Nest Thermostat. So if this family used to pay $300, now they pay $250 with a Nest Thermostat.

D. Finally, it’s good to understand that there are different kinds of interactions between these savings numbers. For example, savings from having an aggressive schedule doesn’t affect how much savings you’d get from Heat Pump Balance. And the amount of Auto-Away savings doesn’t affect Airwave savings. However, Auto-Schedule and Auto-Away savings both come from schedule changes, so you have a great energy-efficient schedule that’s set to save energy when you’re gone, but Auto-Away savings won’t look large. And if you don’t get much savings from your temperature schedule because your real-life schedule is always changing, then Auto-Away could result in big savings.