You have probably seen APS and SRP promote switching your utility plan to a time-of-use plan if you are not currently a part of one. It is painful to almost double your current rate (up to $0.21 a kWh compared to $0.11 a kWh for off-peak) during on-peak hours so it can save on your energy bills but you have to manage it correctly. I have switched and switched back and as a Phoenix home energy auditor I can recommend some tips to help you make an informed decision.
For the time-of-use plans, I have found them beneficial only if:
1. You leave home for work on a regular schedule, especially during the summer when daytime rates are higher.
2. OR you install a major high efficiency upgrade like a variable speed pool pump or water heater timer.
3. OR if can be diligent, and to some degree, put up with, managing your energy usage during the day. That means precooling your home to 72 degrees an hour before on-peak time starts, then bumping up the thermostat until 6 pm and off-peak begins. It also means to a lesser degree doing laundry (the dryer more so than the washer) and running the dishwasher at night... but don't worry if you don't follow this one so closely, the most important energy user is your heating and cooling system.
During the fall, winter and spring when APS rates are lower, the savings benefit will not be as great and the effort you put into trying to manage your usage I have found, not to be worth the small savings if not done on a regular basis. So if you are not on a regular work schedule where you leave the house in the morning then return at night, it is difficult to realize energy savings.
As a side note you are allowed by SRP to switch your plan twice without penalty, so you could try a time-of-use plan and see if you can manage the thermostats where you hardly notice the temperature setback, then if you find you don't like it you can switch back to a normal plan anytime (i.e. 1 day later) at no cost. If you find you want to try a time-of-use plan again though, then SRP will let you but if you want to go back, you will have to wait a several months or pay a penalty fee.
The setback range will vary depending if you ask an HVAC contractor or APS or SRP, but 10% of where your thermostat is set now is a good rule of thumb. So if you are at 78 degrees, an 8 degree setback is good. An important consideration is if your AC system(s) are sized correctly for your house. If it is sized properly they should be able to handle cooling the house back down within 1 hr without a problem. If they were oversized, it would be faster to cool your home but you would be wasting energy from short cycling the rest of the day when you were at a steady 78 degrees. Short cycling is wasteful and uncomfortable because an oversized AC system will reach it’s thermostat set point quickly, so the AC runs for only 5 minutes then shuts off. This causes temperature fluctuations between hot and cold, wastes energy because of the shut up of the motors requires an energy jump-start and uses more energy over the long run. If the AC units were undersized, I would only set it back a couple of degrees because it would take longer for the house to cool. A perfectly sized system will run continuously for 1 hour during the hottest part of the day. I always recommend trying it and adjusting as you go because we can do heat load calculations and theory all day but until you apply it to your lifestyle, home and duct leakage we never truly know how the system will react.
A variable speed pool pump has 8 different settings available to help homeowners take full advantage of APS and SRP's time-of-use plans. SRP time-of-use has on-peak hours from 1 pm - 9 pm during the summer and 5 am - 9 am and 5 pm - 9 pm during the winter. With a variable speed pool pump you want to turn your pool over only once to maintain a clean pool, so you can program different speed settings and times to run the pump that coordinate with your pool's volume and the off-peak hours.
You can learn more about thermostat set points and what SRP and Energy Star have to say at the link below.
You may also want to try a water heater timer, where the water heater is turned on during the night and off during the day, and see if you can match it to you lifestyles. Ideally, you won't notice any change and your recharge on the water heater is completely in the background. It is a good idea to save some more money on your utility bills because the tanks are constantly turning themselves on to keep the water at 130-140 degrees, even when you are not at home.
Here are some important links:
APS Time of Use Rate Plan
Having a teenage daughter and a day-time nanny makes it hard for me to take advantage of the time-of-use programs but I do encourage it for our clients. Good luck and let us know if you have any questions.
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